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Predation and the distribution and abundance of tropical sea urchin populations
Abstract
Nine species of herbivorous sea urchin commonly inhabit shallow-water coral reefs of the Indian Ocean. The abundance, distribution and diversity of these species were studied in seven reef lagoons over a 7-year period to determine the factors that control population variation in time and space and the role that predation plays in their ecology at a scale ≥0.1 hectare. Field studies of predation included experimentally tethering sea urchins (Predation Index), visual transects of fish to estimate the predator biomass and line transects of reef substrate rugosity (bottom contour complexity) which allowed for five possible measures of the hypothesized limiting resource — refuge from predators. The amount of refuge was used to predict the abundance of individual species of sea urchin, species richness and diversity. It was predicted from pervious manipulative species interaction studies that the top competitor in the guild (Echinometra mathaei (de Blainville)) would be directly and closely controlled by predation because competitive dominance may often reflect an evolutionary and energetic trade-off with predator susceptibility and, therefore, E. mathaei abundance would decrease either linearly or logarithmically with measures of predation. Competitively-subordinate species were predicted to have unimodal (“hump-shaped”) abundance relationships with refuge because these species should make an evolutionary trade-off between increased predator resistance and decreased competitive ability. Spatial and temporal variation in wet-weight estimates was high for all species except the numerical and competitive dominant, E. mathaei, and for all species combined. Spatial variation in species abundance was highest under conditions of high predation and low species dominance and decreased as the total wet weight of urchins increased and the competitive dominant occupied resources. The most common subordinate species frequently exhibited the predicted unimodal relationships with the best measure of refuge from predation but variation was high and the best-fit second-order polynomials predicted less than 55% of the variation in sea urchin biomass. There was evidence for partitioning of the spatial refuge resource among species particularly for E. mathaei (de Blainville), Tripneustes gratillaLinn. and Diadema setosum Leske at the low to intermediate levels of predation. There was, however, a large overlap in the peak abundance of Diadema savignyi (Michelin), Echinothrix diademaLinn., Toxopneustes pileolusLam. and Stomopneustes variolarisLam. at the highest levels of predation. Nonetheless, the partitioning of refuge among many species occurs more at the smaller scale of reef crevices (<10 m2) than at the larger scale of ∼0.1 hectare sites or whole sections of reef. Diversity and number of species, however, were strongly associated with measures of refuge and exhibited unimodal curves.
... The relationship between urchin abundance and spatial distribution on coral reefs is complex (e.g. McClanahan and Kurtis 1991;McClanahan 1998;Dumas et al. 2007;Burt et al. 2010;Graham and Nash 2013). The size, composition and distribution of urchin populations are affected by a diverse set of environmental variables, including structural complexity, coral and macroalgal cover, sedimentation, predation and disease (Lessios 1988;Feehan and Scheibling 2014) across multiple scales (Sánchez-Jérez et al. 2001; see review by Dumas et al. 2007). ...
... Numerous studies have investigated the association among urchin distribution, abundance and habitat (e.g. McClanahan and Kurtis 1991;McClanahan 1998;Dumas et al. 2007;Burt et al. 2010;Graham and Nash 2013), each highlighting the complexities relating to urchin spatial distribution on coral reefs. Here, we discuss the unique factors that may contribute to the sympatry observed in these two urchin species. ...
... Of the .500 species of fish that inhabit the reef and deeper offshore waters (Department of Biodiversity, Conservation and Attractions 2019), many, including balistids, labrids and lethrinids, are identified as predatory species of urchins (McClanahan and Shafir 1990;McClanahan 1998;Johansson et al. 2013), with numerous species of the latter two families being targeted by recreational fishers (Ryan et al. 2017). Whereas conflicting reports of urchin abundances within MPAs at Ningaloo Reef exist (e.g. ...
Sea urchins can play a critical ecological role in the functioning of marine benthic ecosystems, mediating competitive interactions between corals and algae. Yet, little is known about factors affecting urchin distribution in intact coral reef systems. This study aims to determine the spatial distribution of two sympatric urchin species, Echinometra mathaei and Echinostrephus molaris, and potential factors contributing to this, within the intact coral reef system of Ningaloo Marine Park, north-western Western Australia. Benthic photographs and surveys were conducted on SCUBA at 126 sites across the Park to determine urchin presence, rugosity, substrate cover, water velocity, and fish predation for each site. Generalised additive models found that E. mathaei presence was positively related to algal cover, rugosity and non-sanctuary zones, suggesting that distribution may be driven by foraging behaviour, habitat complexity and predation. Echinostrephus molaris presence was positively related to habitat and region, suggesting its distribution may be largely driven by hydrodynamics, feeding strategy and regional variation. This study highlighted species-habitat associations and the complexities of these in structuring urchin communities. Although occupying similar niches, the predominantly non-overlapping feeding preferences, and morphological and behavioural differences between E. mathaei and E. molaris enable these species to coexist within the intact reef system of Ningaloo Marine Park.
... A long-term monitoring study of E. mathaei on coral reefs in Kenya found that spatial variability in population densities is strongly and positively related to predation levels (McClanahan, 1998) (Fig. 28.1). In contrast, studies on the intertidal rocky shore of Hawaii suggest that water movement and the availability of algal drift positively influence the density of Echinometra as well as the ratio of E. mathaei to E. oblonga (Russo, 1977). ...
... This has been largely attributed to varying levels of postsettlement predation in these reefs (McClanahan and Muthiga, 1989;Muthiga and Jaccarini, 2005). Populations on most reefs have been fairly stable (<10% annual variation) in reefs with moderate population densities (10-70 ind 10 m À2 ), but population densities are more variable at low and high densities (McClanahan, 1998). In the high-density reefs there was a synchronous mortality event for two sites separated by $ 100 km in 1994. ...
... In contrast to the Caribbean studies, crevice competition studies in Kenya found that E. mathaei is a superior interference competitor over both Diadema setosum and Diadema savignyi (McClanahan, 1988;McClanahan and Shafir, 1990). For shallow fringing reefs with low levels of predators, E. mathaei lives outside of cervices, and other species are found only at very low population densities (McClanahan, 1998). This suggests that E. mathaei may be the superior resource competitor. ...
... One prediction is that the abundance and community composition of predators will reflect the intensity of fishing (McClanahan 2000). Secondly, the sea urchin species composition will be influenced by their differential susceptibility to predation and interspecific competitive interactions (McClanahan 1988(McClanahan , 1998. Finally, reducing sea urchin predators will change the competitive dynamics between fish and sea urchin herbivores and subsequent changes in grazing will influence the calcifying and noncalcifying organism dominance McClanahan 2010, O'Leary et al. 2013). ...
... In these sites, sea urchin biomass was estimated by haphazardly tossing a stone tied to a rope, using the rope as the radius of a 10-m 2 plot, identifying sea urchins to the species and counting their numbers within this circular plot. The species-specific density was multiplied by a mean body mass from field measurements and summed across all taxa to estimate total sea urchin biomass (McClanahan 1998). In some sites, we tethered the sea urchin Echinometra mathaei and measured its death due to predators over a 24-h period using the methods described in McClanahan (1998). ...
... The species-specific density was multiplied by a mean body mass from field measurements and summed across all taxa to estimate total sea urchin biomass (McClanahan 1998). In some sites, we tethered the sea urchin Echinometra mathaei and measured its death due to predators over a 24-h period using the methods described in McClanahan (1998). This method produces a relative predation index where 0 is the lowest and 1 the highest rate, corresponding to no and all urchins eaten over the 24-h period. ...
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... A long-term monitoring study of E. mathaei on coral reefs in Kenya found that spatial variability in population densities is strongly and positively related to predation levels (McClanahan, 1998) (Fig. 23.1). In contrast, studies on the intertidal rocky shore of Hawaii suggest that water movement and the availability of algal drift positively influence the density of Echinometra as well as the ratio of E. mathaei to E. oblonga (Russo, 1977). ...
... This has been largely attributed to varying levels of post-settlement predation in these reefs (McClanahan and Muthiga, 1989;Muthiga and Jaccarini, 2005). Populations on most reefs have been fairly stable (< 10% annual variation) in reefs with moderate population densities (10 to 70 ind 10 m −2 ), but population densities are more variable at low and high densities (McClanahan, 1998). In the high-density reefs there was a synchronous mortality event for two sites separated by ∼100 km in 1994. ...
... In contrast to the Caribbean studies, crevice competition studies in Kenya found that E. mathaei is a superior interference competitor over both D. setosum and D. savignyi (McClanahan, 1988;McClanahan and Shafir, 1990). For shallow fringing reefs with low levels of predators, E. mathaei lives outside of cervices, and other species are found only at very low population densities (McClanahan, 1998). This suggests that E. mathaei may be the superior resource competitor. ...
Echinometra is the most common and abundant sea urchin in the tropics and the focus of a broad array of evolutionary, biological and ecological studies. There are an estimated eight species that have evolved primarily by isolation associated with distance and sea level change. Echinometra has a small body size and low resource requirements, which allow it to live in cryptic, stressful and low predation environments. Echinometra flexibly allocates resources primarily to reproduction, and its high reproduction and recruitment rates make it able to colonize low predation habitats quickly. Triggerfish, larger-bodied wrasses, emperors and porgies are the main predators of Echinometra. Echinometra has two feeding modes, one consuming benthic algae and the other reliant on detritus. Echinometra can reach very high densities (> 100 m−2), particularly when detritus is abundant, and they can be a major contributor to reef erosion when feeding on benthic algae.
... Predation is another major mortality factor for T. gratilla and can change its density and population structure (Dafni and Tobol 1986;McClanahan 1998;Eklöf et al. 2009). Especially small (<40 mm) individuals of T. gratilla are heavily predated (Dafni and Tobol 1986). ...
... Small rock crevices (∼5-20 cm) that fit sea urchin bodies can also be shelters, but available crevices are usually limited in environments and competed among sea urchins (McClanahan 1988). McClanahan (1998) reported that the degree of predation pressure changes the partitioning of those shelters between several sea urchin species, and T. gratilla adapts to environments at a relatively low level of predation. Predation on sea urchins is also reduced by removal of predators by overfishing (McClanahan and Shafir 1990). ...
The life expectancy (i.e., an average lifespan in a population) of Tripneustes gratilla has been reported as short as 1 or 2 years in natural environments, which is relatively short among sea urchins. However, some T. gratilla individuals can live for up to 5 years under rearing and 6.85 years in the field, according to previous reports. Until present, the maximum longevity that a certain individual of T. gratilla can physiologically live remains unknown. The present study conducted long-term rearing during 12.2 years of hatchery-produced T. gratilla in tanks to investigate its longevity. We especially focused on whether the maximum longevity among T. gratilla individuals exceeds a decade, as in the case of most sea urchin species. At the end of the rearing period, eight T. gratilla individuals were still alive. In last 9.5 years of the rearing period, during which dead individuals were counted, the mortality was 41% (7 out of 17 individuals). During growth, the body weight was related to the transverse test diameter to a power of 2.84. The asymptotic transverse test diameter and body weight, determined by fitting to the von Bertalanffy growth function, were 95.9 mm and 344 g, respectively. The results of the present study suggest that T. gratilla has the maximum longevity of more than a decade under rearing conditions, and the short-lived characteristics of this species in natural environments are mainly attributed to ecological factors.
... Knowledge of the behavioral mechanisms of echinoid feeding preferences in relation to the physiological aspects of digestion are fundamental to understanding sea urchin biology and ecology (Trenzado et al. 2012, Lawrence et al. 2013, Rodríguez-Barreras et al. 2016. Several studies have reported on the ingestion of hard substrata during grazing activity, suggesting constraints strongly imposed by the limited mobility of tropicalsubtropical Echinometra species, such as E. viridis (McPherson 1969, McClanahan 1998), E. mathaei (McClanahan & Kurtis 1991, Mills et al. 2000, and E. lucunter (McLean 1967, Grunbaum et al. 1978, Ventura et al. 2003, Mendes & Tavares 2006. All populations living in beachrock and reef environments have their guts filled with a high proportion of calcium carbonate from biogenic sediments as well as endolithic Cyanobacteria (Lawrence & Sammarco 1982, Lewis & Storey 1984, Blevins & Johnsen 2004. ...
... One of the most characteristic descriptions of E. lucunter feeding behavior is the avoidance of less palatable forms of fleshy (terete) algae (Prince 1995, McClanahan 1998, Reyes-Luján et al. 2015. ...
This investigation analyzed the food contents of Echinometra lucunter specimens from Trindade, an oceanic island in the Southwestern Atlantic, between 2014 and 2016. Forty-one macroalgae taxa represented by Rhodophyta (18), Chlorophyta (11), Phaeophyceae (11), four species of Cyanobacteria, and one species of Bacillariophyta were registered. Faunal taxa richness was incipient, mostly represented by Littoriniidae. The echinoids (39.8 ± 6.2 mm test diameter and 31.4 ± 12.4 g wet weight) showed a predominantly herbivorous diet, mostly for filamentous algae. Seasonality in the sea urchin diet may be strongly associated with macroalgae availability. The feeding behavior of E. lucunter in the Brazilian territory suggests a typical trophic generalism with distinct preferences depending on the food availability or morphofunctional groups among distinct populations. Studies focusing on sea urchin feeding habits in remote islands can advance research on the marine ecology of benthic communities in isolated environments.
... D. setosum had on average higher densities than D. savignyi and fished reefs also had higher densities than marine protected areas (MPAs). McClanahan (1998) suggested that changes in D. setosum densities may express a weak inverse density dependence influence that dampened population growth between years and possibly competition with Echinometra mathaei. Continued annual monitoring showed marked variability and a slight downward trend in density for D. savignyi The major factor controlling the abundance of sea urchins was the level of predation, with protected sites having the highest abundance of predators and the highest estimates of predation showing the lowest densities of sea urchins (McClanahan and Shafir, 1990;McClanahan and Mutere, 1994;McClanahan, 1998McClanahan, , 2000. ...
... McClanahan (1998) suggested that changes in D. setosum densities may express a weak inverse density dependence influence that dampened population growth between years and possibly competition with Echinometra mathaei. Continued annual monitoring showed marked variability and a slight downward trend in density for D. savignyi The major factor controlling the abundance of sea urchins was the level of predation, with protected sites having the highest abundance of predators and the highest estimates of predation showing the lowest densities of sea urchins (McClanahan and Shafir, 1990;McClanahan and Mutere, 1994;McClanahan, 1998McClanahan, , 2000. This relationship between sea urchin abundance and predation remained robust across geographical regions, as comparisons between reefs in Kenya and Tanzania showed similar responses (McClanahan et al., 1999). ...
... Observation can be carried out either by divers or using an underwater camera. In some cases it is also possible to observe and document the predatory activity of some species (McClanahan, 1998;Sala, 1997). These observations are generally successful when large-sized predators are observed. ...
... Using this method it has been possible to identify predators, to assess their guild, the way they attack and eat prey and the prey size selection of each predator. The rate of sea urchin predation has been estimated by McClanahan (1998; in a series of tethering experiments using several sea urchins. Sea urchin individuals were pierced with a hypodermic needle and threaded with monofilament line. ...
The present guide was prepared in the framework of the ECOMARE project, a concerted action of the European Community Marine Science and Technology Programme (MAST III) concerned with the ecological effects of Marine Protected Areas (MPAs) in the Mediterranean. The overall aim of ECOMARE was to unite and coordinate the efforts of a broad group of research teams involved in the assessing the effects of protection (cessation of fishing) in Mediterranean littoral ecosystems. The specific objectives of this project were:
- To establish the state of knowledge of the responses of marine communities to protective measures in Mediterranean littoral ecosystems.
- To identify the main research needs and steps forward to progress from the assessment of effects on exploited populations to assessment of effects at the ecological level.
- To review and recommended working methodologies so that future investigations could be carried out in a coordinated and comparable manner.
... physical structure, Hernández and Russell, 2009 wave exposure, Tuya et al., 2007), and biotic factors (e.g. predators abundance, McClanahan, 1998). Quantitative studies regarding species/habitat relationships have shown that the urchin-habitat relationship is often species-specific and highly dependent on the assessed scale (Andrew, 1993;Dumas et al., 2007;Entrambasaguas et al., 2008). ...
... Despite obvious differences in origin and formation, reefs of both regions were covered mainly by EAM, with the differentiation between them being related to the presence of reef building organisms (scleractinian corals and milleporids) in Recife de Fora. SM along with Complexity were important variables associated to sea urchins abundance for all reefs indicating that, in general, the reef spatial structure is crucial to sea urchin species due to resources provided, such as refuge from predation, shelter and food resources, as cited by previous authors (Benedetti-Cecchi and Cinelli, 1995;McClanahan, 1998;Clemente and Hernández, 2008). ...
