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Habitat modification drives benthic trophic diversity in an intertidal soft-bottom ecosystem
... Moreover, whether non-trophic interactions typically affect specific species, trophic levels, or functional groups within the food web, or, alternatively, indiscriminately mediate species and their trophic interactions throughout the network has yet to be resolved. While multiple studies suggest that sessile species with a generally low trophic level benefit more than others from non-trophic facilitation [17,21], other work suggests that facilitation also benefits higher trophic levels and more mobile species [20,22,23]. ...
... Additionally, we test that any change in food web properties caused by foundation species occurs via random facilitation of species throughout the entire food web or via targeted facilitation of specific species that belong to certain trophic levels or functional groups. Although foundation species are part of the food web like any other species (e.g. as prey or predator), numerous studies have shown that they strongly facilitate the associated community by creating new habitat and alleviating physical stress [12,13,[21][22][23][27][28][29][30]. This form of non-trophic facilitation by foundation species has been found to occur across a wide range of ecosystems and environmental conditions [31,32]. ...
... Our study included three coastal ecosystems: (1) intertidal seagrass beds dominated by Zostera noltii in Banc d'Arguin, (Mauritania), (2) cordgrass-dominated (Spartina alterniflora) fringing marshes growing on the cobble beaches of Rhode Island (USA), and (3) intertidal blue mussel beds (Mytilus edulis) in the Wadden Sea (the Netherlands). Apart from enhancing habitat complexity, the foundation species of these coastal ecosystems all attenuate current and waves, stabilize the substrate, and provide shelter and attachment for other species [22,[54][55][56]. In addition, seagrass and mussels trap large quantities of suspended particles from the water column, whereas cordgrass provides shading and stabilization of cobble stones [57]. ...
Food webs are an integral part of every ecosystem on the planet, yet understanding the mechanisms shaping these complex networks remains a major challenge. Recently, several studies suggested that non-trophic species interactions such as habitat modification and mutualisms can be important determinants of food web structure. However, it remains unclear whether these findings generalize across ecosystems, and whether non-trophic interactions affect food webs randomly, or affect specific trophic levels or functional groups. Here, we combine analyses of 58 food webs from seven terrestrial, freshwater and coastal systems to test (1) the general hypothesis that non-trophic facilitation by habitat-forming foundation species enhances food web complexity, and (2) whether these enhancements have either random or targeted effects on particular trophic levels, functional groups, and linkages throughout the food web. Our empirical results demonstrate that foundation species consistently enhance food web complexity in all seven ecosystems. Further analyses reveal that 15 out of 19 food web properties can be well-approximated by assuming that foundation species randomly facilitate species throughout the trophic network. However, basal species are less strongly, and carnivores are more strongly facilitated in foundation species' food webs than predicted based on random facilitation, resulting in a higher mean trophic level and a longer average chain length. Overall, we conclude that foundation species strongly enhance food web complexity through non-trophic facilitation of species across the entire trophic network. We therefore suggest that the structure and stability of food webs often depends critically on non-trophic facilitation by foundation species.
... In addition, a rapidly increasingly number of studies have recently demonstrated that non-trophic interactions play a key role in mediating food web structure and resilience (Compton et al., 2013;Kefi et al., 2015;van der Zee et al., 2016). The effects of habitat modifying species on their environment and biodiversity are well studied (Tylianakis et al., 2007;Lemieux and Cusson, 2014;van der Zee et al., 2015;Donadi et al., 2015), however to date only few studies have assessed the effects of these species on food web structure, function, and resilience (van derZee et al., 2016;de Fouw et al., 2016). In this study we explore how intertidal shellfish reefs – dominated by habitat-structuring blue mussels (Mytilus edulis) – affect community and food web structure in an intertidal ecosystem that is heavily impacted by human activity, the Wadden Sea. ...
... In this study we explore how intertidal shellfish reefs – dominated by habitat-structuring blue mussels (Mytilus edulis) – affect community and food web structure in an intertidal ecosystem that is heavily impacted by human activity, the Wadden Sea. Similar to many temperate soft-bottom intertidal ecosystems, mussel beds in the Wadden Sea form reefs that increase benthic trophic diversity as shellfish provide shelter and settlement substrate for many species, reduce hydrodynamic stress, stabilize sediment and facilitate other connected ecosystems (Gutierrez et al., 2003;Donadi et al., 2013;Donker et al., 2013;van der Zee et al., 2012van der Zee et al., , 2015). Mirroring declines of coastal ecosystems worldwide (Lotze et al., 2006), however, the Dutch Wadden Sea lost virtually all (~4000 ha) its intertidal mussel beds around 1990 due to overfishing in combination with storms and recruitment failure. ...