Sea urchins exhibit close linkages with the substrate, derived from their life habits such as locomotion and feeding. The main objectives of this study were to evaluate the distribution and abundance patterns of urchins and their relationships with habitat characteristics (habitat complexity, depth, and benthic percent cover) at the microhabitat scale in Brazilian reefs. In situ sampling was performed during scuba diving, using 0.5mx0.5m quadrat counts and the percent cover of the microhabitat variables. Six species of urchins were found in subtropical reefs (Santa Catarina), including a new record of the species Tripneustes ventricosus. At the tropical coral reef (Recife de Fora) four species were found but E. lucunter was by far the most abundant species (12.7 ± 1.1 ind.m-2). At the subtropical rocky reefs, E. lucunter mean density in shallow areas was 5.12 ± 2.1 ind.m-2. Other species were also representative in these reefs, such as Arbacia lixula (1.67 ind.m-2) and Paracentrotus gaimardi (1.34 ind.m-2). The structure of the assemblages of sea urchins was different between biogenic and rocky reefs, with the latter showing higher species richness but lower abundances of sea urchins. Despite intrinsic differences in studied reefs, the sea urchins abundance was mainly related to structural complexity (reef building organisms, holes and crevices) indicating that, in general, the reef spatial structure is crucial to sea urchin species due to direct and indirect resources provided.
... Th e boom and bust nature of these species suggests a tight coupling between predators and prey (Uthicke et al. 2009), and an alleviation of consumer pressure (via anthropogenic disturbances such as overfi shing) is likely to result in a dramatic increase in sea urchin population size and phase-shifts away from algal-dominated systems (e.g. C. rodgersii outbreak in Ling et al. 2009). Echinometra mathaei was also strongly impacted by predators, in keeping with the fi ndings of McClanahan (1998). Echinometra mathaei is a competitive dominant on coral reefs in the Indian Ocean but this comes at a cost, with increasing susceptibility to predation (McClanahan 1998). ...
... Echinometra mathaei was also strongly impacted by predators, in keeping with the fi ndings of McClanahan (1998). Echinometra mathaei is a competitive dominant on coral reefs in the Indian Ocean but this comes at a cost, with increasing susceptibility to predation (McClanahan 1998). In contrast, a species that appeared to benefi t from the presence of predators was the rock-boring sea urchin E. molaris , whose feeding range does not extend beyond the mouth of its burrow (Campbell et al. 1973, Coppard et al. 2012. ...
Latitudinal gradients in the strength of biotic interactions have long been proposed, but empirical evidence for the expectation of more intense predation, herbivory and competition at low latitudes has been mixed. Here, we use a meta-analysis to test the prediction that predation pressure on sea urchins, a group of consumers with a particularly strong influence on community structure in the world's oceans, is strongest in the tropics. We then examine which biotic and abiotic factors best correlate with biogeographic and within habitat patterns in sea urchin responses to predation. Consistent with expectations, predator impacts on sea urchins were highest in tropical coral reefs and decreased towards the poles in rocky reef habitats (>25° absolute latitude). However, latitude and temperature were weakly correlated with effect sizes, and the strongest predictor of predator impacts was sea urchin species. This suggests an important role of prey identity (i.e. traits including behaviour, physical, and chemical defences) rather than large scale abiotic factors in determining variation in interaction strengths. Ecosystem-shaping sea urchins such as Tripneustes gratilla, Diadema savignyi and Centrostephanus rodgersii were strongly impacted by consumers, indicating a tight coupling between predators of these species and their boom and bust prey. Anthropogenic activities such as over-fishing, climate change and habitat destruction are causing rapid environmental change, and understanding how predation pressure varies with temperature, across habitats and among prey species, will aid in predicting the likelihood of ecosystem wide effects (via trophic cascades). This article is protected by copyright. All rights reserved.
... One prediction is that the abundance and community composition of predators will reflect the intensity of fishing (McClanahan 2000). Secondly, the sea urchin species composition will be influenced by their differential susceptibility to predation and interspecific competitive interactions (McClanahan 1988McClanahan , 1998). Finally, reducing sea urchin predators will change the competitive dynamics between fish and sea urchin herbivores and subsequent changes in grazing will influence the calcifying and noncalcifying organism dominance (O'Leary and McClanahan 2010, O'Leary et al. 2013). ...
... In these sites, sea urchin biomass was estimated by haphazardly tossing a stone tied to a rope, using the rope as the radius of a 10-m 2 plot, identifying sea urchins to the species and counting their numbers within this circular plot. The species-specific density was multiplied by a mean body mass from field measurements and summed across all taxa to estimate total sea urchin biomass (McClanahan 1998). In some sites, we tethered the sea urchin Echinometra mathaei and measured its death due to predators over a 24-h period using the methods described in McClanahan (1998). ...
Trophic cascades caused by a reduction in predators of sea urchins have been reported in Indian Ocean and Caribbean coral reefs. Previous studies have been constrained by their site-specific nature and limited spatial replication, which has produced site and species-specific understanding that can potentially preclude larger community-organization nuances and generalizations. In this study, we aimed to evaluate the extent and variability of the cascade community in response to fishing across ~23° of latitude and longitude in coral reefs in the southwestern Indian Ocean. The taxonomic composition of predators of sea urchins, the sea urchin community itself, and potential effects of changing grazer abundance on the calcifying benthic organisms were studied in 171 unique coral reef sites. We found that geography and habitat were less important than the predator– prey relationships. There were seven sea urchin community clusters that aligned with a gradient of declining fishable biomass and the abundance of a key predator, the orange-lined triggerfish (Balistapus undulatus). The orange-lined triggerfish dominated where sea urchin numbers and diversity were low but the relative abundance of wrasses and emperors increased where sea urchin numbers were high. Two-thirds of the study sites had high sea urchin biomass (>2,300 kg/ha) and could be dominated by four different sea urchin species, Echinothrix diadema, Diadema savignyi, D. setosum, and Echinometra mathaei, depending on the community of sea urchin predators, geographic location, and water depth. One-third of the sites had low sea urchin biomass and diversity and were typified by high fish biomass, predators of sea urchins, and herbivore abundance, representing lightly fished communities with generally higher cover of calcifying algae. Calcifying algal cover was associated with low urchin abundance where as noncalcifying fleshy algal cover was not clearly associated with herbivore abundance. Fishing of the orange-lined triggerfish, an uncommon, slow-growing by-catch species with little monetary value drives the cascade and other predators appear unable to replace its ecological role in the presence of fishing. This suggests that restrictions on the catch of this species could increase the calcification service of coral reefs on a broad scale.
... This is contrary to the findings of this study, with D. setosum appearing to be the dominant species and E. mathaei exhibiting a much lower abundance in LC and DC biotypes. A later study conducted (McClanahan, 1998) found that E. mathaei is the most susceptible to predation compared to other Indo-Pacific sea urchin species. It is likely that this has an influence on the abundance of E. mathaei observed. ...
... However, this may also be a result of limited food resources, or likely a combination of the two influences. Variation of species dominance has been seen to vary among reefs (McClanahan, 1997;McClanahan, 1998), and the reason for the dominance of D. setosum on this particular reef and others in the area requires further exploration. ...
Marine science report, Phase 153, Frontier Madagascar. Assessment of fish and invertebrate assemblages, and marine debris using citizen science.
... Echinometra mathaei are common inhabitants of coral reefs and breakwaters assemblages and have been implicated as important herbivores throughout much of the southern Gulf (John and George, 2003), yet their abundance and distribution patterns have been poorly described (George, 2012). The abundance of E. mathaei in Dubai showed a high degree of spatial and temporal variation consistent with patterns reported from Indo-Pacific reefs at a range of latitudes (McClanahan, 1998;Appana et al., 2004;Dumas et al., 2007). Results revealed spatial heterogeneity in population abundance of E. mathaei, with greatest abundances being recorded on shallow (3 m) exposed sites; abundances substantially decreased with both increased depth and decreased wave exposure. ...
... Alternatively, seasonal changes in E. mathaei abundance in Dubai may also reflect seasonal variation in the density and/or feeding rates of predators. Urchin predators, when present in sufficient densities, can strongly influence the abundance of urchins (e.g., McClanahan, 1998). In Dubai, there are strong seasonal shifts in the abundance of fishes, including those capable of predating on E. mathaei (e.g. ...
... data). Changes in D. setosum densities over the 20-year period suggest a weak inverse density dependence influence that dampened population growth between years and possibly competition with Echinometra mathaei (McClanahan, 1998). ...
... The major factor controlling the abundance of sea urchins was the level of predation, with protected sites having the highest abundance of predators and the highest estimates of predation showing the lowest densities of sea urchins (McClanahan and Shafir, 1990;McClanahan and Mutere, 1994;McClanahan, 1998McClanahan, , 2000. This relationship between sea urchin abundance and predation remained robust across geographical regions, as comparisons between reefs in Kenya and Tanzania showed similar responses (McClanahan et al., 1999). ...
Diadema is a common and abundant sea urchin in the tropics with localized large influences on the grazing and benthic communities. There are six species, of which four are closely related morphologically and genetically; but only two species, D. setosum and D. savignyi, have frequently overlapping distributions. Diadema has long spines that allow it to graze in more open areas than most tropical sea urchins. Diadema has variable reproduction patterns with considerable differences in local environments in terms of their lunar patterns and in their reproduction in the presence of conspecifics. Recruitment patterns are also quite variable and only seldom associated with adult densities. Complex interactions between Diadema, its predators and competitors, as mediated by human fishing, are frequently a critical influence on the ecology of shallow coral reef and rocky habitats.
... However, sea urchin fishery has reportedly collapsed in certain regions (Johnson et al., 2012;Scheibling & Mladenov, 1987;Talaue-McManus & Kesner, 1995). Sea urchins play an important role in the marine ecosystem as they consume algae and prevent it from covering coral reefs (Humphries et al., 2020;McClanahan, 1998;Williams, 2022). Additionally, sea urchins serve as a crucial food source for various marine creatures such as triggerfish, wrasse, large sea conch, and starfish Tertschnig, 1989). ...
Many marine organisms broadcast gametes for external fertilization, thus conspecifics need to concentrate the time of gamete release through synchronization with environmental factors. We investigated the timing of spawning in two sea urchin species, Tripneustes gratilla and Toxopneustes pileolus , by assessing the spawning potential of individuals using both surveys and experiments in southern Taiwan from March to November 2010. We developed a new method to observe the spawning potential without sacrificing any individuals, defining those individuals‐initiated spawning within 30 min after 10 s of exposure to air as having the spawning potential. We discovered that the percentage of individuals with spawning potential, that is, the spawning ratios of the population, varied greatly from 0% to 80% in To. pileolus and from 0% to 50% in Tr. gratilla within the reproductive seasons. We evaluated the possibility of a lunar/semilunar rhythm of spawning ratios of the population and tested three environmental factors, that is, light intensity, tide, and temperature that could potentially affect the spawning potential of the sea urchins by experiments. Individuals with spawning potential were neither randomly nor evenly distributed among days but were aggregated on certain days; however, no lunar/semilunar rhythm of the spawning ratios was detected within the reproductive season. The possible role of the tidal factor within a day was not found in experiments in either species. However, when water temperature was >27°C in our surveys, significantly higher spawning ratios were observed in To. pileolus , but not in Tr. gratilla in the reproductive season. A further temperature experiment in To. pileolus supported the positive role of high temperature on the spawning ratios of individuals. In addition, To. pileolus was more likely to spawn during the day than at night, and the light/dark experiments suggested that the spawning potential was controlled by an endogenous biological clock rather than by the ambient light conditions directly. The lack of rhythmic pattern and yet concentrated spawning of these sea urchins may be adaptive to avoid predation.
... Different sea urchin species may partition resources and coexist within the same habitat (Shulman 1990). Population responses to biotic factors, such as predator abundance, are also significant, indicating the presence of top-down control (McClanahan 1998). Quantitative studies have shown that the relationship between sea urchins and their habitat is often species-specific and highly contingent on the scale of assessment (Andrew 1993;Dumas et al., 2007;Entrambasaguas et al., 2008). ...
... First, the sea urchin diversity values are likely to be underestimates because sampling was conducted in the day and nocturnal sea urchin species are underrepresented in our data set. Second, this study did not investigate other factors in the distribution and density of sea urchin assemblages, especially predation level, which has proved to greatly affect the spatial heterogeneity of echinoids (Carreiro-Silva and McClanahan, 2001;McClanahan, 1998;Sala and Zabala, 1996). ...
... The collapse of algal-dominated communities through over-grazing has given rise to so-called urchin barrens as an alternative stable state in many places (Filbee-Dexter & Scheibling, 2014), often with a sharp "grazing front" between kelp beds and urchin barrens (Gagnon et al., 2004). Overgrazing is frequently associated with dramatic increases in urchin abundances through reduced urchin predation, reflecting an inverse relationship between predator and prey abundances (Brown-Saracino et al., 2007;McClanahan, 1998). ...
Abstract We hypothesized congruence in the spatial structure of abundance data sampled across multiple scales for an ecological guild of consumers that exploit similar nutritional and habitat resources. We tested this hypothesis on the spatial organization of abundance of an herbivorous guild of sea urchins. We also examined whether the amount of local along‐shore rocky habitat can explain the observed spatial patterns of abundance. Standardized estimates of abundance of four intertidal sea urchins—Diadema cf. savignyi, Echinometra mathaei, Parechinus angulosus, and Stomopneustes variolaris—were determined by six observers at 105 sites across 2,850 km of coast of South Africa. For each species and observer, wavelet analysis was used on abundance estimates, after controlling for potential biases, to examine their spatial structure. The relationship between local sea urchin abundance and the amount of upstream and downstream rocky habitat, as defined by the prevailing ocean current, was also investigated. All species exhibited robust structure at scales of 75–220 km, despite variability among observers. Less robust structure in the abundances of three species was detected at larger scales of 430–898 km. Abundance estimates of sympatric populations of two species (D. cf. savignyi and E. mathaei) were positively correlated with the amount of rocky habitat upstream of the site, suggesting that upstream populations act as larval sources across a wide range of scales. No relationship between abundance and habitat size was found for P. angulosus or S. variolaris. Within the range of scales examined, we found robust congruence in spatial structure in abundance at the lower, but not the larger, range of scales for all four species. The relationship between abundance and upstream habitat availability in two species suggests that larval supply from upstream populations was probably the mechanism linking habitat size and abundance.
... ). Accurately defining variation in both indices over time is statistically complex and direct comparisons of temporal trends in population densities and community composition are likely to be inaccurate in datasets that contain high spatial variation(Stewart-Oaten et al. 1995, McClanahan 1998, Thompson and Mapstone 2002. Using dageraad as an example, the influence of spatial dependency and autocorrelation in temporal analyses was highlighted in this study. ...
Tsitsikamma National Park (TNP) possesses the oldest (established 1954), and one of the largest (350 km 2) 'no-take' marine protected areas (MPA) in South Africa. A long-term monitoring (LTM) programme to observe the subtidal reef fishes in the TNP MPA was established in 2007. To date, 243 angling replicates have been completed, and a total of 2,751 fish belonging to 41 different species have been caught and released. In an era of unprecedented global biodiversity loss, data that can be used to monitor ecosystems and gauge changes in biodiversity through time are essential. This thesis aims to improve the methodological and statistical processes currently available for LTM of subtidal reef fish by providing an evaluation of the TNP MPA LTM programme. Angling data revealed definitive spatial structuring, in the form of spatial autocorrelation, and a shift in viewing spatial dependency as a statistical obstacle to a source of ecological information created a new avenue of data inference. Species-specific distribution maps identified localized habitat as the main predictor variable for species abundance, emphasizing the need for accurate a priori bathymetric information for subtidal monitoring. 'Random forest' analyses confirmed spatial variables are more important than temporal variables in predicting species abundance. The effectiveness of Generalized Linear Mixed Models (GAMMs) to account for spatial autocorrelation was highlighted, and evidence that disregarding spatial dependencies in temporal analyses can produce erroneous results was illustrated in the case of dageraad (Chrysoblephus cristiceps). Correlograms indicated that the current sampling strategy produced spatially redundant data and the sampling unit size (150 m 2) could be doubled to optimize sampling. Temporal analyses demonstrated that after 50 years of 'no take' protection the TNP MPA ichthyofauna exhibits a high level of stability. Species-specific size structure was also found to be highly stable. Dageraad was the only species to exhibit a definitive temporal trend in their size structure, which was attributed to recruitment variation and the possibility that large individuals may migrate out of the study area. The inadequacy of angling as a method for monitoring a broad spectrum of the fish species was highlighted, particularly due to its selectivity towards large predators. As a result, a new sampling iii technique known as Stereo Baited Remote Underwater Videos (stereo-BRUVs) was introduced to the LTM programme in 2013. Stereo-BRUVs enabled sampling of 2640 fish belonging to 52 different species, from 57 samples collected in less than two years. A comparison of the sampling methods concluded that, compared to angling, stereo-BRUVs provide a superior technique that can survey a significantly larger proportion of the ichthyofauna with minimal length-selectivity biases. In addition, stereo-BRUVs possess a higher statistical power to detect changes in population abundance. However, a potential bias in the form of 'hyperstability' in sites with unusually high fish densities was identified as a possible flaw when using stereo-BRUVs. In an attempt to provide a more rigorous method evaluation, simulation testing was employed to assess the ability of angling and stereo-BRUVs to accurately describe a decreasing population. The advantage of this approach is that the simulated population abundances are known, so that each sampling method can be tested in terms of how well it tracks known abundance trends. The study established that stereo-BRUVs provided more accurate data when describing a distinct population decline of roman (Chrysoblephus laticeps) over 10-and 20-year periods. In addition, spawner-biomass was found to be a more accurate population estimate than relative abundance estimates (CPUE and MaxN) due to the inclusion of population size structure information, highlighting the importance of length-frequency data. The study illustrated that an evaluation framework that utilizes simulation testing has the potential to optimize LTM sampling procedures by addressing a number of methodological questions. This includes developing a procedure that aligns data collected from different sampling methods by applying correction factors, thus ensuring LTM programmes are able to adapt sampling strategies without losing data continuity. iv
... predation, settlement, recruitment, migration, disease, etc., may play a crucial role in this sense (e.g. Watanabe & Harrold, 1991;Hagen, 1995;McClanahan, 1998;Scheibling et al., 1999;Tomás et al., 2004). Colonization of new habitats by organisms with planktonic larvae, e.g. ...
Although sea-urchins can play an important role affecting the community structure of subtidal bottoms, factors controlling the dynamics of sea-urchin populations are still poorly understood. We assessed the seasonal variation in recruitment of three sea-urchin species (Diadema africanum, Paracentrotus lividus and Arbacia lixula) at Gran Canaria Island (eastern Atlantic) via monthly deployment of artificial collectors throughout an entire annual cycle on each of four adjacent habitat patches (seagrasses, sandy patches, 'urchin-grazed' barrens and macroalgal-dominated beds) within a shallow coastal landscape. Paracentrotus lividus and A. lixula had exclusively one main recruitment peak in late winter-spring. Diadema africanum recruitment was also seasonal, but recruits appeared in late summer-autumn, particularly on 'urchin-grazed' barrens with large abundances of adult conspecifics. In conclusion, this study has demonstrated non-overlapping seasonal recruitment patterns of the less abundant species (P. lividus and A. lixula) with the most conspicuous species (D. africanum) in the study area.
... Coral reefs in the intertidal and subtidal zones provide diverse and complex ecosystems for marine fauna. Sea urchin populations are particularly dependent on the relative abundances of other species that coexist in these tidal communities (McClanahan, 1998). The increase of sea urchin predators such as fish (McClanahan and Shafir, 1990), crab, lobster, or even sea otters (Erlandson et al., 2005) will lead to marked population declines. ...
Archaeological excavations of prehistoric sites in proximity to the coast often recover the remains of various marine resources such as fish, shell, barnacle, crab, and sea urchin. Among these resources, sea urchin is one of the least studied. The absence of sea urchin remains in coastal sites may not only be the results of paucity of sea urchin species in the coastal waters or taphonomic factors destroying the remains in the sites, but also due to archaeologists using large sized sieves or a lack of knowledge concerning sea urchin body parts. Lately, sea urchins have been recovered with increasing regularity in various archaeological excavations, especially throughout the Indo-Pacific region. An in-depth analysis involving sea urchin quantification methods, taxonomical identification, taxa and size distribution over-time, and palaeoenvironmental impacts, will be of great benefit for future research of coastal sites.
Makpan is a cave on Alor Island which preserves an occupation record reaching back to ca. 40.3 ka cal BP. It is one of the various archaeological Wallacean sites to recover massive amounts of marine faunal remains. In particular, the Makpan excavation recovered ~14.8 kg of sea urchin from 68 spits excavated over four main phases of occupation from a 1 x 1 m excavation area. This research project analysed a sub-sample of nine spits selected from all phases of occupation. The purpose of this analysis was to determine the role of sea urchins in the prehistoric diet, taxonomical distribution, and detect any changes in exploitation patterns over time.
This study found that sea urchins played a significant dietary role at Makpan, especially during the early stage of occupation. Together with barnacles, sea urchins were the most abundant marine resource exploited at Makpan between ca. 40.3 to 21.4 ka cal BP. In the Terminal Pleistocene to Early Holocene (13.3 – 11.1 ka cal BP) there is a rapid increase in occupation intensity at the site, coinciding with the build-up of a dense shell midden layer where most faunal remains, including sea urchins, appear in high numbers. Various different taxa are present, including Echinothrix spp., Colobocentrotus atratus, Heterocentrotus spp., Tripneustes gratilla, and Mespilia globulus, with Echinometra spp. the most heavily exploited taxa in the Makpan sea urchin assemblage.
Statistical analysis suggests that sea urchin exploitation is correlated to some degree with other marine resources. The strongest positive correlation occurred between sea urchin and barnacle, followed by shell, with fish exploitation the least related. A negative correlation was recorded between sea urchin and crab. Generally, sea urchin abundance is higher in the Pleistocene and crab is higher in the Holocene layers. A similar trend was also documented from Here Sorot Entapa, Kisar. This may not only reflect a change in subsistence strategies related to dietary preference and technologies, but may also reflect changes to urchin habitat caused by environmental or ecological change.
... Fluctuation in abundance of the animals may be due to change in their distribution in response to change in substrate type, food availability, change in dispersion from aggregated to random, a restricted to specific locality due to the lack of suitable habitat, and heavy predation (Miller, 1985). Wave activity, water depth, substratum composition, food, predation, and behavior have been reported to contribute to the distribution of echinoid species on coral reefs (Regis & Thomassin 1982, Lawrence 1983, Tegner & Levin, 1983, McClanahan 1998, Dotan 1990). In the present study, it seems that the sharp decline in density from El-Giesum site (Red Sea) to Beer Odeeb site (Gulf of Suez) is the result of the effect of substrate, where live and dead coral substrates are much flourished in El-Giesum site than Beer Odeeb site. ...
... Today echinoids form a successful group of marine invertebrates living in a wide range of marine habitats from the equator to the polar seas and from the intertidal zone to abyssal depths and have left an extensive fossil record, dating back to the Ordovician (Pisera, 1994;Smith and Saville, 2001;Kroh and Smith, 2010;Smith and Kroh, 2011). The diversity, abundance, and distribution of echinoids depend on numerous factors including, among others, temperature, hydrodynamic regimes, substrate types and complexity, nutrient availability, and distribution of predators (see Ernst et al., 1973;Smith, 1984;McClanahan, 1995McClanahan, , 1998Sala and Zabala, 1996;Guidetti and Mori, 2005;Cordeiro et al., 2014;Labbé-Bellas et al., 2016;Petovićand Krpo-Ćetković, 2016). ...
A rich echinoid fauna within the middle Miocene carbonate sedimentary succession cropping out along the coast between Santa Caterina di Pittinuri and S'Archittu (central-western Sardinia) allows the comparison of faunal gradients and preservation potentials from both hard and soft substrata. Three echinoid assemblages are recognized. Faunal composition, as well as taphonomic and sedimentological features and functional morphological interpretation of the echinoid test indicate an outer sublittoral setting. Assemblage 1 represents a highly structured environment within the photic zone, with mobile substrata occupied by infaunal irregular echinoids, mainly spatangoids, and localized hard substrata, provided by rhodolith beds, with epibenthic regular echinoids represented by the co-occurrence of the diadematid Diadema Gray, 1825 and the toxopneustids Tripneustes L. Agassiz, 1841 and Schizechinus Pomel, 1869. Assemblage 2 shows a higher diversity of irregular echinoids, dominated by the clypeasteroids Echinocyamus van Phelsum, 1774 and Clypeaster Lamarck, 1801 and different spatangoids, with the minute trigonocidarid Genocidaris A. Agassiz, 1869 among regular echinoids. This assemblage points to a soft-bottom environment with moderate water-energy conditions, periodically affected by storms. A low-diversity echinoid fauna in Assemblage 3, dominated by the spatangoids Brissopsis L. Agassiz, 1840 and Ova Gray, 1825, documents a deeper, soft-bottom environment, possibly below storm-wave base. These results indicate that the diversity of echinoid faunas originating in sublittoral environments is related to: (1) the presence of both soft and hard substrata, (2) differential preservation potentials of the various echinoid taxa, (3) intense bioturbation, and (4) sediment deposition by sporadic storm events.
... Comparisons between inshore and offshore reefs (here on referred to as location) encompassed the potential influence that both water depth and distance from shore may have had on the epibenthic communities, which were indistinguishable given the study design. Because habitat profiles differed greatly between natural and artificial reefs, complexity was measured to investigate the relationship between structural heterogeneity and urchin densities (McClanahan, 1998). Due to the low-rugosity of the study reefs, the traditional chain-and-tape method (Risk, 1972) was not deemed appropriate for measuring habitat complexity. ...
Composition of marine epibenthic communities are influenced by both physical and biotic processes. For instance, the larval supply and cues that influence colonization (physical), as well as the growth and mortality of individuals (biotoic), may differ across location and reef type. Determining the relative influence of these processes is important to understanding how epibenthic communities can develop in a region. Using both a partial caging experiment that controlled grazing by urchins and in situ photographic surveys of epibenthic communities, this study examined the relationship between urchin grazing and the composition of epibenthos on natural limestone and artificial reefs in the eastern Gulf of Mexico (eGOM). In the experiment, tiles that were open to urchin grazing had lower percent cover of algae (−12%) and higher cover of crustose coralline algae (CCA) (13%) than those that excluded urchins. Patterns in tile cover were likely the result of CCA either resisting grazing mortality or recolonizing exposed areas after algae were removed. Variation in colonization was observed between inshore and offshore reef groups. Urchin density was positively correlated with the structural complexity of the habitats, which was higher on artificial reefs than natural ones, a factor that potentially had important effects on several observed patterns. Results from photographic surveys indicated that natural reef communities had higher algal cover and lower cover of invertebrates (e.g., corals and hydroids) than artificial reefs. These findings were consistent with previous work conducted in both temperate and tropical ecosystems, and suggested that grazing from urchins plays an important role in shaping epibenthic community structure in the subtropical eGOM.
... Natural Product Communications Vol. 13 (12) [54]. In the same way, their ecological distributions are also similar [55]. These observations could be due to their similar way of life, which could then lead them to a convergent evolution. ...
Sea urchin pigments, also known as spinochromes or polyhydroxynaphthoquinones (PHNQ) have been well studied for their bioactive properties like anti-bacterial, fungicidal, antioxidant and pro-inflammatory effects. Moreover, many studies have analyzed and identified these pigments in sea urchin tests and spines but, and to the best of our knowledge, never in their body compartments or in a quantitative manner. The aim of this work was to investigate the diversity and concentration of pigments in three sea urchins localized in the same coral reef in Madagascar (Diadema savignyi, Tripneustes gratilla and Toxopneustes pileolus). Their pigment distribution patterns were compared and linked to their behavior in order to understand better the implication of spinochromes in a sea urchin’s life. Analyses and quantifications by LC-MS showed the presence of 11 different spinochromes with principally two kinds of patterns. First, D. savignyi showed a high concentration in all its body compartments with a predominance for Echinochrome A and Spinochrome D – Iso 3, a pattern close to the E. mathaei distribution found in our previous study. The presence of these pigments has been linked to their bioactive properties and suggested their implications in UV radiation protection, in reproduction, in feeding as well as in their immune systems. Secondly, T. gratilla and T. pileolus presented few spinochromes, which could explain their restricted diet and their covering behavior facing sunlight. Moreover, the absence of pigments in their gonads could be explained by their seasonal spawning occurring before our collection period. Nevertheless, the near absence of spinochromes may be counterbalanced by other bioactive compounds like their toxic proteins, which may act as either a defense mechanism or immune agents, or like carotenoid pigments already shown in sea urchin and acting as defense agents. In conclusion, spinochromes, and pigments in general, seem to play many roles in sea urchin ecology. Their highly reactive structures confer them with strong bioactive properties involved in most parts of their metabolic system and clearly participate in the survival of the sea urchins.
... These results are indicative of an early juvenile plasticity related to the substrate composition. Previous studies have identified extensive spine growth plasticity in juvenile sea urchins in areas of high wave action (shorter spines − -Bennett et al., 1995) or in response to interspecific competition (McClanahan, 1988). However, neither of these factors apply to our experiment and future studies will have to determine which signal from the substrate is responsible for this plastic response. ...
Some sea urchins, including the purple sea urchin Strongylocentrotus purpuratus, have been successfully used in aquaculture, but their slow growth and late reproduction are challenging to overcome when developing efficient aquaculture production techniques. S. purpuratus develops via an indirect life history that is characterized by a drastic settlement process at the end of a larval period that lasts for several weeks. During this transition, the bilateral larva is transformed into a pentaradial juvenile, which will start feeding and growing in the benthic habitat. Due to predation and other ecological factors, settlement is typically associated with high mortality rates in juvenile populations. Additionally, juveniles require several days to develop a functional mouth and digestive system. During this perimetamorphic period, juveniles use up larval resources until they are capable to digest adult food. Mechanisms underlying the onset of juvenile feeding and metabolism have implications for the recruitment of natural populations as well as aquaculture and are relatively poorly understood in S. purpuratus. The insulin/insulin-like growth factor signalling (IIS)/Target of Rapamycin (TOR) pathway (IIS/TOR) is well conserved among animal phyla and regulates physiological and developmental functions, such as growth, reproduction, aging and nutritional status. We analyzed the expression of FoxO, TOR, and ILPs in post-settlement juveniles in conjunction with their early growth trajectories. We also tested how pre-settlement starvation affected post-settlement expression of IIS. We found that FoxO provides a useful molecular marker in early juveniles as its expression is strongly correlated with juvenile growth. We also found that pre-settlement starvation affects juvenile growth trajectories as well as IIS. Our findings provide preliminary insights into the mechanisms underlying post-settlement growth and metabolism in S. purpuratus. They also have important implications for sea urchin aquaculture, as they show that pre-settlement nutrient environment significantly affects both early growth trajectories and gene expression. This information can be used to develop new biomarkers for juvenile health in sea urchin population ecology and aquaculture aquaculture.
... identifying sea urchins to the species and counting their numbers within this circular plot. The species-specific density was multiplied by a mean body weight estimated from field measurements and summed across all taxa to calculate total sea urchin biomass (McClanahan 1998). ...
A priority for modern conservation is finding and managing regions with environmental and biodiversity portfolio characteristics that will promote adaptation and the persistence of species during times of rapid climate change. The latitudinal edges of high-diversity biomes are likely to provide a mixture of environmental gradients and biological diversity that meet the portfolio criteria needed for adaptive systems. Northern Mozambique and the Quirimbas Islands represent the edge of a coral reef diversity center with limited potential to expand because of geologic and oceanographic limits on the southern edges. This region does, however, have the potential to be its own discrete adaptive center if it contains climate refugia and there are environmental gradients that promote acclimatization, ecological reorganization, and natural selection. Consequently, to evaluate this potential we tested for strong regional environmental spatial heterogeneity that might indicate a climate-adaptive center. Additionally, we evaluated human influences and environmental and demographic data on finfish, coral, and sea urchins in 66 reefs across ∼4° of latitude to evaluate ecological changes and human threats. A number of clear gradients in environmental and human influences were observed. For example, temperature increased and became more centralized and right-skewed, while water quality decreased to the south. Coral communities susceptible to thermal stress were found in the north where dispersed temperatures indicated a location with either tolerance to or refugium from recent thermal disturbances. Nevertheless, high coral diversity was found in southern deep-water channels. Further, spatial patterns for corals and fish differed indicating complex geographicfishing-biodiversity gradients. Consequently, environmental conditions for an adaptive portfolio exist and include refugia for preserving climate-sensitive and for numbers of coral taxa. Fishing and urban threats were observable as reduced fish biomass, diversity, and body sizes but higher biomass of sea urchins. We observed that many remote and protected areas had fish biomass values lower than expected or near maximum sustainable yields. This indicates low compliance and widespread migratory fishing, which is reducing fish diversity below maximum levels. Recommendations to sustain this adaptive center are to maintain fish biomass >500 kg/ha by increasing fisheries restrictions and compliance.
... This leads to a higher proportion of smaller-bodied mid-level fishes (trophic position 3-3.5; p < 0.001) that are often invertebrate feeders (e.g., species of wrasse and triggerfish) able to feed directly or scavenge on juvenile sea urchins [32]. These fish species are less targeted in conventional or artisanal fisheries and may be under weaker predation pressure when total biomass is low [6]. ...