... The mussel beds in our study clearly modified local habitat conditions by increasing space for shelter and attachment for organisms, and reducing the hydrodynamic stress for organisms (Table 1). Our results, combined with earlier work on mussel beds demonstrate that habitat modification enhances species diversity and alters predator-prey interactions (Donadi et al., 2015;van der Zee et al., 2015). These findings support theoretical work (Olff et al., 2009;Kefi et al., 2012) and a thus far very limited number of recent empirical studies that show that trophic interactions are strongly controlled by non-trophic interactions in foundation species-dominated ecosystems (van derZee et al., 2016;Kefi et al., 2015). ...
In conservation strategies of marine ecosystems, priority is given to habitat-structuring foundation species (e.g. seagrasses, mangroves and reef-building corals, shellfish) with the implicit goal to protect or restore associated communities and their interactions. However, the number and accuracy of community level metrics to measure the success of these strategies are limited. Using intertidal shellfish reefs as a model, we tested to what extent foundation species alter community and food web structure, and explored whether basic metrics of food web structure are useful indicators of ecosystem complexity compared to other often-used indices. We found that shellfish reefs strongly modified community and food web structure by modifying habitat conditions (e.g. hydrodynamics, sediment grain size). Stable isotope-based food web reconstruction captured important differences between communities from bare mudflat and shellfish reefs that did not emerge from classic abundance or diversity measures. On shellfish reefs, link density and the number of top predators were consistently higher, while both connectance and the richness of intermediate species was lower. Species richness (+ 42%), species density (+ 79%) and total biomass of benthos, fish and birds (+ 41%) was also higher on shellfish reefs, but this did not affect the Shannon diversity or Evenness. Hence, our results showed that basic food web metrics such as link density and number of top consumers and intermediate species combined with traditional measures of species richness can provide a robust tool to measure conservation and restoration success. We therefore suggest that these metrics are included as Essential Biodiversity Variables (EBV), and implemented as ecosystem health indicators in legislative frameworks such as the Marine Strategy Framework Directive (MSFD).
... Restoration as a conservation practice aims to actively restore habitats to resemble former states [27]. This can happen naturally once anthropogenic pressures have been removed but often requires building a self-sufficient ecosystem [28,29] by reintroducing or supporting the ecosystem engineers [30][31][32]. ...
... Owing to the catastrophic decline and modern-day rarity of O. edulis reef habitats, and the historical context within which these declines occurred, it is extremely difficult to re-construct the biodiversity characteristics of a fully restored reef [39]. By examining the biodiversity of oyster habitats recovering from fishing activity [40], a greater understanding of the faunal community composition associated with oyster reef habitats can be gained [2,10,31] and used to manage expectations of biodiversity outcomes and progress towards biodiversity goals. ...
Reefs formed by Ostrea edulis, the European native oyster, are among many biogenic habitats that have declined globally. European oyster habitats are now rare, and undisturbed examples have not been described. As more is understood of the ecosystem services provided by the reefs, oyster restoration efforts are on the rise, becoming a more prominent component of Europe's portfolio of marine conservation practices. It is therefore important to establish the relationship between the development of oyster reefs and their associated biotic community if the biodiversity benefits are to be accurately predicted and the progress of restoration projects assessed. The Loch Ryan oyster fishery in Southwest Scotland is the last of its type and uses a rotational harvest system where different areas are fished each year and then left for six years before they are fished again. This provided an opportunity to study the effect of oyster reef development and biodiversity gain at different stages of habitat recovery. In this study three treatments were surveyed for faunal biodiversity, oyster shell density and oyster shell percentage cover. Treatments were plots that had been harvested one year before, two years before, and six years before the study. The treatments were surveyed with SCUBA using a combination of video transects and photo quadrats. Oyster shell density, oyster shell percent cover and macrofaunal biodiversity differed significantly between treatments, with the highest values observed in the six-year treatment. Shell density was 8.5 times higher in the six-year treatment compared to the one-year treatment, whilst Shannon-Wiener's diversity was 60.5% higher, and Margalef's richness 68.8% higher. Shell density and percent cover had a significant positive relationship with macrofaunal biodiversity. This is probably due to the provision of increased structural complexity in the matrix of live and dead oyster shells. Projecting forward the trend of biodiversity increase in relation to time since disturbance indicates that full recovery would take approximately ten years in which time diversity (Shannon-Wiener) would probably have doubled. The findings from the present study indicate the probable biodiversity benefits of oyster habitat restoration and a cost-effective metric (shell density) to judge progress in restoration projects.
... One of the sites was located on a sandy intertidal flat, characterized by sediments with large grain sizes and low natural cockle densities. The other site was located in the wake of a mussel bed, where previous work showed that the bed alters sediment conditions through attenuation of hydrodynamics and pseudofaeces deposition, resulting in silty sediment with a high organic matter content Donadi et al., 2013a;Donadi et al., 2013b;van der Zee et al., 2015;Eriksson et al., 2017). These environmental modifications in turn enhance the settlement of large numbers of cockles, making this site an important feeding ground for many avian predators Donadi et al., 2013a;Donadi et al., 2013b). ...
... E) (Donadi et al., 2013a). Both sites were chosen based on previous studies that successfully demonstrate that this area is highly suitable to investigate the effects of habitat characteristics on multiple interaction types Donadi et al., 2013a;Donadi et al., 2013b;Donadi et al., 2015;van der Zee et al., 2015). At the start of the experiment in May 2010, the mean background cockle density was 65 nm -2 at the mussel site and 0 nm -2 at the sandy control site. ...
... Specifically, a recent study in which 178 species were sampled at 839 locations across the entire Dutch Wadden Sea, found that diatoms are the most important energy source for the majority of consumers at higher trophic levels, including many species of worms, molluscs, crustaceans, fish, and birds (Christianen et al., 2017a). Hence, in modifying their habitat epibenthic bivalve reefs increase niche diversity through both trophic and non-trophic pathways, making these reefs, and the areas they affect beyond their boundaries, biodiversity hotspots of the intertidal ( Van der Zee et al., 2015;Christianen et al., 2017b). In fact, as a consequence of their extended habitat modification effect, it has been estimated that although these epibenthic bivalve reefs currently cover only 5% (Folmer et al., 2017a), they effect around 20% of the intertidal mudflats (Nieuwhof et al., 2018). ...
... Therefore, the degradation of mussel beds will surely contribute to biodiversity loss in marine ecosystems. In this way, and as suggested in some previous studies [e.g., 64,65], our results also demonstrate that ecosystem engineers should be considered important target species for conservation issues. Particularly, mussel beds could be useful indicators of hotspots with great biodiversity [47]. ...
Biodiversity loss is considered one of the main threats to marine ecosystems. In this framework of biodiversity decline, organisms that provide biogenic habitat play a relevant role by their capacity to structure assemblages and influence ecological processes. The Mediterranean mussel Mytilus galloprovincialis is considered an ecosystem engineer because it alters local environmental conditions maintaining habitat suitability for other organisms, and enhancing local biodiversity. Although it is widely recognized that mussel beds increase diversity, the drivers shaping these assemblages are poorly explored. We investigate whether mussel size homogenisation shapes the abundance, richness and structure of macrobenthic assemblages associated with mussel beds in two shores of the Galician coast (NW Spain). At each shore, two sites, 10 m apart, were selected and at each site, faunal assemblages were compared between mussel clumps showing shells of various sizes (control), and mussel clumps with closely similar-sized mussels, considered as homogenised. Homogenised clumps showed, in general, higher values in total number of individuals and species than control clumps. Regarding the effect of mussel size homogenisation on the multivariate structure of the assemblages, significant differences between control and homogenised clumps were found in three out of the four sites. Most relevant associated species usually reached higher abundances in homogenised clumps than control ones. Therefore, mussel size homogenisation influenced the structure of the macrofaunal assemblages associated with mussel beds but, its effect was context dependent (i.e., varied with sites). Information about the species contribution to dissimilarities among homogenised and control clumps was provided and the potential influence of sediment and algae on mussel clumps was discussed.
... As ecosystem engineers increase biodiversity locally, they are considered valuable conservational targets [3,38]. However, to understand how environmental disturbances modify the diversity of assemblages harbored by ecosystem engineers, empirical data are still needed. ...
Nowadays, coastal urbanization is one of the most serious and prevalent pressures on marine ecosystems, impacting their biodiversity. The objective of this study was to explore differences in attributes and biodiversity associated with an intertidal ecosystem engineer, the mussel Mytilus galloprovincialis Lamarck, 1819 between urban and non-urban shores. For this, mussel attributes and their associated macrofauna were compared between urban and non-urban rocky shores in the north of Portugal. Results showed that the largest sized mussels were more frequent on urban shores, whereas the smallest size class was only present in non-urban shores. Regarding macrofauna associated with mussels, the number of taxa was significantly higher on non-urban shores. Moreover, the structure of the macrobenhic assemblages was significantly different between urban and non-urban shores. Most important taxa responsible for differences were more abundant on non-urban shores except for Nucella lapillus, Idotea pelagica and Oligochaeta that were more abundant on urban shores. Therefore, our results showed that the mussel size frequency and the structure of the associated macrobenthic assemblages changed in urban shores. Considering the relevance of mussel beds for biodiversity and human well-being, our results indicate the need of adopting proper management plans to minimize these effects on urban intertidal ecosystems.