The distribution of biomass among trophic levels provides a theoretical basis for understanding energy flow and the hierarchical structure of animal communities. In the absence of energy subsidies [1], bottom-heavy trophic pyramids are expected to predominate, based on energy transfer efficiency [2] and empirical evidence from multiple ecosystems [3]. However, the predicted pyramid of biomass distribution among trophic levels may be disrupted through trophic replacement by alternative organisms in the ecosystem, trophic cascades, and humans preferentially impacting specific trophic levels [4-6]. Using empirical data spanning >250 coral reefs, we show how trophic pyramid shape varies given human-mediated gradients along two orders of magnitude in reef fish biomass. Mean trophic level of the assemblage increased modestly with decreasing biomass, contrary to predictions of fishing down the food web [7]. The mean trophic level pattern is explained by trophic replacement of herbivorous fish by sea urchins at low biomass and the accumulation of slow-growing, large-bodied, herbivorous fish at high biomass. Further, at high biomass, particularly where fishers are not selectively removing higher trophic level individuals, a concave trophic distribution emerges. The concave trophic distribution implies a more direct link between lower and upper trophic levels, which may confer greater energy efficiency. This trophic distribution emerges when community biomass exceeds ∼650 kg/ha, suggesting that fisheries for upper trophic level species will only be supported under lightly fished scenarios.
... Pertumbuhan dan rekruitmen karang akan semakin cepat dikarenakan competitor mereka, alga, dimakan oleh echinoid. Echinoid juga memakan substrat dasar perairan (bioeroder ) sehingga dapat melepaskan nutrientnutrien penting yang terperangkap didalam substrat perairan (McClanahan dan Muthiga, 1998). Echinoids mampu merubah sedimen menjadi lebih halus dan membalikkan posisi sedimen dari bawah keatas sehingga oksigen dapat berdifusi kedalam sedimen. ...
Reef Check is a coral reef monitoring protocol that was widely used. The technique was simple can be done by everyone who had scientific. Invertebrate was defined as animals which lack of vertebral coloumn and evolved a hard skeleton (shell). The aims of this study were to identify and to calculate abundance of the target invertebrate. The study was conducted on Teluk Semut, Sendang Biru, South Malang. Data were taken from two stations and used the belt transect 20 x 5 m with 4 replications, and then the total survey was 800 m2. The results showed that Stenopus hispidus has the highest abundance in this location (7). Station 1 has a more diverse than station 2. Healthy of coral reef always indicated by less or view amount of predator such triton and a crown of thorn fishes. The least amount of invertebrate discovered show the ecological balance between the coral cover and fish populations. Although, the region is also an area of fishing with nets disturbing evidence of coral life. The high primary productivity of waters causing coral these waters is often a place of spawning, rearing and foraging. That’s area needs further action, so that the ecosystem can be maintained and restored with high diversity.
... Sea urchins of 3 to 5 cm (test diameter, TD) were collected in rocky habitats near the study site, measured, and then tethered using the piercing technique [62]. Each sea urchin was randomly assigned to one cell (Fig 2), then hooked with a 50 cm-long fishing line to a labelled brick placed in the center of each cell, and left fairly free to move around the brick to look for shelter within a 50 cm radius area [62,63]. The effects of prey manipulation associated with this tethering technique under the conditions of the experiment was negligible, as only 8.1% of prey died within a few hours (loss of all spines) due to stress (12.2% in L1, 4% in L2, 8.1% in L3; see results). ...
Habitat structure plays an important mediating role in predator-prey interactions. However the effects are strongly dependent on regional predator pools, which can drive predation risk in habitats with very similar structure in opposite directions. In the Mediterranean Sea predation on juvenile sea urchins is commonly known to be regulated by seagrass structure. In this study we test whether the possibility for juvenile Paracentrotus lividus to be pre-dated changes in relation to the fragmentation of the seagrass Posidonia oceanica (four habitat classes: continuous, low-fragmentation, high-fragmentation and rocks), and to the spatial arrangement of such habitat classes at a landscape scale. Sea urchin predation risk was measured in a 20-day field experiment on tethered individuals placed in three square areas 35×35 m 2 in size. Variability of both landscape and habitat structural attributes was assessed at the sampling grain 5×5 m 2. Predation risk changed among landscapes, as it was lower where more 'rocks', and thus less seagrass, were present. The higher risk was found in the 'continuous' P. oceanica rather than in the low-fragmentation, high-fragmentation and rock habitats (p-values = 0.0149, 0.00008, and 0.0001, respectively). Therefore, the expectation that juvenile P. lividus survival would have been higher in the 'continuous' seagrass habitat, which would have served as shelter from high fish predation pressure, was not met. Predation risk changed across habitats due to different success between attack types: benthic attacks (mostly from whelks) were overall much more effective than those due to fish activity, the former type being associated with the 'continuous' seagrass habitat. Fish predation on juvenile sea urchins on rocks and 'high-fragmentation' habitat was less likely than benthic predation in the 'continuous' seagrass, with the low seagrass patch complexity increasing benthic activity. Future research should be aimed at investigating , derived from the complex indirect interactions among species, how top-down control in marine reserves can modify seagrass habitat effects.
... Sea urchins were collected from rocky reefs near the study sites using SCUBA. Ten individual sea urchins per size class (small and medium) were marked by tethering (Ebert 1965, Aronson and Heck 1995, McClanahan 1998 and placed randomly inside the habitat, to evaluate predation risk by habitat-associated predators (inside, n 10 per size class and habitat), at the edge of the habitat, to evaluate predation risk by both habitat-associated and roving predators (edge, n 10 per size class and habitat) and on bare sandy spaces, to evaluate predation risk by roving predators alone (sand, n 10 per size class). Urchins were tied with a fishing line to metal pegs firmly fixed to soft substrates or attached to pieces of concrete brick on rocky substrates. ...
Structural complexity strongly influences the outcome of predator–prey interactions in benthic marine communities affecting both prey concealment and predator hunting efficacy. How habitat structure interacts with species-specific differences in predatory style and antipredatory strategies may therefore be critical in determining higher trophic functions. We examined the role of structural complexity in mediating predator–prey interactions across several macrophyte habitats along a gradient of structural complexity in three different bioregions: western Mediterranean Sea (WMS), eastern Indian Ocean (EIO) and northern Gulf of Mexico (NGM). Using sea urchins as model prey, we measured survival rates of small (juveniles) and medium (young adults) size classes in different habitat zones: within the macrophyte habitat, along the edge and in bare sandy spaces. At each site we also measured structural variables and predator abundance. Generalised linear models identified biomass and predatory fish abundance as the main determinants of predation intensity but the efficiency of predation was also influenced by urchin size class. Interestingly though, the direction of structure-mediated effects on predation risk was markedly different between habitats and bioregions. In WMS and NGM, where predation by roving fish was relatively high, structure served as a critical prey refuge, particularly for juvenile urchins. In contrast, in EIO, where roving fish predation was low, predation was generally higher inside structurally complex environments where sea stars were responsible for much of the predation. Larger prey were generally less affected by predation in all habitats, probably due to the absence of large predators. Overall, our results indicate that, while the structural complexity of habitats is critical in mediating predator–prey interactions, the direction of this mediation is strongly influenced by differences in predator composition. Whether the regional pool of predators is dominated by visual roving species or chemotactic benthic predators may determine if structure dampens or enhances the influence of top–down control in marine macrophyte communities.
... Fish predation on echinoids has been the subject of several studies (e.g. McClanahan & Muthiga 1989;Sala & Zabala 1996;McClanahan 1998;Nebelsick 1999;Kowalewski & Nebelsick 2003;Young & Bellwood 2012). Predation marks on echinoids produced by various fish species can often be recognized as bite traces (Gripp 1929;Kier & Grant 1965;Bishop 1975). ...
Stingrays (Order: Myliobatiformes) are well-known predators that feed on a variety of marine species including crustaceans, teleosts, molluscs and annelids. Here, a predatory attack by a stingray on the large spatangoid echinoid Meoma ventricosa (Lamarck) is reported from the shallow-water carbonate platform of San Salvador, Bahamas. A single stingray was observed feeding on a large adult specimen of M. ventricosa near Sand Dollar Beach, San Salvador Island. The partially crushed and partially eviscerated test of the attacked echinoid was collected from the feeding site. In addition, a crushed test of a freshly killed, smaller irregular clypeasteroid echinoid Leodia sexiesperforata (Leske) was also recovered at the same site. In both cases, the test was broken across the test plates and not along plate boundaries. Meoma ventricosa showed major plate loss on the oral side; however, bite traces and spine loss were not observed in the damaged areas. Similarly, L. sexiesperforata showed no evidence of spine removal; however, bite marks were identifiable. The observed test damage in both taxa is morphologically distinct and could be potentially identifiable in the fossil record.
... GR max and GR min in Eq. (1) were calculated as +0.05 and −0.05 from GR mean , respectively. Rugosity, the structural complexity of a coral reef, strongly increases the availability of habitat and hiding places for various organisms (Friedlander and Parrish, 1998;McClanahan, 1998) and has a significant influence on the abundance of herbivorous organisms. In this study we only consider rugosity by corals, because although macroalgae may provide 3-dimensional structure its role for grazer refuge is not fully clear (Hoey and Bellwood, 2011). ...
... Mesmo o ambiente marinho sendo ideal para os equinodermos, algumas espécies desenvolveram mecanismos para explorar as vantagens que um ambiente alternativo, particularmente estuarino, pode prover (Diehl, 1986). Ao mesmo tempo, fatores como dispersão larval e recrutamento (Clemente et al. 2009), interações bióticas (Hagen and Mann, 1992;McClanahan, 1998;Alves et al., 2001), temperatura (Tyler et al., 2000; O'Hara e Tittensor, 2010), pesca (Hasan, 2005) e até mesmo doenças (Dumont et al., 2004) também afetam sua distribuição e abundância. ...
... Fishing pressure on finfish may also indirectly be influencing the distribution of the reef-top invertebrate communities of Rarotonga. For example, large sea urchin numbers are often associated with the removal of predatory fish by fishers (e.g., McClanahan and Muthiga 1989;McClanahan and Shafir 1990;McClanahan 1994McClanahan , 1998. Large increases in the size of Echinometra mathaei populations have been followed by the competitive exclusion of herbivorous fish and other sea urchin species, high reef substratum bioerosion and decreased reef topographic complexity (McClanahan and Muthiga 1989;McClanahan and Shafir 1990;McClanahan 1994). ...
Marine habitats are often used as surrogates of biodiversity for marine park planning. In this study we investigate the significance of both habitat and wind exposure to reef-top macroinvertebrates to determine whether habitats alone provide a surrogate of biodiversity. Detailed habitat maps of the reef flat of Rarotonga (Cook Islands) and historical wind data, were used to assess the distribution and abundance of the macroinvertebrate fauna of the reef-top. Macroinvertebrates were surveyed along transects at 128 sites in four main, broad habitat types (rubble/rock, sand/ coral, sand and algal rim) and two exposure categories (windward/leeward). Analyses of variance on the most abundant species and multivariate ordination (nMDS) found that habitat was more important than exposure per se in explaining the abundance of species and the composition of assemblages. The rubble/rock habitat had the greatest substratum heterogeneity and structural complexity and supported the largest number of species and individuals. Five of the abundant holothurian species and two echinoids (Echinometra mathaei and Tripneustes gratilla) were most abundant in the rubble/rock, while two holothurian species (Holothuria cinerascens and Actinopyga mauritiana) and the gastropod Trochus niloticus, were most abundant in the algal rim. Spatial data for the five most abundant species and two of the key fisheries species highlighted the significance of the south-eastern, windward coast to all species, an area that is most likely to be impacted by an increase in the frequency and magnitude of major storm events under future climate change scenarios.
... Haphazard collections of~20 individuals of each species were taken and their wet weights measured for converting population densities to wet weight biomass. Predation on sea urchins was measured by tethering the sea urchin Echinometra mathaei and recording mortality due to predators either fish or invertebrates, over a 24-hour period using the methods described in McClanahan [22]. This method produces a relative predation index where 0 is the lowest and 1 the highest rate, corresponding to no and all urchins eaten over the 24 hour period. ...
The coral reefs of Tanga, Tanzania were recognized as a national conservation priority in the early 1970s, but the lack of a management response led to damage by dynamite, beach seines, and high numbers of fishers until the mid 1990s. Subsequently, an Irish Aid funded IUCN Eastern Africa program operated from 1994 to mid 2007 to implement increased management aimed at reducing these impacts. The main effects of this management were to establish collaborative management areas, reduce dynamite and seine net fishing, and establish small community fisheries closures beginning in 1996. The ecology of the coral reefs was studied just prior to the initiation of this management in 1996, during, 2004, and a few years after the project ended in 2010. The perceptions of resource users towards management options were evaluated in 2010. The ecological studies indicated that the biomass of fish rose continuously during this period from 260 to 770 kg/ha but the small closures were no different from the non-closure areas. The benthic community studies indicate stability in the coral cover and community composition and an increase in coralline algae and topographic complexity over time. The lack of change in the coral community suggests resilience to various disturbances including fisheries management and the warm temperature anomaly of 1998. These results indicate that some aspects of the management program had been ecologically successful even after the donor program ended. Moreover, the increased compliance with seine net use and dynamite restrictions were the most likely factors causing this increase in fish biomass and not the closures. Resource users interviewed in 2010 were supportive of gear restrictions but there was considerable between-community disagreement over the value of specific restrictions. The social-ecological results suggest that increased compliance with gear restrictions is largely responsible for the improvements in reef ecology and is a high priority for future management programs.
The sea urchin Heliocidaris crassispina has northern expanded in geographic range to Toga Bay, Oga Peninsula in the Sea of Japan due to the global warming. To demonstrate the gonad traits and age structure of H. crassispina in the central range and compare the difference with those in northern extended range, we collected 100 H. crassispina individuals from a barren off Shitsumi, Fukui (central range), in August 2018. Their growth, gonad traits (size, development and color), and sex ratio were investigated and compared with those in Toga Bay, in August 2014 (extended range). The successive 2012–2016 year classes of H. crassispina indicated successive juvenile recruitment in the central range, as found in the extended range. The individuals at three years old in central range were obviously larger than those in the extended range, showing large test diameter and body weight, possibly due to the higher temperature in central range than that in northern extended range. In addition, a balanced sex ratio in the central range was found, in contrast to the female-skewed sex ratio found in the extended range. Gonadal development was sex- and site-dependent. The testicular development in the central range was not only delayed in comparison to ovarian development, but also less delayed than those in the extended range, which possibly attribute to the higher water temperature in the central range. A significantly lower male gonad index and L* (lightness) value was recorded in central ranges compared to that in extended range, possibly due to the releasement of sperm of H. crassispina in central ranges.
The study of echinoid evolution, diversity, and ecology has always suffered from the fact that they are represented by taxa showing widely differing architectural designs of their multi-plated skeletons, inhabiting a large range of marine paleoenvironments, which result in highly varying taphonomic biases dictating their presence and recognition. This Element addresses the taphonomy of echinoids and includes: a general introduction to the morphological features of echinoids that play a role in their preservation; a review of processes which play an important role in the differential preservation of both regular and irregular echinoids including predation and transport; a summary of taphonomic pathways included in actualistic studies for recent sea urchins and then reconstructed for fossil taxa; and finally, a case study of the variation of echinoid taphonomy across a shelf gradient using the rich Miocene echinoid fauna of Sardinia.
Anthropogenic and ecological impacts affect not only the environment, but also the livelihoods of the fishers and communities reliant on them. It is, therefore, crucial to study the entire chain of well-being for an ecosystem, from changing environmental conditions, to the impacts of humans and social processes, to the effect of high-level policies and ecosystem services. As part of the Sustainable Poverty Alleviation from Costal Ecosystem Services project (SPACES), this research addresses the important question: how can we best develop sustainable, effective fisheries decisions while preserving the livelihoods of fishers and their dependents?
This volume outlines the steps taken and data sources used for the development of a representative food web model for the Nyali-Mombasa ecosystem, reproducing and quantifying main energy flows, and holistically investigating dominant food-web dynamics, and the role of fisheries on the system.
Specifically, the work described herein is intended as a preliminary step towards the estimation of the broad community, ecological, and economic effects of anthropogenic impacts as well as targeted fisheries management decisions (e.g., ban of specific fishing gears) with a view to inform future policies in support of responsible and sustainable fisheries, as well as to identify gaps in the data, and guide future research programmes.