... Affects: Trophic structure, resource use and energy transfer, biogeochemical processes (Rosenberg, 1995;Norling et al., 2007) Responds to: Hydrodynamics, resource availability, habitat stability and heterogeneity (Rosenberg, 1995;Simboura et al., 2000;Rossi et al., 2001;van der Zee et al., 2015) Suspension/filter feeder Responds to: Environmental stability, resource availability (Pearson and Rosenberg, 1978;Beauchard et al., 2017) type of the larval settlement and early development after metamorphosis Affects: Recolonization potential, resilience, nutrient cycling (Thrush and Whitlatch, 2001;Degen et al., 2018;Bolam et al., 2020) Responds to: Environmental stability and resource availability (Fauchald, 1983) Benthic Pelagic Fecundity ...
Pennatulacean octocorals (sea pens), one of the primary conservation targets of the Laurentian Channel Marine Protected Area (MPA), in eastern Canada, occur at high densities in some deep-sea sedimentary habitats. Considered important habitat-forming organisms for many megafaunal organisms, the effect of sea pens on nearby sedimentary macrofaunal communities remains unexplored. During two cruises in September 2017 and July 2018, we sampled 9 stations within the MPA, including sea pen fields and bare sedimentary habitats (336–445 m depth), targeting individual sea pens in a subset of the cores to assess small-scale effects. We evaluated macrofaunal density, taxonomic diversity, vertical distribution, community composition, and biological trait expression, and investigated variation between sea pen fields and other (bare) sedimentary habitats, as well as between cores with and without sea pen specimens. Using multivariate analyses, we identified abiotic and biotic drivers of macrofaunal community composition and biological trait expression. Enhancement of macrofaunal density and taxonomic diversity and higher percentages of organisms in the upper sediment layers characterized sea pen fields in autumn, with more variable results in summer. Community composition and biological trait expression consistently differed in sea pen fields compared to bare sedimentary habitats, with Pennatula sea pen density as one of the primary drivers of variation especially for community composition, along with other environmental drivers (depth, grain size, and organic matter quantity and quality). We also detected small-scale enhancement of macrofaunal diversity in cores containing sea pens at stations characterized by predominantly bare sediments. Our results indicate that macrofaunal communities within sea pen fields differ from those in bare sediments and we propose sea pens play a role in influencing those patterns by increasing food availability, stability, and small-scale heterogeneity in sedimentary habitats, acknowledging that other environmental +factors might also play a role in determining the observed patterns. Characteristics of macrofaunal communities within sea pen fields also suggested potentially higher sensitivity to disturbance, which amplifies the need for protection of sea pen fields in deep-sea sedimentary environments.
... a) À travers ces effets sur les producteurs et les consommateurs, c'est donc tout le réseau trophique de l'écosystème qui peut potentiellement être modifié. En effet, outre la compétition directe entre une espèce invasive et les espèces natives (Zwerschke et al., 2016), les effets indirects dus à la transformation de l'habitat sont susceptibles d'influer sur le fonctionnement trophique de l'écosystème (Beekey et al., 2004 ;Donadi et al., 2013 ;van der Zee et al., 2015). Par exemple, la colonisation par la moule zébrée Dreisseana polymorpha a engendré un changement complet du réseau trophique d'un lac par l'intermédiaire de la stimulation du microphytobenthos par ses biodépôts (Ozersky et al., 2012). ...
La crépidule (Crepidula fornicata) est une espèce invasive des côtes européennes. Hermaphrodite protandre, cette espèce grégaire forme des chaines d’individus qui s’accumulent en forte densité sur les fonds. Longtemps considérée comme un envahisseur néfaste, la crépidule est aussi un ingénieur de l’écosystème, modifiant physiquement et biologiquement son habitat. Elle constitue un modèle biologique pour étudier comment les espèces invasives et ingénieurs peuvent structurer et modifier l’écosystème qu’elles colonisent. Dans le cadre de cette thèse, ces effets ont été examinés à travers le fonctionnement trophique des habitats à crépidules, en lien notamment avec la production primaire benthique. Il a été mis en évidence expérimentalement une stimulation du microphytobenthos (MPB) subtidal grâce à l’activité biologique de la crépidule. La niche trophique de C. fornicata a été redéfinie suite à la découverte de la présence de sphérules de carbonates dans ses tissus, surestimant la contribution du MPB dans son régime alimentaire. L’utilisation de différents marqueurs trophiques (pigments, acides gras, isotopes) a révélé que les jeunes individus mobiles étaient susceptibles de brouter le MPB associé au biofilm présent sur les coquilles. Les adultes sessiles, sont des filtreurs opportunistes, où la matière détritique mais aussi du MPB contribuent à leur régime alimentaire. L’analyse de différents suspensivores inféodés à cet habitat, a démontré que la crépidule n’était pas un compétiteur trophique pour des espèces commercialement importantes (pétoncle noir et huître plate) dû à leurs mécanismes de sélection trophique. À l’échelle du réseau trophique, la crépidule en très forte densité peut conduire à une homogénéisation du réseau trophique global dû à l’enrichissement en matière organique. Au contraire, un banc mort de crépidules montre une complexité trophique similaire à celle d’un banc de maërl, écosystème à forte biodiversité. Cette thèse, en plus d’avoir caractérisé le fonctionnement trophique des bancs de crépidules, montre qu’une espèce invasive, en facilitant certains compartiments biologiques (MPB, suspensivores), peut contribuer à la richesse d’un écosystème comme celui la rade de Brest.
... Previous work from Angelini et al. (2015) suggests that together with the live oak on which it grows, Spanish moss forms a facilitation cascade, in which the moss acts as a secondary foundation species. In general, foundation species are suggested to enhance species richness by increasing habitat structure and mitigating physical stress (Bertness et al. 1999, Govenar 2010, van der Zee et al. 2015, 2016. Although our results show that Spanish moss indeed reduces temperature and humidity fluctuations, we found this mitigating effect to be relatively minor, that is, <0.5°C temperature reduction in deviation from the average compared to bare branch and <3% Fig. 4. Invertebrate abundances of different feeding guilds for all festoon patch sizes and both festoon types (living or mimic) with statistics results; no interactive effects were found (statistics are reported in Appendix S1: Table S2). ...
Foundation species are typically suggested to enhance community diversity non‐trophically by increasing habitat structure and mitigating physical stress, while their trophic role is considered of minor importance. Yet, there is little experimental evidence on the relative importance of trophic and non‐trophic effects and the interaction with patch size. Here, we transplanted different festoon sizes of living Tillandsia usneoides (Spanish moss) and structural mimics assessing the trophic and non‐trophic roles of this habitat‐forming epiphyte in mediating the invertebrate community. Compared to bare branches, mimics enhanced species and feeding guild richness and abundances, but living festoons even more so, demonstrating that trophic and non‐trophic effects jointly stimulated the community. Specifically, our results show that, independent of patch size, 40% of the total species richness and 46% of total guild richness increase could be contributed to habitat structure alone, while Spanish moss trophically stimulated these metrics by another 60% and 54%. As detritivores were particularly enhanced in living festoons, our findings suggest that trophic stimulation occurred primarily through the provisioning of Spanish moss detritus. Our results highlight that foundation species can facilitate their associated communities through both trophic and non‐trophic pathways, calling for studies addressing their indirect trophic role via the brown food web.
... Like other dominant suspension-feeding bivalves (Cummings et al. 1998, Cummings et al. 2001, Norkko et al. 2001, Hewitt et al. 2002, Beadman et al. 2004, Borthagaray & Carranza 2007, Commito et al. 2008, the presence of Mercenaria mercenaria resulted in significantly different benthic communities. Communities stocked with hard clams contained different functional structure, as has been seen elsewhere when dominant bivalves are restored (Rodney & Paynter 2006), in bivalve ecosystem engineer stocking experiments (van der Zee et al. 2015), and studies of bivalve engineered/nonengineered habitats (Christianen et al. 2017). Although communities were different between stocked and unstocked sites, identification of possible drivers of change had mixed evidence. ...