A comprehensive study of the palaeoenvironmental and post-mortem conditions of Cenozoic clypeasteroids from the Miocene (Aquitanian) of the Qom Formation in the Esfahan–Sirjan Basin (central Iran) is based on test morphology, taphonomy, test surface preservation, echinoid abundance and echinoid sedimentary fabrics (density, orientation, and cluster), and facies analysis. The deposit-feeding clypeasteroids lived in deeper open-marine, shallow open-marine and shoal settings in a sublittoral environment. The low–moderate energy, deeper open-marine facies contain the lowest population of clypeasteroids, including rare shallow infaunal Parascutella and rare semi-infaunal Clypeaster. The moderate to high energy shallow open-marine facies contains the highest abundance and diversity of Clypeaster. The shallow open-marine and high-energy shoal facies are dominated by epibenthic Clypeaster individuals with robust, inflated and commonly dome-shaped tests. Differences in the abundance and diversity of the living clypeasteroids were related to water depth, food sources and energy levels. Different food sources, life-styles and burrowing depths are indicated by the particular morphologies of the echinoids. Dead echinoids, especially the robust and inflated Clypeaster individuals, were used as substrates and domiciles by skeletozoans and other epibiontic organisms. Disarticulation, fragmentation, bioerosion, test outline distortion and radial cracking affected the preservation of the dead clypeasteroid tests. The clypeasteroid shells in the deposits accumulated as 1—an almost autochthonous assemblage (Parascutella), 2—a moderately transported and reworked assemblage (Clypeaster and Parascutella) and 3—transported and multiple reworked assemblages (Clypeaster) affected by storms.
O mergulho recreativo (MR) é uma das atividades turísticas que mais cresce no mundo, gerando emprego e renda, além de poder contribuir com a conservação dos ambientes recifais, impulsionando a criação de unidades de conservação (UC) e auxiliando com insumos financeiros a manutenção de UCs já existentes. Apesar das vantagens socioeconômicas, quando realizado sem controle, o MR pode provocar impactos negativos ao ecossistema recifal, como alterações na estrutura das comunidades, danos físicos aos recifes e redução na resisliência do recife. Diversos estudos em recifes de corais no mundo têm caracterizado tais impactos propondo estratégias de manejo e indicadores biológicos para monitoramento. As peculiaridades dos recifes brasileiros, com baixa cobertura coralínea e elevada cobertura de algas e esponjas, torna necessário identificar e definir estratégias de manejo próprias para esse tipo de uso. O presente estudo avaliou o impacto que o mergulho recreativo provoca em ambientes recifais tropicais do Brasil. O primeiro capítulo é uma análise cienciométrica que indica as tendências dos estudos sobre os impactos do MR nos ambientes recifais brasileiros. Os resultados mostram o crescimento do número de publicações com esse tema ao longo do tempo e aponta a carência de uma análise integrada dos impactos que o MR causa na biota recifal. Além disso, observou-se a necessidade de delineamento do perfil, preferências e do comportamento dos mergulhadores. Nosso trabalho indica ainda que os recifes do Ceará e recifes urbanos da Paraíba e Alagoas são áreascom maior carência de estudos no tema. No segundo capítulo foi caracterizado o comportamento de mergulhadores recreativos em uma Área de Proteção Ambiental Marinha (AMP) no nordeste brasileiro, quantificando-se a frequência de toques no substrato recifal relacionado ao perfil do mergulhador e características do tipo de mergulho realizado. A frequência média de toques (FT) observada foi notadamente inferior no recife estudado, quando comparado a outros estudos em recifes de corais do mundo. As variáveis que influenciaram na FT foram: tipo de mergulho (scuba > snorkel), sexo (homens > mulheres) e na faixa etária (acima de 50 anos, maior que nas demais faixas etárias). A discussão deste capítulo aborda como as características físicas do local de estudo pode ter influenciado na redução da FT, e também a postura dos profissionais do turismo. O terceiro capítulo trata dos impactos do MR na ictiofauna e na comunidade bentônica em um ambiente recifal tropical do Brasil. Nesse capítulo a realização de uma análise integrada de múltiplas variáveis da biota marinha identificou importantes alterações na estrutura da comunidade recifal em função do MR. Essa abordagem permitiu a observação de possíveis relações de causa e efeito que a avaliação de apenas um grupo biológico poderia não detectar. As principais alterações documentadas foram: maior frequência relativa de areia e cascalho na cobertura do substrato e menores valores das categorias coral duro, coral mole e algas filamentosas nas áreas de Alto uso. A densidade média de corais não variou entre as áreas, contudo, a espécie de coral Favia gravida apresentou menor densidade nas áreas turísticas. As áreas de Alto uso exibiram ainda maior abundância relativa de ouriços pretos e invertebrados sésseis. As áreas de Baixo uso apresentaram maior cobertura de algas folhosas. Com relação a ictiofauna observou-se maior abundância, menor diversidade e dominância da espécie Haemulon aurolineatum nas áreas de alto uso. Nessas áreas destacaram-se as maiores abundâncias dos grupos tróficos invertívoros móveis, onívoros e carnívoros, com predominância de peixes da categoria de tamanho entre 11 – 20 cm. As áreas Controle apresentaram um padrão diferente com menor abundância total de peixes, maior diversidade e homogeneidade, com maior abundância de herbívoros, especialmente os territoriais, e destaque para as categorias de tamanho de 6 – 10 cm e de 21 – 30 cm. Nas áreas de Baixo uso observou-se valores intermediários e de maior semelhança com as áreas Controle. Essas alterações podem ser causadas diretamente pelo MR ou por efeitos indiretos, que é resultado da complexa forma como os diferentes táxons respondem aos danos oriundos da atividade turística. Apesar disso, essas modificações são espacialmente pontuais e com mais estudos, monitoramento, fiscalização e manejo adequado, acredita-se que os impactos possam ser reduzidos nas áreas turísticas. Neste trabalho ainda são sugeridas medidas de manejo para redução do impacto do MR e indicadores biológicos que podem otimizar o monitoramento de áreas recifais submetidas a atividade turística O quarto e último capítulo relata como o uso de uma abordagem metodológica diversificada em um projeto de Educação Ambiental (EA), cujo tema foi “os impactos do mergulho recreativo”, foi eficaz em uma escola pública do entorno de AMP alvo da especulação turística. Os resultados indicam que a combinação do conhecimento científico, conhecimento local, uso de mídias e visitas à ambientes não-formais são fundamentais para a eficácia de um projeto de EA. Esta tese apresenta dados pioneiros sobre os impactos que o mergulho recreativo gera em ambientes recifais brasileiros, contribuindo efetivamente para o manejo e desenvolvimento de um turismo sustentável em áreas protegidas marinhas.
Natural and anthropogenic stressors can cause phase shifts from coral-dominated to algal-dominated states. In the Caribbean, over-fishing of large herbivorous fish and disease among the long-spined urchin, Diadema, have facilitated algal growth on degraded reefs. We found that diminutive species of urchin and parrotfish, which escaped die-offs and fishing pressure, can achieve abundances comparable to total herbivore biomass on healthier, protected reefs, and exert sufficient grazing function to pre-empt macroalgal dominance following mass coral mortality. Grazing was highest on the most degraded reefs, and was driven by small herbivores that made up >93% of the average herbivore biomass (per m²). We suggest that previously marginal species can achieve a degree of functional redundancy, and that their compensatory herbivory may play an important role in ecosystem resilience. Management strategies should consider the potential role of these additional herbivore functional groups in safeguarding natural controls of algal growth in times of increased uncertainty for the world’s reefs.
Oral presentation about Ecosystem responses reveal critical thresholds of fish biomass and set tangible targets for ecosystem-based fisheries management of coral reefs in the Indian Ocean
Our study investigated the carbon:nitrogen:phosphorus (C:N:P) stoichiometry of mangrove island of the Mesoamerican Barrier Reef (Twin Cays, Belize). The C:N:P of abiotic and biotic components of this oligotrophic ecosystem was measured and served to build networks of nutrient flows for three distinct mangrove forest zones (tall seaward fringing forest, inland dwarf forests and a transitional zone). Between forest zones, the stoichiometry of primary producers, heterotrophs and abiotic components did not change significantly, but there was a significant difference in C:N:P, and C, N, and P biomass, between the functional groups mangrove trees, other primary producers, heterotrophs, and abiotic components. C:N:P decreased with increasing trophic level. Nutrient recycling in the food webs was highest for P, and high transfer efficiencies between trophic levels of P and N also indicated an overall shortage of these nutrients when compared to C. Heterotrophs were sometimes, but not always, limited by the same nutrient as the primary producers. Mangrove trees and the primary tree consumers were P limited, whereas the invertebrates consuming leaf litter and detritus were N limited. Most compartments were limited by P or N (not by C), and the relative depletion rate of food sources was fastest for P. P transfers thus constituted a bottleneck of nutrient transfer on Twin Cays. This is the first comprehensive ecosystem study of nutrient transfers in a mangrove ecosystem, illustrating some mechanisms (e.g. recycling rates, transfer efficiencies) which oligotrophic systems use in order to build up biomass and food webs spanning various trophic levels.
Sea urchins may be the single most important consumer affecting shallow marine communities worldwide. Vast seagrass beds and kelp forests have been denuded by foraging aggregations of sea urchins resulting in loss of habitat, food and changes to the physical environment. On coral reefs, sea urchins shift algal communities from fleshy algae, hostile to corals, to encrusting coralline algae that facilitates coral recruitment and reef resilience. Sea urchins evolved unique foraging capabilities in the Mesozoic Era but their relatively small size, limited mobility and modest defenses made them susceptible to large Cenozoic vertebrate predators including sea otters and large fish. Global fishery-induced declines of predators over the past century may have contributed to sea-urchin hyperabundances. This trend reversed rapidly in recent decades by the global sea-urchin fishery. Fishing on predators and sea urchins altered the strength of interactions in food webs and in some places has created alternative stable states from which sea urchin population are slow or unable to recover.
Tripneustes appears to be a generalist in habitat and food in tropical and subtropical, shallow water habitats subject to disturbance. It can have major effects on seagrass and algal beds and cause barren grounds. Tripneustes has rapid growth, early maturation, high fecundity, sporadic and potentially high recruitment and short longevity. All are characteristics of a ruderal species.
We studied the influence of piscivorous fishes and prey refuges on assemblages of fishes occupying 52 model reefs in a large seagrass bed off St. Thomas, U.S. Virgin Islands. We conducted three experiments: two involving 6 reefs each, lasting 2 and 5 yr, and one involving 40 reefs, lasting 1 yr. Each experiment included replicate reefs in various combinations of five structural treatments: holeless controls, 12 and 24 small holes, and 12 and 24 large holes. Tagging studies indicated that the reefs were sufficiently isolated from each other to comprise statistically independent replicates, and that resident piscivores occupied home reefs. We observed 97 species on or near the reefs, representing all major foraging guilds, and each holed reef supported hundreds of individuals. We examined four categories of fish: (1) large reef associates (too large for the small holes; most of these fish were both predators on smaller fish and prey for larger transient piscivores), (2) moray eels (piscivores that could fit into the small holes), (3) small reef associates (potential prey that could fit into the small holes), and (4) juvenile grunts (potential prey that sporadically were extremely abundant). We tested five a priori predictions of the general hypothesis that predation is an important process structuring reef-fish assemblages. The first two predictions dealt with the role of prey refuges. First, if reef holes function as prey refuges, then prey fish should be most abundant on reefs providing holes near their body diameters, because such holes would make the prey fish safest from predation. Seven of eight experimental comparisons supported this prediction, and five of them were statistically significant. Second, if refuge availability limits prey abundance, then prey fish should be more abundant on reefs with 12 holes than those with no holes, and should be more abundant on reefs with 24 holes than those with 12 holes. The first part of this prediction was verified by all nine experimental comparisons, seven of which were statistically significant. However, there were no strong differences between 12-hole and 24-hole reefs. Thus, between 0 and 12 holes per reef, holes limited local prey populations; between 12 and 24 holes per reef, the number of holes was not limiting. Several lines of evidence suggested that the latter pattern was due to temporary saturation of the study area with refuges when we added 40 reefs to 12 existing reefs. The remaining three predictions dealt directly with the community-level role of predation. First, predators should affect local prey abundance either chronically, in which case a negative relationship among reefs is predicted between the average abundances of predators and prey, or sporadically, in which case a negative relationship is predicted between the abundance of predators and the maximum number of co-occurring prey ever observed at each predator abundance. The former prediction was falsified, whereas the latter was verified. Observations of extreme type III survivorship of recruit cohorts on reefs with many piscivores and occasional direct observations of piscivory bolstered the conclusion that this relationship was causal. Finally, we predicted that predators should affect the number of prey species on a reef. We observed a significant negative relationship among reefs between predator abundance and maximum prey-species richness. Comparing species' relative abundances on reefs at the extremes of this regression, piscivores appear to have nonselectively reduced and extirpated both common and rare prey species, although this relationship remains purely correlative. In our model system, high local species diversity appears to have been maintained despite rather than because of predation. We propose a conceptual model where the local abundances of coral-reef fishes are determined by the relative magnitudes of recruitment by larvae, colonization by juveniles and adults, predation, and competition for refuges, each of which varies through time and space. Multifactorial field experiments will be necessary to test such pluralistic hypotheses.
To increase our understanding of how large numbers of similar species of reef fishes coexist, we have determined the components of feeding-niche separation among the members of a feeding guild of coral reef fishes. The seven West Indian species of shallow-water squirrelfishes (Holocentridae) comprised >99% of the nocturnally active, benthic-crustacean-feeding fishes at five sites off St. Croix, Virgin Islands. Resources utilization frequencies were determined for food and foraging habitat. Food was partitioned by taxon between the four species that consumed predominantly shrimps and the three species that ate mainly crabs. Food was secondarily partitioned by body size of prey items, particularly in the principal food category. Differences in foraging microhabitat (position within a reef zone) were as important as food differences in separating species. In all cases but one, overall feeding niche overlap was @?.25 or less, with a mean value of .13. This low value contradicts statements that reef fishes are generalists with broadly overlapping resource utilization and, compared with overlap values in other guilds, suggests that the mechanism permitting great local diversity within coral reef fish guilds are not basically different from those operating in other ecosystems. The mean overlap in food among the 6 commonest species was significantly different from the mean overlaps of 50 randomly generated competition-free communities (Sale 1974), suggesting that interspecific competition has played a role in the evolution of this assemblage.
A common measure of the temporal variability of a population is the standard deviation of the logarithms of successive estimated population sizes, In(D-t). This measure overestimates true temporal variability (the standard deviation of the logarithms of true population density, In[Delta(t)]) because it is contaminated by spatial variance (variability among samples taken on the same date). The random error in D-t causes an overestimation of temporal variance, both directly and also indirectly, by causing ln(D-t) to underestimate ln(Delta(t)). Both problems are more severe if spatial variance is large or the sample size, on a date, is small. We develop an alternative estimator, which uses an estimate of spatial variance to correct for both problems. To evaluate it, we sampled from simulated populations with a wide range of clumping. The results show that the standard estimate can be badly biased. The new estimator is much better and is quite accurate over a broad range of conditions. Our results suggest a reanalysis of some ecological studies that have estimated temporal variability to attack theoretically important questions. In particular, the apparently greater average temporal variability of terrestrial arthropods compared with terrestrial vertebrates could be an artifact caused by the fact that, typically, clumping is weaker and density estimates are more accurate in vertebrates.
Spinyhead blennies occupy abandoned worm holes in coral heads on Caribbean reefs. We conducted a series of short-duration field experiments off St. Thomas, US Virgin Islands, which indicated that such holes are a limiting resource and that larger fish competitively dominate smaller fish for access to holes. Few fish that were added to coral heads lacking vacant holes were able to secure holes, and did so only by displacing larger residents after severe combat. However, when vacant holes were added to coral heads before adding fish, transplanted fish readily occupied the new holes. When holes were added to coral heads without also adding fish, the new holes were colonized by immigrants from the surrounding habitat. Similarly, when resident fish were removed from coral heads, the emptied holes were colonized. Immigrants were smaller than removed residents, and, in cases where resident fish changed holes following removals of neighbors, they moved to sites previously occupied by larger fish. Fish displaced up to 5 m returned to their original holes, consistent with the possibility that spinyhead blennies may occasionally leave their holes and search for sites of better quality. We conclude that intraspecific competition for shelter holes may limit the number of spinyhead blennies occupying a coral head.
Alternative ecological models make different predictions about the long-term influences of recruitment, mortality and density upon the demography and abundance of populations of coral reef fishes. Recent validation of protocols for aging tropical fish to year-class provided the basis for testing some of these predictions. Specifically, we assessed the influence of recruitment variability on populations by comparing age structures for two species of damselfish (Pomacentrus moluccensis and P. wardi) with long-term (9 year) recruitment records from seven coral reefs located on the southern Great Barrier Reef. Detailed comparisons are given for two reefs: Wistari, which was a high recruitment reef for P. wardi and a low recruitment reeffor P. moluccensis, and Lady Musgrave, which displayed the opposite pattern. Both species lived longer on Wistari relative to Lady Musgrave suggesting that longevity is controlled by local predator populations independent of conspecific density. On Wistari, where turnover was lower, the age structures of both species were positively correlated with the initial sizes of the corresponding year-classes (for the 9 youngest cohorts). On Lady Musgrave, clear relationships were only evident for the 4/5 most recent year-classes because of the depletion of older cohorts. Most age structures contained evidence of one or more dominant cohorts among the older age classes attributable to strong recruitment in the past. Age-specific mortality schedules varied between reefs; a linear model provided the best fit in all cases although those from Wistari had little slope and high variability relative to populations from Lady Musgrave. The mortality schedules for P. wardi contained possible evidence of compensatory responses among cohorts but these patterns were weak and ambiguous compared to the influence of temporal and spatial variability in recruitment on age structure and abundance. The latter was able to account for 90% of the variability in abundance observed across species and reefs for the full data set. These comparisons indicate that the abundances of both species were determined by variable patterns of recruitment interacting with relatively uniform and non-compensatory mortality. Consequently, coral reefs that received higher recruitment of either species simply accumulated more fish with no evidence of saturation from the highest levels observed over a 9-year period.
We examined patterns of distribution, persistence, and growth of groupers (especially Nassau grouper, Epinephelus striatus), which recruited to or colonized 52 one cubic-meter concrete-block reefs and 10 natural patch reefs off St. Thomas, USVI. The artificial reefs comprised five shelter treatments: 24 large holes, 12 large holes, 24 small holes, 12 small holes, and holeless controls. Although small individuals (<15 cm TL) of the two most abundant species, E. striatus and E. afer, were distributed evenly among these treatments, larger individuals were usually more abundant on large-hole reefs, E. striatus and E. afer partitioned the artificial reefs by depth, ranging from 6 to 12 m, with E. striatus occurring deeper than E. afer. Tagged individuals demonstrated persistence on and homing to specific reefs. Overall, Nassau grouper was the most abundant of six species of Epinephelus observed on the artificial reefs. However, this was not the case on natural reefs or in commercial catches, which were dominated by E. cruentatus, E. fulvus, and E. guttatus. Growth rates of a distinct recruit cohort of Nassau grouper on the artificial reefs were comparable to those estimated in other recent field studies. Mean monthly growth rates ranged from 0.84 to 1.17 cm·month−1, and mean growth from the first month of observation (mean size = 8.7 cm) through the eleventh month (19.5 cm) was 10.8 cm. Persistence of the cohort during the year was low, with an 85.4% decline from 158 to 23 individuals over 8 months, presumably due to predation. Due to apparent recruitment overfishing caused by decimation of the local spawning aggregation off St. Thomas, Nassau grouper catches have declined dramatically in recent years. Given the differential distribution of adult Nassau grouper between artificial and natural reefs, we conclude that artificial reefs of the appropriate design may enhance the local abundance of this commercially valuable species.
To test a hypothesis that resource partitioning is less important to the small- and large-scale coexistence of similar species of reef fishes than are the chance elements involved in patterns of mortality, recruitment, and space-creation, the patterns of spatial distribution of a set of damselfishes were examined on both large and small scales. The data show a greater degree of within and between habitat segregation than was previously thought to exist. Space partitioning is thus of significance to large- and small-scale coexistence in many cases. While the “chance” hypothesis remains relevant to those cases in which resource partitioning is not evident among actively competing species, the existing data also fit alternative, more conventional hypotheses. Competitively interacting species may also differ in the extent to which their populations are space limited and reliant on external agencies to create new space.
Aspects of the habitat distributions of eight species of territorial Caribbean damselfishes of the genera Eupomacentrus and Microspathodon were examined at Puerto Rico and at Panama. Substrate and depth partitioning by various species was demonstrated with the use of transects in various reef and depth zones in Puerto Rico, although there was much multispecific use of habitats. Habitats of exclusive usage were defined for some Eupomacentrus species in Panama and, in most, for certain size classes of the one species. Removal experiments suggest differences in the preference of different species for a given habitat. Habitat partitioning can account for the large-scale coexistence of most Eupomacentrus species. The coexistence of adult Microspathodon chrysurus and several Eupomacentrus spp. depends on the ability of M. chrysurus to aggressively dominate them and force them to share feeding areas with it.
The benthic grazer Diadema antillarum Philippi (Echinoidea) has been demonstrated experimentally to contribute to the control of coral community structure in shallow water. In Discovery Bay, Jamaica, West Indies, Diadema densities were manipulated over a range of 0-64/m2 with the aid of enclosures. Grazing by Diadema under primary and post-primary succession conditions were compared.Algal percent-cover decreased as Diadema density was increased. Despite the presence of high algal cover. highest coral recruitment and diversity occurred at lowest Diadema densities, with planular settlement occurring predominantly in openly exposed micro-habitats. However, since algal growth rates greatly exceeded those of corals, space was rapidly monopolized by the former, resulting in intense competition and high coral mortality. This was particularly evident in Agaricia and Porites spp. At high Diadema densities, coral recruitment was greatly depressed in at least a genus-specific manner by intense levels of biological disturbance resulting from the echinoid's abrasive grazing activities. Favia Fragum (Esper) was especially susceptible to this perturbation. The surviving coral spat were found generally in cryptic, protected areas. Here they suffered some competitive losses to other sessile epifauna and -flora, particularly coralline algae, polychaetes and forams, which were well adapted to these physical and biological conditions. Increased sedimentation also depressed coral recruitment, replacing grazing as a limiting factor for successful settlement.Optimal conditions for coral survival, competitive success, and possibly growth were found at intermediate densities due to a balance between competition for space and biological disturbance. Diadema antillarum plays an important role in controlling the distribution and abundance of coral spat in the shallow reef community.
Numbers offish and their wet weights were estimated in Kenyan coral-reef lagoons on seven reefs over 6 years. Two sites were protected from fishing for over 20 years, whereas the other five sites were heavily fished in recent years. A heavily fished site was converted into a marine park (Mombasa Marine National Park, approximately 10 km2 no fishing allowed), and the number of fishers allowed was slowly decreased between August 1991 and August 1992. The area adjacent to the park was converted into a marine reserve (only fishing traps, lines, and gill nets allowed) that provided fishing grounds for fishers excluded from the park. Data from a fish-landing site adjacent to the newly created marine park were collected for 3 years and analyzed to determine the effect of the park’s creation on fish catches. Results suggest that fishing in the reserve reduced fish wet weight by about a factor of 10 and reduced fish numbers and species richness by a factor of two. Both field studies and landing data suggest harvesting at a bionomic equilibrium. For example, approximately 65% of the landing site’s fishing grounds were protected with the creation of the park, and 65% of the fishers quit the studied landing, leaving nearly the same density of fishers in the remaining area (∼ 12 fishers/km2). Further, fishers using pull seines were excluded from the reserve, and their numbers were replaced by fishers using other gear (mostly basket traps). Although the overall catch per unit effort increased by about 110% after the park’s creation, the total fish landed decreased by 35% and the catch per unit effort decreased toward the end of the study period despite increasing fish abundance in the park. Although establishment of small parks elsewhere have increased the total catch, the large park we studied did not; one reason may have been the lower ratio of edge to park area of the large park. Alternatively, the park’s edge may have provided a good fishing area, so fishing effort may have been highest along the park’s edge. Consequently, a barrier may have been created that restricted fish dispersal to most of the reserve. Therefore, the area that had an increased catch was small (< 1 to 2 km from the edge) and could not compensate for the lost fishing area. Most fish species within the park showed recovery after fisher exclusion. Total fish wet weights 3 years after the fishers’ exclusion were 25% below the older marine parks. Poor recovery of the herbivoroas parrot and surgeonfish can account for much of this shortfall. Competition for resources with sea urchins appear to be slowing recovery of these two groups. A study site 2.5 km from the park’s southern boundary, in the reserve section of the protected area, showed no changes in fish abundance over the study period, despite changing gear regulations.
A lo largo de sies años se estimaron los números y el peso húmedo de peces en lagunas de siete arrecifes de Kenia. La pesca esta restringida en dos de los sitios desde hace 20 años, mientras que en los otros cinco se ha intensificado en años recientes. En uno de estos sitios fue decretado un parque marino (Parque Nacional Marino Mombasa, con extensión aproximada de 10 km2 y vedado a la pesca) y lentamente se redujo el número de pescadoes entre agosto de 1991 y agosto de 1992. El área adyacente al parque fue declarada reserva marina (solo se permiten trampas, líneas y redes agalleras) proporcionando asíáreas para la pesca a los pescadores excluidos del parque. Se colectaron datos durante tres años en los sitios de desembarco de las capturas adyacentes al parque marino recientemente creado para determinar el efecto de la creacíon del parque sobre la captura de peces. Los resultudos sugieren que la pesca en la reserva ha reducido el peso húmedo de los peces por un factor aproximado de diez y el número de peces y la riqueza de especies por un factor de dos. Tanto los estudios de campo como los datos de desembarco sugieren que la cosecha se ubique en un equilibrio bionómico. Por ejemplo, aproximadamente 65% de las áreas de captura de los sitios de desembarco fueron protegidas con la creación del parque y 65% de los pescadores ahandonaron el sitio estudiado, quedando casi la misma densidad de pescadores en el área restante (≈12 pescadores/km2). Además, los pescadores que utilizaban redes de arrastre fueron excluidos de la reserva y su número fue reemplazado inmediatamente por pescadores que utilizaban otro tipo de arte de pesca (en su mayoría trampas de canasta). Aunque la captura general por unidad de esfuerzo se incrementó aproximadamente un 110% después de la creación del parque, el total de peces disminuyó en un 35% y la captura por unidad de esfuerzo disminuyó hacia el final del período de estudio a pesar de un incremento de la abundancia de peces en el parque. A unque el establecimiento de parques pequeños en otros lugares ha incrementado la captura total el parque estudiado, de dimensiones grandes, no lo hizo. Una razón pudo haber sido la menor proporcíon bordesuperficie del parque grande. Alternativamente, los bordes del parque pudieron haber proporcionado una área de buena pesca de tal forma que el esfuerzo de captura pudo haber sido mayor a lo largo de los bordes del parque. Consecuentemente se pudo haber creado una barrera que restringió la dispersión de los peces hacia la mayor parte de la reserva. Por lo tanto, el área que tuvo incremento en la captura fue pequeña (< 1 a 2 km del borde) y pudo no haber compensado la pérdida de área de pesca. La mayoría de las especies de peces dentro del parque mostraron recuperación después de la exclusíon de los pescadores. El peso total de los peces tres años después de la exclusión de los pescadores fue 25 % menor que el de otros parques marinos más antiguos. La pobre recuperación de peces herbívoros, como el pez loro y el cirujano, puede explicar en mucho las dismuniciones. La competencia por recursos con los erizos de mar parece estar disminuyendo la recuperación de estos dos grupos. Un sitio de estudio 2.5 km al sur del parque en la sección de reserva marina no mostró cambios en la abundancia de peces a los largo del periodo de estudio a pesar de los cambios en la regulación de las aretes de pesca.
The assumption that parrotfishes represent a single group of grazing herbivores is addressed by morphological, functional and ecological analyses. This assumption is rejected. The 24 scarine parrotfishes from the Great Barrier Reef, Australia, are divided into two functional groups: excavators and scrapers. The osteology and myology of the jaws of all 24 species were examined, and a detailed description of a representative species from each group is provided. The relative weights of the major jaw structures of five representative species were quantified. Morphological differences between species were interpreted in a functional context and were assessed based on observations of feeding in the field. Species in the two functional groups show major group-related differences in their bite speeds, microhabitat utilization patterns, and form and extent of substratum excavation during grazing. Group-related differences are also apparent in feeding rates, foray sizes and bite rates. The division of the species into two functional groups is supported by both morphological and behavioural observations. The ecological significance of two functional groups within the family is discussed in relation to the role of parrotfishes on reefs, particularly in terms of bioerosion.
Blades of Thalassia testudinum Banks ex Knig were used in a field bioassay to assess the impact of herbivorous fishes and urchins in different habitats, at different depths, and at different times of day on a fringing reef at Galeta Point, Panama. Algal transplants were then performed to determine the effect of herbivory on algal distribution and on potential plant-plant interactions in the absence of grazers.
Thalassia loss on the reef flat (0%/h) and sand plain (0%/h) was very low relative to that on the shallow reef slope (14%/h). Within the reef slope habitat, herbivore activity decreased linearly with depth. On the shallow reef slope, herbivory was highest in habitats that were most topographically complex. Herbivory was reduced in microhabitats where algal turfs had decreased structural heterogeneity by overgrowing small branching corals and filling in the cavities that these corals normally provide. On the shallow reef slope, grazing of Thalassia was lowest at night (0.3%–1%/h), rose rapidly in the morning to peak between 0900–1300 h (27%–45%/h), then dropped gradually throughout the afternoon and returned to nocturnal rates at 1900–2000 h.Herbivorous fishes did 97% of their fedding during the daytime and were responsible for more than 90% of the Thalassia removed from the reef. Urchins accounted for only 9% of the Thalassia removal, and day and night grazing rates were not markedly different.Algal species from the reef flat and sand plain are excluded from the reef slope by herbivores, not physical parameters, and should outcompete reef-slope species in the absence of herbivory. On this tropical reef, plants in the more physically benign environment appear to be selected primarily for herbivore resistance. Selection for competitive ability among seaweeds becomes increasingly important in habitats where herbivores are more exposed to their own predators or where herbivore populations are low due to periodic physical disturbances.
Spinyhead blennies (Acanthemblemaria spinosa) and roughhead blennies (A. aspera) are planktivorous hole-dwelling fishes that live in dead coral skeletons. Both species are known to choose shelters high above the reef surface (although spinyheads displace roughheads downwards). To test the hypothesis that this preference is due to greater plankton availability in higher locations, fish were placed on artificial habitats located 15 cm and 100 cm above the surface of a natural reef. Both species experienced higher feeding rates, growth rates, and fecundities in high locations, and spinyhead rates generally exceeded roughhead rates at a given height. Under laboratory conditions, oxygen consumption by spinyheads was 1.6 times greater than that of roughheads and this corresponds well with the 1.8 ratio of feeding rates under controlled aquarium conditions. This information provides a partial explanation for the observed microhabitat distribution and resulting coexistence of these competing species: it is hypothesized that spinyheads have an advantage in agonistic interactions because of their higher metabolic rates, thus excluding roughheads from high sites, but that roughheads can persist at low sites because their lower metabolic rates result in lower food demands. A model is presented that predicts varying occurrences and vertical distributions of these species in locations with different zooplankton densities.
(1) We investigate problems associated with the measurement and interpretation of population density variability that have confounded most, if not all, previous studies of the subject. (2) The most commonly used measure of variability, S.D.(log[N + 1]), is subject to potentially very large bias, and other measures based on untransformed densities can be affected by the long-tailed frequency distributions of population data. (3) The fact that variability often depends on the mean density is usually ignored, yet it affects the generality of any statement based on a single estimate of variation. (4) When comparing variability between species, the generality of any conclusions is also limited by the choice of spatial and temporal scale of sampling. This potentially confounds many studies. (5) We discuss the patterns that emerge from considering spatial and temporal Taylor power plots, and suggest that combinations of such plots for individual species allow a classification according to markedly different types of dynamics; the underlying mechanisms of which are not well understood. (6) Finally, we consider ways of making meaningful comparisons of spatial and temporal variability across species.
One of the major questions in ecology is, what controls the structure of communities? We used projection matrix models to examine community dynamics and patterns of succession. The inputs of the model are transition probabilities of species replacements that were measured repeatedly during a long-term (1962-1989) study of diverse coral assemblages on Heron Island, Great Barrier Reef. Transitions varied strikingly among species and sites, reflecting differences in recruitment, growth, longevity (persistence), and the rate of replacement of one species by another. Species that had a poor ability to persist (e.g., algae and Pocilloporid corals) were generally good colonists. The observed number of transitions expressed as a proportion of the maximum number possible provides an index of the complexity of interactions in an assemblage, analogous to the concept of connectance in food-web analysis. Transitions occurred to and from nearly every species group, indicating that there was no competitive dominant in this system. We use the models in simulations to track transitory changes in species abundance and community composition following a major disturbance (e.g., due to a cyclone or outbreak of crown-of-thorns starfish). Some species showed a rapid initial increase followed by a decline to lower equilibrium levels, while others increased smoothly to a generally higher equilibrial abundance. The length of time required to reach a climax assemblage using the same matrix recurrently (approximate to 20 yr) is far greater than the observed interval between major disturbances, supporting nonequilibrium theories of coral reef communities. Climax assemblages were highly diverse and varied in composition from site to site. The ''intermediate disturbance hypothesis'' does not fully predict successional changes in these shallow-water coral assemblages since diversity remained very high at equilibrium (i.e., long after a major disturbance), Competitively inferior species were not eliminated because routine mortality ensured that some space always remained available for colonization. We also present a novel method for quantifying the relative importance of each species interaction to community composition and the rate of succession, based on a sensitivity analysis of the transition matrix. The analysis shows that the importance of a species to the dynamics of a community may be unrelated to its abundance at equilibrium, with some rare species groups having a greater impact than more common ones. Sensitivity analysis of this type will provide a powerful means of identifying ''keystone'' species in complex assemblages where experimental manipulation of each species is impossible.