Overharvest of the once highly abundant northern quahog, or hard clam [Mercenaria mercenaria (Linnaeus, 1758)], has decimated local populations on Long Island, NY, and the ecosystem services they provide. Among the potential ecological consequences of the loss of hard clams, impacts on benthic community structure and function have not been well documented. This study took advantage of a large-scale hard clam restoration effort by The Nature Conservancy in Great South Bay, NY, to investigate these parameters. The benthic communities between areas planted with high densities of adult clams (i.e., spawner sanctuaries) and adjacent sites with no transplants were compared. In clam-stocked areas, there were greater trophic and taxonomic group densities, especially for some environmentally sensitive groups such as crustaceans, and increased species richness, which in turn altered community structure of the resident infauna. Differences between paired sites for macrofaunal diversity and community structure were also observed, likely reflecting larger scale differences in invertebrate communities. Minimal differences were observed in sediment porosity, carbon and nitrogen content, and total organic matter between adjacent areas with and without clams, although all of these parameters were significantly associated with community structure. Although shell density was significantly higher in clam-stocked plots, it explained little of the variation in macrofaunal composition on its own or in combination with other environmental parameters, but in combination with the living clams may have added enough environmental complexity to enhance densities of different trophic groups. Within the short period of 3 y, differences in the benthic community with the presence versus the absence of clams were already detectable. More long-term data incorporating the effect of processes at multiple scales are needed to gain insight into the complexity of rehabilitating coastal benthic environments following the removal of important species such as clams. The results of this work support the notion that M. mercenaria acts as an ecosystem engineer, modifying the environment by creating habitat and enhancing the abundance of other species.
... Shellfish have been shown to have positive effects on biodiversity in the system, by altering both macrofauna and microbial communities. A number of papers (nine) have shown the link between shellfish and community composition alterations in general, showing changes in community assemblages (Kluger et al., 2016), species richness, abundance and biomass (Quan et al., 2012;van der Zee et al., 2015). The majority of the papers focused on the changes in macrofauna (invertebrate) or meiofauna communities or fish population dynamics (Boldina et al., 2014;Winberg and Davis, 2014;Van Colen et al., 2015). ...
The supply of ecosystem services (ES) that benefit humanity are derived from multiple, interacting ecological functions and processes. Focusing on the ecological mechanisms that underpin ES delivery allows bundles of services to be identified, bridging a critical gap with management. Work in marine systems has not yet progressed to the identification of ES bundles, as a result of data scarcity and complications arising from system complexity and connectivity, as opposed to terrestrial systems where ES bundles have been more widely applied based on spatial clustering. To demonstrate the approach, identification of ES bundles provided by shellfish is used as a case-study. Shellfish provide a number of known ES that need to be strategically managed to ensure sustainable use. As a result of global degradations in shellfish beds ES have been lost, and restoration efforts emphasize the importance in regaining these services. A literature review, including 146 papers aimed specifically at linking shellfish to either ecosystem functions or ES, was conducted to establish key linkages between processes, functions and services. Based on co-occurrence of services and shared linkages, four bundles of services are identified, including Marine resources, Coastal health and quality, Habitat modification, and Biological structuring. Our study emphasizes the underpinning ecological mechanisms and the importance of interactions between services, expressed in the formation of bundles by mutual drivers and processes, as well as between services in different bundles, as either synergies or trade-offs. The approach enables the translation of ecological knowledge and creates generality to inform policy making and management, thereby providing a format useful for ecologists, managers and other stakeholders.
... Significant differences are indicated with an asterisk (Mann-Whitney test, n = 12) Godet et al. 2011). It has been indicated that the presence of high densities of polychaetes and the stability of these structures may also lead to changes in sediment type (Van der Zee et al. 2015); accordingly, we observed that in the biogenic structures, the proportion of very fine sands was higher than in the surrounding area. There are three records of polychaete reefs on the Argentina coast; Ficopomatus enigmaticus built calcareous tubes (Schwindt and Iribarne 1998), Sabellaria nanella (Bremec et al. 2013) and Boccardia proboscidea (Jaubet et al. 2011) form reefs of sand tubes. ...
This work analyzed the variation in composition and abundance of benthic assemblages in an intertidal environment of the Río Gallegos estuary, Argentina (51°35′ S - 69° 01′ W). The macrofauna was sampled in a tidal flat adjacent to the city in late summer of 2006 and 2013 (March). In total 27 macrofauna taxa were identified; the most abundant species were Darina solenoides (44%) in 2006 and Fabricinuda sp. (89%) in 2013. Biogenic structures composed of aggregates of polychaete (Fabricinuda sp.) tubes and sediment were observed for the first time in December 2006; they are spherical or of irregular shape, 1 to 3 m² in area, and up to 50 cm in height. In 2013 benthic composition was compared between biogenic aggregates (“patch”) and the area outside them (“non-patch”). Species richness and total abundance of individuals were higher in the patch. The associated benthos was different; Fabricinuda sp. was dominant in the patch (107,780.7 indiv.m-², SD = 44,772.6), and Kinbergonuphis dorsalis (2139 indiv.m-², SD = 840) and Notocirrus lorum (1830 indiv.m-², SD = 516.4) in the non-patch. In 2013, four taxa that were not found in 2006 were recorded only in the patch samples, two bivalves (Sphenia hatcheri and Malletia cumingii) and two polychaetes (Fabricinuda sp. and Ophioglycera eximia). Polychaete aggregations have produced changes in number and abundance of benthic species at the lowest level of the tidal flat.