Data have been drawn together to demonstrate that reef fishes by and large are food and habitat generalists with a large amount of overlap in requirements among coexisting species. Suitable living space is the resource most likely to be in short supply for them, and their environment, although benign, is one in which the supply of living space is both spatially and temporally unpredictable. The argument is developed that reef fishes are adapted to this unpredictable supply of space in ways which make interspecific competition for space a lottery in which no species can consistently win. Thus, the high diversity of reef fish communities may be maintained because the unpredictable environment prevents development of an equilibrium community
Species diversity of assemblages of the gastropod genus Conus occurring on tropical Indo-West Pacific reef platforms increases in the following order of habitat types: (1) extensive regions of sand substrate (mean number of species N = 3.0; mean species diversity H = 0.5); (2) intertidal smooth limestone plantforms (mean N = 7.6; mean H = 1.3); (3) topographically complex subtidal coral reefs (mean N = 13.7; mean H = 2.1). The census data show a close correlation between N and H (Spearman r = 0.93). The absence of a longitudinal gradient of species number and diversity in the uniform Type II habitats over 160⚬ of longitude, despite reports of larger total numbers of species near the "faunistic center" of the Indo- West Pacific region, also supports the conclusion that habitat complexity is an important determinant of species diversity. Type III habitats near the "faunistic center" have more diverse assemblages than those of outlying areas east and west, but it has not been possible to distinguish the role...
Coral reef herbivores were classified into three functional groups according to criteria describing the frequency and intensity of disturbance to the algal community created by their herbivory. These herbivore groups included: microherbivores, which have small foraging ranges (1-100 cm^2) and a high frequency of grazing any point within their range; the echinoid Diadema antillarum, which has intermediate-size foraging ranges (0.5-1 m^2) and intermediate frequencies of grazing any one area within an individual's range; and herbivorous fishes, with large foraging ranges (up to 0.5 ha) and the lowest grazing frequencies at any one site. Algal communities on experimental coral-settling plates were subjected to treatments designed to detect the effects of grazing by representatives of each functional herbivore group on algal biomass, community structure, and primary productivity in a backreef/reefcrest habitat in St. Croix, United States Virgin Islands. Abundances of microherbivores and herbivorous fishes (mainly juvenile scarids) fluctuated over time, while Diadema antillarum abundance remained relatively constant. Grazing intensity (as measured by the rate of algal biomass removal and algal standing crop under different grazing regimes) was greatest for D. antillarum; grazing by D. antillarum resulted in low-biomass, highly productive algal turfs (O"2 release per unit chlorophyll a per unit time: 11.3-22.4 @mg@?@mg^-^1@?h^-^1). Algal turfs grazed by D. antillarum were dominated by epilithic and endolithic filamentous and crustose species. Grazing by herbivorous fishes led to algal communities with algal turfs of higher biomass than in D. antillarum-grazed treatments; these communities became dominated by a single algal species (Sphacelaria tribuloides). Fish-grazed algal communities had lower biomass-specific productivity rates than algal turfs grazed by D. antillarum (2.4-8.4 @mg@?@mg^-^1@?h^-^1). Microherbivore-grazed and ungrazed treatments had the lowest biomass-specific productivity rates and eventually became dominated by a few macroalgal species. Although algal biomass in sea urchin-grazed treatments was only 25-50% of the biomass values in treatments not grazed by sea urchins, productivity per unit area was not significantly lower in the former treatments, suggesting a positive effect of sea urchin grazing on algal productivity. This positive effect is a result of the reduction of algal self-shading when grazing is intense and/or the input of regenerated nutrients from D. antillarum excretions. Coral reef plant-herbivore interactions are similar to those in terrestrial grassland-herbivore systems. As in many terrestrial systems, plant-herbivore interactions result in higher overall ecosystem primary productivity and apparently facilitate the flow of energy and materials in this case from the highly productive algal turf component to higher levels in the reef trophic web.
Diffusive limitation of C and nutrient uptake can be an important factor regulating metabolic processes in aquatic plants. Relationships between the flow regime and both primary productivity and nitrogenase activity (ethylene production) of coral reef algal components were studied via a productivity chamber that simulates oscillatory flow. Net primary productivity of algal turfs was increased significantly by flow speeds (steady flow equivalent) up to 16 cm s-l. The mode of water flow also was important; primary productivity of algal turfs increased by 2 1% under an oscillatory flow regime as compared to vortex flow created by stirring. Acunthophora spicifera exhibited increased primary productivity at flow speeds up to 10 cm s-l, while primary productivity of another rhodophyte, Coelothrix irregularis. increased up to 4 cm s- I. Rates of nitrogenase activity exhibited by algal turfs also were affected significantly by flow speed. These data indicate that diffusive limitation is important to varying degrees in reef algal turfs and macroalgae and suggest that primary productivity and rates of nitrogenase activity depend on ambient flow speeds and boundary-layer dynamics.
This study compares shallow leeward coral reef sites in East Africa. I compared protected and unprotected and fringing and patch reefs of southern Kenya and northern Tanzania to determine the effect of reef structure and fishing on the abundance and species richness of fish. Studies of the benthic community including coral, algae and sea urchins were also completed. Fishing is the strongest of the two factors. Fishing had its largest effect in Kenya's fringing reef lagoon where fish wet weights were reduced from 1100 to 75 kg/ha and the number of species was reduced by 50%. The studied patch reefs showed less effect from fishing with fished reefs generally maintaining a total fish wet weight of around 200 kg/ha and only small reductions in species richness at the scale of 1.0 ha. Sea urchins were much more abundant in fished reefs regardless of the reef type and was attributable to lower predation and triggerfish abundance in fished reefs. Patch reefs had lower abundance of the rock-boring sea urchin (Echinometra mathaei) than fringing reef sites. This may, in part, explain the lesser indirect effects of fishing in patch compared to fringing reefs. The pristine patch reef had a lower abundance of coral and fish, especially trigger fish and goatfish, but more scavengers than the studied fringing reefs. Reef structure can modify the effects of fishing because the abundance and distribution of important species (seagrasses, the rock boring sea urchin, and triggerfish in this study) are influenced by habitat.
Rates and factors affecting predation on the omnivorous burrowing sea urchin Echinometra mathaei (de Blainville) were studied on four Kenyan coral reefs, two with a history of heavy fishing and two protected from fishing. Three hypotheses concerning predation were tested; these were that predation should be lower (1) on fished reefs compared to unfished reefs, (2) in shallower water and therefore between sites within reefs as a function of water depth and (3) with increasing sea urchin body size. Results support the above hypotheses with fishing being the most important variable followed by depth and individual body size. Finfish accounted for 90%, asteroids 5% and gastropods 5% of the predation which suggests that the removal of finfish is more important than shelling in restricting E. mathaei populations. Depth was less important within fished sites than the protected site where very small differences in depth corresponded to increases in predation. Reef flat sites consistently had the lowest predation rates which accounts for E. mathaei natural (unfished) distribution on reef flats. Fishing pressure leads to a reduction in predators which allows E. mathaei to increase its density and expand its distribution to deeper areas. As E. mathaei is a major bioeroder of coral reefs we suggest that bioerosion is higher and at greater depths on heavily fished reefs.
Many heavily fished coral reefs have a high abundance of sea urchins that may suppress the recovery of fish and coral populations once fishing effort is reduced or eliminated. Restoration of these reefs may be accelerated by intervening and reducing sea urchin populations—particularly those of long-lived species. We studied three Kenyan coral-reef lagoons to determine the influence of reductions in sea urchin populations on coral, algae, and fish populations. Populations were monitored seven times over a 1-year period in 50 × 50 m unmanipulated control plots and experimental sea urchin reduction plots where sea urchin populations were reduced by about 85%. Census of the most-abundant fish families found the greatest positive population responses to sea urchin reduction in plots protected from fishing. The wet weights of fish nearly tripled, population density increased by 65%, and species richness increased by 30% compared with adjacent control plots. Parrotfish, wrasse, scavenger, and snapper families showed the greatest population and wet-weight increases. In the two fished reefs, fish populations also increased but to a lesser degree than in the unfished reef—particularly when comparing the wet-weight estimates of fish. Small-bodied species of the dam-selfish and wrasse families and juvenile parrotfish exhibited the largest population increases in these fished reefs. In fished reefs algae and seagrass cover exhibited the greatest increases following sea urchin reduction. Seagrass colonized bare sand, whereas fleshy brown algae colonized hard substrate (dead coral). The tall canopy-forming alga Sargassum latifolium became the dominant alga in the fished reefs, whereas the more prostrate genus Padina dominated in the unfished reef. Fleshy algae cover in the unfished reef was about half that of the two fished reefs—attributable to the greater abundance of parrotfish in the unfished reef. This suggests that greater herbivory in the unfished reef kept the algae from reaching an algal forest climax typified by Sargassum dominance. In the fished reefs the high S. latifolium cover and the increased thickness of the algal turf reduced hard coral cover by around 30%. In the unfished reef coral cover was reduced by around 13%, but by the end of the experiment coral cover and genera richness were the same in both the control and the sea urchin reduction plots. We conclude that, primarily, fishing and, secondarily and indirectly, high sea urchin abundance are reducing fish numbers and diversity in Kenyan reefs and that sea urchin reduction has the potential to increase reef fisheries production and recovery from overfishing. Sea urchin reduction is not recommended, however, on moderately fished reefs due to the possible loss of coral cover and diversity. If reefs are either severely degraded or if fishing effort is reduced or eliminated, then sea urchin reduction is recommended for the restoration of fish numbers, feeding importance, species diversity, and fisheries production.Muchos arrecifes de coral sujetos a una pesca intensa tienen una alta abundacia de erizos marinos que podrían detener la recuperación de las poblaciones de peces y corales una vez que el esfuerzo pesquero es reducido o eliminado. La restauración de estos arrecifes podría ser acelerada interviniendo y reduciendo la abundancia de las poblaciones de erizos marinos, particularmente de las especies longevas. En el presente trabajo, estudiamos tres lagunas de arrecifes de coral para determinar la influencia que las reducciones de las poblaciones de erizos marinos tienen sobre las poblaciones de corales, algas y peces. Las poblaciones fueron monitoreadas 7 veces a lo largo de un año en parcelas de control de 50 × 50m sin manipular y parcelas experimentales donde las poblaciones de erizos marinos fueron reducidas en aproximadamente un 85%. Los censos de las familias de peces más abundantes encontraron las respuestas poblacionales positivas más grandes a la reducción en el número de erizos marinos en las parcelas protegidas de la pesca. El peso húmedo de los peces casi se triplicó, la densidad poblacional aumentó en un 65% y la riqueza de especies aumentó en un 30% en comparación con las parcelas de control adyacentes. Las familias de viejas, lábridos, barrenderos y lutjánidos exhibieron los incrementos poblacionales y en peso húmedo más pronunciados. En los dos arrecifes sometidos a la pesca, las poblaciones de peces también aumentaron, pero en menor grado, que en los arrecifes no sujetos a la pesca; particularmente cuando se compararon las estimaciones de los pesos húmedos de los peces. Las especies de menor tamaño de las familias castañuelas y lábridos y las viejas juveniles exhibieron los aumentos poblacionales más pronunciados en los arrecifes sujetos a la pesca. En los arrecifes sujetos a la pesca, la cobertura de algas y fanerógamas marinas exhibió el incremento más pronunciado luego de la reducción en el número de erizos. Las fanerógamas marinas colonizaron las zonas arenosas mientras que las algas pardas carnosas colonizaron el sustrato duro (i.e., coral muerto). El alga Sargassum latifolium que forma doseles altos se transformó en dominante en los arrecifes sujetos a la pesca mientras que el genero Padina, más postrado, dominó en los arrecifes no sujetos a la pesca. La cobertura de algas carnosas en los arrecifes no sujetos a la pesca fue la mitad de aquella de los dos arrecifes sujetos a la pesca, lo cual es atribuíble a la mayor abundancia de viejas en estos arrecifes. Esto sugiere que el mayor herviborismo en los arrecifes no sujetos a la pesca evitó que las algas alcanzaran el climax algal caracterizado por la dominancia de Sargassum En los arrecifes sujetos a la pesca, la alta cobertura de S. latifolium y el espesor del cesped algal redujo la cobertura de corales duros en aproximadamente un 30%. En los arrecifes no sujetos a la pesca, la cobertura de coral fue reducida en un 13% pero al final del experimento la cobertura de coral y la riqueza de géneros fue la misma tanto en las parcelas de control como en aquellas en las cuáles se redujo la población de erizos marinos. Concluímos que la pesca en primer lugar y la abundancia de erizos marinos en forma secundaria e indirecta están reduciendo el número y diversidad de los peces en los arrecifes de Kenia. La reducción en el número de erizos marinos tiene el potencial para aumentar la producción de las pesquerías de los arrecifes y permitir la recuperación luego de la sobrepesca. Sin embargo, no se recomienda la reducción en el número de erizos marinos en los arrecifes sometidos a una pesca moderada debido a la posible pérdida de la cobertura y diversidad de los corales. Si los arrecifes estan seriamente degradados o se reduce o elimina el esfuerzo pesquero, se recomienda la reducción en el número de erizos marinos para la restauración del número de peces, la importancia alimenticia, la diversidad de especies y la producción pesquera.
Large differences in community structure of sea urchins and finfish have been observed in Kenyan reef lagoons. Differences have been attributed to removal of finfish predators through human fishing activities. This study attempted to determine (i) the major sea urchin finfish predators, (ii) the effect of predation on sea-urchin community structure, and (iii) the possible effect of sea urchin increases and finfish decreases on the lagoonal substrate. Six reefs, two protected and four unprotected, were compared for differences in finfish abundance, sea urchin abundance and diversity and substrate cover, diversity and complexity. Comparisons between protected and unprotected reefs indicated that finfish populations were ca. 4 x denser in protected than unprotected reefs. Sea urchin populations were >100 x denser and predation rates on a sea urchin, Echinometra mathaei, were 4 x lower in unprotected than in protected reefs. The balistidae (triggerfish) was the single sea-urchin finfish predator family which had a higher population density in protected than in unprotected reefs. Balistid density was positively correlated with predation rates on tethered E. mathaei (r=0.88; p
Coexistence between the coral reef inhabiting sea urchins Echinometra mathaei, Diadema savignyi and D. setosum was studied by comparing differences in body morphology, distribution, diet, susceptibility to predators, intra- and interspecific competition and settlement. The three species share similar diets and broad within-habitat distributions but differ in their microspatial preferences. E. mathaei is the smallest species, has the highest settlement rates and lives territorially within small burrows or crevices. D. savignyi is intermediate in size and lives frequently in intermediate size crevices or occassionally in social groups. D. setosum is the largest species and occassionally lives in large crevices or more frequently in social groups. Both Diadema have similarily low settlement rates. Competition experiments showed that E. mathaei was consistently the top competitor for crevice space. Diadema species shared larger crevices but competition occured within smaller crevices and was frequently won by the largest individual, regardless of species. D. savignyi may be the top competitor for crevice space between the Diadema species due to a reduced spine length/test size ratio which gives it a larger test for the same crevice size requirement. Predation rates were high for E. mathaei and low for both Diadema species. Coexistence is mediated by predation on the competitive-dominant while predation coupled with different body morphologies and behavior allows spatial resource partitioning of the reef's variable topography. Consequently, the three variables of predation, topographic complexity and differing body shapes create the observed species diversity. A reduction in predators due to stochastic fluctuations or from fishing pressure can lead to E. mathaei population increases and competitive exclusion of Diadema.