... Calculating functional diversity involves grouping organisms based on traits that represent their function in the environment to understand communities and ecosystems based on what organisms do, rather than on their evolutionary history (Petchey & Gaston 2006). This approach, commonly used for terrestrial plants, has relatively recently been used to describe diversity of benthic macrobenthos (Bazairi et al. 2005, Savage et al. 2012, van der Zee et al. 2015, and the impact of habitat on functional diversity of marine organisms has been identified as a research need (Airoldi et al. 2008). In addition, although species are often grouped based on trophic position, most studies relate functional diversity only to bottom-up processes, omitting top-down processes that are likely to influence distribution and diversity of marine prey (though see Griffin et al. 2008, Clemente et al. 2010, Gauzens et al. 2016. ...
Habitat loss is occurring rapidly in coastal systems worldwide. In Chesapeake Bay, USA, most historical oyster reefs have been decimated, and seagrass loss is expected to worsen due to climate warming and nutrient pollution. This loss of habitat may result in declining diversity, but whether diversity loss will equate to loss in ecosystem function is unknown. A bivalve survey was conducted in a variety of habitat types (seagrass, oyster shell, shell hash, coarse sand, detrital mud) in 3 lower Chesapeake Bay sub-estuaries from spring 2012 through summer 2013 to examine the correlation between bivalve densities, habitat type, habitat volume (of material retained on 3 mm mesh), and predator density. Bivalves were analyzed as functional groups based
on feeding mode, living position, and predator defense strategy. On average, seagrass supported one additional functional group, and diversity was increased 68–94%, in seagrass compared to the other habitats examined. Species richness and functional group richness were positively correlated with habitat volume. The greatest densities of deposit-feeding bivalves were in detrital mud habitats, the greatest densities of thin-shelled and surface-dwelling bivalves were in seagrass habitats, and the greatest densities of armored bivalves were in oyster shell habitats. Small, thin-shelled bivalves were negatively correlated with densities of predators, including blue crabs Callinectes sapidus and cownose rays Rhinoptera bonasus. Overall, bivalve diversity was associated with habitat type, habitat volume, and predator densities. These results suggest that all
habitats, and particularly seagrass, play a role in maximizing bivalve functional diversity in Chesapeake Bay.
... Aggregating species can play an important role as autogenic ecosystem engineers, modifying the physical structure of their environment (Jones et al., 1997(Jones et al., , 1994. Molluscs are particularly important in this context because their shells provide additional habitat for other species (Gutierrez et al., 2003a), which can be extremely important in soft-sediment dominated areas, where suitable habitat for sessile species can be scarce (Buschbaum et al., 2008;De Smet et al., 2015;Thrush et al., 2001;van der Zee et al., 2015). The horse mussel Modiolus modiolus is a relatively slow growing, longlived species, known to form large biogenic reefs (Lindenbaum et al., 2008;Rees et al., 2008;Brown, 1978, 1977) and is considered an important ecosystem engineer facilitating biodiversity hotspots and playing an important role in terms of ecosystem functioning contributing to nutrient cycling, potentially including carbon storage in the form of shell deposits (Brown and Seed, 1976;Burrows et al., 2014;Cook et al., 2013;Fariñas-Franco et al., 2013;Geraldi et al., in press;Navarro and Thompson, 1997;Ragnarsson and Burgos, 2012;Rees et al., 2008). ...