Given the importance attributed to the occupation of space in benthic coral reef communities, this study asks the question: are any particular microhabitat types limiting resources for an assemblage of worm-eating gastropods on Heron reef (Great Barrier Reef). Microhabitat resource use was measured on three occasions, separated by 12 and 20-month periods. The gastropod populations were typical of those of other Indo-Pacific sites with respect to mean shell size and density. Fluctuations in species' size and density are assumed to have not significantly influenced availability of microhabitat resources. Gastropods occurred mainly in the structurally complex refuge microhabitats during the day and showed an increased abundance in smooth, exposed, foraging microhabitat nocturnally. Nassarius gaudiosus is the most extreme microhabitat specialist diurnally and the most extreme microhabitat generalist nocturnally. A similar, although less pronounced trend was exhibited by other gastropod species. Microhabitat niche overlap was high for Conus coronatus, C. miliaris, C. flavidus, Vasum turbinellus and N. gaudiosus at night and was also high during the day for all these species except N. gaudiosus, which showed little overlap with other gastropod species diurnally. Using gastropod abundance data from all samples, and independently derived microhabitat abundance data, multiple regression analysis demonstrated:1)
A significant relationship between the abundances of N. gaudiosus, C. coronatus, and C. flavidus and the abundance of microhabitat 2 (sand under rocks=refuge).
2)
No positive association between gastropod abundance and the abundance of microhabitat 7a (thin layer of algal-bound sand on reef limestone).
Only N. gaudiosus is abundant in microhabitat 2. Therefore it is concluded that, with some exceptions, microhabitat abundance does not have a significant influence, directly or indirectly, on gastropod abundance. It is possible that density-independent mortality is maintaining gastropod densities below that at which competitive interactions, with respect to microhabitats, have significant effects on the gastropods' use of those resources.
I investigated the ability of predators to influence the patterns of species richness and abundance of non-piscivorous fishes on small, artificial reefs replenished by natural recruitment. Periodic removal of predators effectively reduced the species richness and abundance of predators on removal reefs. The difference between the number of predators on control and removal reefs was greatest immediately following the removal of predators and attenuated between removals. During periods of recruitment, species richness and total abundance of recently-recruited, non-piscivorous fishes were generally greater on predator-removal reefs than on control reefs. Species richness and total abundance of resident non-piscivorous fishes were not affected by the removal of predators in the first year of the experiment. Both abundance and species richness of residents, however, were greater on the removal reefs during the second year of the experiment. The difference in the responses of the two age classes to the removal of predators suggests that predators may affect community patterns of older age classes through time-lagged effects on the survivorship of younger age classes. At the end of the experiment, species richness was positively related to abundance for recruits and residents. The effects of removing piscivorous fishes on the abundance of non-piscivorous fishes were similar for species considered separately. A greater number of species of recruit and resident fishes were more abundant on reefs from which predators had been removed. These data suggest that predators can play an important role in structuring communities of fishes on coral reefs.
Patterns of hard coral and sea urchin assemblage structure (species richness, diversity, and abundance) were studied in Kenyan coral reef lagoons which experienced different types of human resource use. Two protected reefs (Malindi and Watamu Marine National Parks) were protected from fishing and coral collection, but exposed to heavy tourist use. One reef (Mombasa MNP) received protection from fishermen for one year and was exploited for fish and corals prior to protection and was defined as a transitional reef. Three reefs (Vipingo, Kanamai, and Diani) were unprotected and experienced heavy fishing and some coral collection. Protected and unprotected reefs were distinct in terms of their assemblage structure with the transitional reef grouping with unprotected reefs based on relative and absolute abundance of coral genera. Protected reefs had slightly higher (p<0.01) coral cover (23.6 8.3 % S.D.) than unprotected reefs (16.7 8.5), but the transitional reef had the highest coral cover (30.8 6.4) which increased by 250% since measured in 1987: largely attributable to a large increase inPorites nigrescens cover. Protected reefs had higher coral species richness and diversity and a greater relative abundance ofAcropora, Montipora andGalaxea than unprotected reefs. The transitional reef had high species richness, but lower diversity due to the high dominance ofPorites. Sea urchins showed the opposite pattern with highest diversity in most unprotected reefs. Coral cover, species richness, and diversity were negatively associated with sea urchin abundance, but the relative abundance ofPorites increased with sea urchin abundance to the point wherePorites composed >90% of the coral cover at sites with the highest sea urchin abundance. Effects of coral overcollection was only likely for the genusAcropora (staghorn corals). A combination of direct and indirect effects of human resource use may reduce diversity, species richness, and abundance of corals while increasing the absolute abundance of sea urchins and the relative cover ofPorites.
Population density, number of species, diversity, and species-area relationships of fish species in eight common coral reef-associated families were studied in three marine parks receiving total protection from fishing, four sites with unregulated fishing, and one reef which recently received protection from fishing (referred to as a transition reef). Data on coral cover, reef topographic complexity, and sea urchin abundance were collected and correlated with fish abundance and species richness. The most striking result of this survey is a consistent and large reduction in the population density and species richness of 5 families (surgeonfish, triggerfish, butterflyfish, angelfish, and parrotfish). Poor recovery of parrotfish in the transition reef, relative to other fish families, is interpreted as evidence for competitive exclusion of parrotfish by sea urchins. Reef substrate complexity is significantly associated with fish abundance and diversity, but data suggest different responses for protected versus fished reefs, protected reefs having higher species richness and numbers of individuals than unprotected reefs for the same reef complexity. Sea urchin abundance is negatively associated with numbers of fish and fish species but the interrelationship between sea urchins, substrate complexity, coral cover, and management make it difficult to attribute a set percent of variance to each factor-although fishing versus no fishing appears to be the strongest variable in predicting numbers of individuals and species of fish, and their community similarity. Localized species extirpation is evident for many species on fished reefs (for the sampled area of 1.0 ha). Fifty-two of 110 species found on protected reefs were not found on unprotected reefs.
Predation on 120 adult sea urchins of the speciesEchinometra mathaei was observed during daylight in shallow-water coral reefs (0.5 to 3 m deep) in a variety of sites in 3 Kenyan marine parks. The predators were few (8 species) and dominated by the triggerfishBalistapus undulatus (65 % of all observations) followed by terminal-male wrassesCoris formosa, C. aygula andCheilinus trilobatus, and lastly the scavengerLethrinus mahsena. Those species that attempted, but failed, to prey onE. mathaei were slightly more numerous (11 species), while scavengers of opened carcasses were the most speciose (20 species). Based on these observations, it is suggested thatB. undulatus is a keystone predator and that fishery regulations that protect this species may be necessary in order to reduce the detrimental consequences of high sea urchin abundance — such as high reef substrate erosion and competitive exclusion of fishes.
Empirical evidence from studies of the sea urchin Diadema antillarum suggests that this organism widely disperses its offspring, that both recruitment and mortality rates are independent of local densities, and that local food availability does not regulate local population sizes. These attributes would indicate that local populations are generally open and recruitment-limited. Given that current populations have been devastated by a 1983–1984 mass mortality event which spread throughout the range of this species, we examine current population trends and evaluate the prospects for population recovery under the assumptions of recruitment-limitation and density-independent rates of recruitment and mortality. Specifically, we evaluate the dynamics of five, local populations at Lameshur Bay, St. John, U.S.V.I. in order to 1) determine current rates of recruitment and mortality, 2) predict population densities based on the above assumptions, 3) compare predicted densities against observed 1984–1988 densities, and 4) predict future population densities based on current trends. We estimate current recruitment rates at 0.02–0.11 individuals/m2/yr and per capita mortality rates at 0.27–0.47 deaths/yr. Over the period 1985–1988, predicted densities based on these annual rates did not differ significantly from actual observed densities. Therefore, the assumptions that recruitment and mortality rates are density-independent and that local populations are recruitment-limited are sufficient, at present, to adequately predict current population trends. These trends indicate no recovery towards pre-mass mortality densities. The above description of the dynamics of open, recruitment-limited populations may be appropriate for a wide variety of organisms. We note the prevalence of animals with extensive larval dispersal capabilities. Populations located near the limits of their distribution, in freshwater streams and ponds, mountain tops, or other similarly isolated populations may also be subject to recruitment-limitation. Remote, recruitment-limited populations are likely to be more susceptible to local extinction than less remote populations. Dispersal distances and the scale of the processes controlling recruitment and mortality are important determinants of the degree of openness of local populations.
We examine changing ecological theory regarding the role of disturbance in natural communities and relate past and emerging paradigms to coral reefs. We explore the elements of this theory, including patterns (diversity, distribution, and abundance) and processes (competition, succession, and disturbance), using currently evolving notions concerning matters of scale (temporal and spatial), local versus regional species richness, and the equilibrium versus nonequilibrium controversy. We conclude that any attempt to categorize coral reef communities with respect to disturbance regimes will depend on the question being asked and the desired level of resolution: local assemblage versus regional species pool, successional versus geological time, and on the taxonomic and tropic affinities of species included in the study. As with many communities in nature, coral reefs will prove to be mosaics of species assemblages with equilibrial and nonequilibrial dynamics.
A literature review indicates that coral reef sea urchins out-compete herbivorous fishes (i.e. scarids and acanthurids), and reach maximum biomass levels an order of magnitude higher than herbivorous reef fishes. Additionally, sea urchins have lower respiration and food consumption rates, and at high biomass levels appear to reduce algal resources to levels lower than herbivorous fishes. A simple biomass-based energetic model was developed to help explain these patterns and to determine the energetic properties of organisms which affect their resource competitive ability and ability to sustain themselves under predation. Simulations of a hypothetical herbivore indicate that herbivore steady-state biomass increases with decreasing consumption rates, but a parabolic relationship was found between herbivore biomass and the level to which the herbivore reduces its food resource (i.e. R∗). Simulations of competition between hypothetical herbivores indicate that only the level to which a herbivore reduces its resource (R∗) was important in determining the superior resource competitor, although biomass dynamics differed depending on feeding, gross production rates, and initial conditions of competitors. High herbivore consumption and gross production increase rates of biomass recruitment. But, unless an herbivore can also keep resources below the level competitors can tolerate, a high rate of gross production does not insure the competitor superior resource competitive ability in the absence of predators. Yet the model also predicts that organisms with high gross production and consumption rates are able to persist better under high levels of harvesting by predators. The model suggests that sea urchins reach high biomass due to low algal consumption rates and out-compete herbivorous fishes, when their predators are reduced (i.e. often attributable to overfishing), by reducing algal biomass to low levels. Sea urchins appear to tolerate low algal biomass due to their low consumption and respiration rates which allows them to persist at low levels of net algal productivity. But, low sea urchin gross production makes them less likely to persist under high levels of harvesting or predation. The model suggests that organisms may frequently need to make energetic trade-offs between traits which will give them superior resource competitive ability and tolerance to predation or disturbance.
The impact of “overfishing” on coral-reef herbivores was studied using Thalassia and Sargassum bioassays at two reefs protected from fishing for over 15 yr, one “transition reef” protected for ~ 2 yr, and three unprotected reefs. The primary goals of this research were to (1) assess the ability of an herbivory assay to distinguish between (a) herbivore types such as sea urchins and herbivorous fishes (i.e. parrotfish and surgeonfish), (b) rates of herbivory by sea urchins and herbivorous fishes, (2) potential impacts of coral-reef herbivores on seagrass species composition and abundance, and (3) the role that fishing plays in mediating competitive interactions between sea urchins and herbivorous fishes. Studied reefs differed in their management regulations and enforcement such that impacts of fishing regulations could be partly distinguished from species interactions. Parrotfishes appear to be the dominant fishes feeding on the assay. The sea urchin Echinothrix diadema (Linnaeus) exhibited a much higher preference for the assay than other sea urchin species. Consequently, the variable species composition of herbivorous fishes and sea urchins and their feeding preferences make between-site comparisons of herbivory problematic. However, if sites with high numbers of Echinothrix are excluded and the majority offish bites on the Thalassia assay are attributabed to a few species of parrotfish, then between-site and between-treatment comparisons of relative herbivory can be made. Experimental reduction of sea urchin abundance led to increased bite rates on herbivory assays by parrotfishes at protected site but not the unprotected sites. These results imply that sea urchins can reduce grazing rates of some species of parrotfishes. The species composition of seagrass communities in protected and unprotected reefs appears to be partially affected by prey choices of the dominant grazers such that parrotfish and Echinothrix sea urchins favor Thallasodendron dominance while other species of sea urchin such as Diadema setosum, D. savignyi (Audoin) and Echinometra mathaei de Blainville favor Thalassia dominance.
The potential effects of food and shelter availability on the recruitment and early survivorship of coral reef fishes were studied on St. Croix, U.S. Virgin Islands. The faunal assemblage studied included diurnally active fishes found in the “rubble/sand” habitat. The most abundant members were: beaugregory, Stegastes leucostictus (Muller & Troschel), goldspotted goby, Gnatholepis thompsoni Jordan, bridled goby, Coryphopterus glaucofraenum Gill, surgeonfishes, Acanthurus bahianus Castelnau and A. chirurgus (Bloch), and French grunt, Haemulon flavolineatum (Desmarest). Comparisons of recruitment to reefs constructed from substrata that varied in morphological characteristics showed that there were differences in the relative abundances of recruits attracted to and/or surviving on the different reef types. Juveniles of most species appeared to prefer the branching coral Porites porites (Pallas), which provided a large number of small crevices between the branches.Manipulations of the availability of shelter sites for fishes demonstrated that recruitment and/or early survivorship were strongly limited by the number of refuges. This result was found in six separate carried out during different years and in different seasons. Shelter site availability presumably limits fish populations through its effects on prédation rates.Experimental manipulations of food availability indicated that food does not directly influence settlement or early survivorship of coral reef fishes. However, it is probable that correlations between habitat characteristics and food availability have influenced the evolution of settling preferences.
Echinometra mathaei (de Blainville) population increases, attributable to removal of their predators by fishermen, results in competitive exclusion of herbivorous fish and other sea urchin species, high reef substratum bioerosion and decreased reef topographic complexity. This study explores the population regulation of E. mathaei under conditions of reduced predator abundance. Experimental density manipulations suggest some density-dependent mortality, yet populations on heavily fished reefs continue to increase despite high biomass and reduced food. On two of three studied reefs, including the site with the highest population density, adult and juvenile densities were positively correlated in 1-m2 quadrats. Individuals on the high population-density reef had smaller gonads and gonad indices (gonad wt · body wt−1), lower relative gut weights, a higher fraction of calcium carbonate in their guts, larger Aristotle's lantern indices (jaw wt · body wt−1), and lower respiration rates than individuals on the low population density reef. Comparisons with individuals starved in aquaria indicate that these are responses to reduced food availability. There was no indication that larger individuals were able to avoid resource limitations more effectively than small individuals. Additionally, there was no indication of reduced body size with increasing population density. The frequency of intraspecific agonistic behavior during experimental manipulations was lower (G test, p < 0.001) on the high than on the low-density reef. Agonistic behavior appears important for defending burrows which reduces predator-induced mortality but not in maintaining a balance between the population food requirements and food resources. In high-density reefs, where predator densities and topographic complexity are low, the lack of predators and agonistic behavior or territorial defense leads to unrestrained population increases and reduced energy availability to individuals. Regulation occurs at the level of the individual organism, allowing high population densities through decreased energy availability to all individuals.
An aggregated energy-based coral reef simulation model was developed and used to perform fishing experiments where fishing intensity and catch selection were varied. The model is composed of two groups of primary producers (coral and algae), herbivores (herbivorous fishes and sea urchins) and carnivores (piscivores and predators of invertebrates). Gross and net primary production are calculated from coral and algal production and respiration parameters, while the calcium carbonate balance is calculated from deposition by coral and algae and erosion by sea urchins and herbivorous fish. Simulation results indicate that fishing affects the coral reef's ecology and the benefits of the fisheries yield must be weighted against impacts on reef structure and processes. The model predicts that removing all fish groups will eventually result in reef dominance by sea urchins once their predators have been removed. This results in a rapid and dramatic drop in fisheries yields and reduced algal and coral biomass and productivity. Net calcium carbonate deposition is particularly sensitive to the effect of sea urchins or fishermen on living coral. Fishing only piscivores results in low fisheries yields but high reef accretion by indirectly releasing coral from competition with algae. A management strategy of fishing only piscivores and herbivorous fishes results in the highest and most stable fisheries yields. However, under this management strategy, high levels of fishing results in increased algae that competitively exclude coral and produces a temporary reduction in calcium carbonate deposition. But, at the highest algal biomass, calcium carbonate deposition is high and solely attributable to algal deposition. Nonetheless, this form of calcium carbonate deposition may not provide the reef structure required for fish habitat and shoreline protection. It may therefore prove beneficial, over the long term, to keep fishing below this level.
It is suggested that local animal species diversity is related to the number of predators in the system and their efficiency in preventing single species from monopolizing some important, limiting, requisite. In the marine rocky intertidal this requisite usually is space. Where predators capable of preventing monopolies are missing, or are experimentally removed, the systems become less diverse. On a local scale, no relationship between latitude (10⚬ to 49⚬ N.) and diversity was found. On a geographic scale, an increased stability of annual production may lead to an increased capacity for systems to support higher-level carnivores. Hence tropical, or other, ecosystems are more diverse, and are characterized by disproportionately more carnivores.