Biogenic reefs are an important component of aquatic ecosystems where they enhance biodiversity. These reefs are often established by dense aggregations of a single taxa and understanding the fundamental principles of biogenic reef formation is needed for their conservation and restoration. We tested whether substratum type and density affected the aggregation behaviour of two important biogenic-reef forming species, the horse mussel, Modiolus modiolus (Linnaeus, 1758), and the blue mussel, Mytilus edulis (Linnaeus, 1758). First, we tested for effects of substratum type on mussel movement and aggregation behaviour by manipulating substrata available to mussels in mesocosms (three treatments: no sediment added, sediment added, sediment and shells added). Both mussel species moved less in treatments with sediment and with both sediment and shells present than when no sediment or shells were added and the percentage of these mussels that aggregated (clumps of two or more individuals) was lower when shells were present compared to treatments without shells, however, fewer M. modiolus attached to shells than M. edulis. There was no effect of different substratum type on patch complexity of either mussel species. In addition, motivated by our interest in the restoration of M. modiolus, we also tested experimentally whether the aggregation behaviour of M. modiolus was density-dependent. M. modiolus moved a similar distance in three density treatments (100, 200 and 300 mussels m− 2), however, their aggregation rate appeared to be greater when mussel density was higher, suggesting that the encounter rate of individuals is an important factor for aggregation. M. modiolus also formed aggregations with a higher fractal dimension in the high and medium density treatments compared to lower density, suggesting that at higher density this increased patch complexity could further facilitate increased recruitment with the enhanced habitat available for settlement. These findings add to the growing evidence showing that adding dead shells to substratum to encourage M. modiolus restoration is not likely to be effective. Our findings suggest that mussel density is a more important driver for patch and subsequently reef formation. Moreover, two seemingly functionally similar mussel species showed some differences in their behaviour (e.g. attachment to shells). This highlights the importance of considering the specific ecology of a target species, such as M. modiolus, when designing restoration methods because information garnered from experimentation on an ostensibly similar species (e.g. M. edulis) may not be appropriate.
... Here, we empirically test the hypothesis that, in ecosystems dominated by organisms that strongly modify their abiotic environment (hereafter called 'habitat modifiers'), overall food web complexity is enhanced by these modifications, beyond previously documented single-species facilitation effects. Habitat modifiers, also described as 'ecosystem engineers' or 'foundation species', are increasingly recognized as important drivers of ecosystem functions [16,2122232425262728. Although habitat modifiers are part of the food web like any other species (e.g. as prey or predator), they also have non-trophic effects on associated species by creating new habitat, altering resource availability and modifying physical environmental conditions. ...
The diversity and structure of ecosystems has been found to depend both on trophic interactions in food webs and on other species interactions such as habitat modification and mutualism that form non-trophic interaction networks. However, quantification of the dependencies between these two main interaction networks has remained elusive. In this study, we assessed how habitat-modifying organisms affect basic food web properties by conducting in-depth empirical investigations of two ecosystems: North American temperate fringing marshes and West African tropical seagrass meadows. Results reveal that habitat-modifying species, through non-trophic facilitation rather than their trophic role, enhance species richness across multiple trophic levels, increase the number of interactions per species (link density), but decrease the realized fraction of all possible links within the food web (connectance). Compared to the trophic role of the most highly connected species, we found this non-trophic effects to be more important for species richness and of more or similar importance for link density and connectance. Our findings demonstrate that food webs can be fundamentally shaped by interactions outside the trophic network, yet intrinsic to the species participating in it. Better integration of non-trophic interactions in food web analyses may therefore strongly contribute to their explanatory and predictive capacity. © 2016 The Author(s) Published by the Royal Society. All rights reserved.
... Recruitment of native species or species that compete less well for space, and avoidance of undesired invasive species, may be enhanced if engineering and maintenance works can be favourably timed with known seasonal recruitment periods (Airoldi and Bulleri, 2011). Alternatively, recovery time may act in a step-change manner if community diversity is dependent upon recruitment of keystone, engineering or facilitator species to catalyse the colonisation of subsequent species (Jackson et al., 2008;Klein et al., 2011;van der Zee et al., 2015). Identifying methods to enhance populations of these key species on coastal structures should improve levels of mitigation effectiveness. ...
Coastal ecosystems globally face pressures, with natural coastal habitats being replaced by engineered structures. While hard structures for navigation‐purposes and coastal defense can negatively impact native communities, they can also be applied in ecological restoration as artificial reefs. This way substrates may facilitate establishment of biogenic (shellfish) reefs and provide habitat heterogeneity in soft‐sediment ecosystems. In a 1.5‐year experiment, we introduced six different types of natural or biodegradable hard substrates in a subtidal soft‐sediment system. We compared the substrates with surrounding soft sediment and evaluated differences among substrate types using biodiversity indices, community composition analyses, and food web indicators. This offers a comprehensive understanding of how the introduced hard substrates affect ecosystem complexity. Overall ecological complexity indicators were higher in the second year of the experiment. We found a significant increase in species richness, community composition, and link density (number of feeding interactions per species) on hard substrates compared to the surrounding sediment. However, Shannon diversity index and food web connectance (measure of food web saturation) did not differ between substrates and the surrounding sediment. Although differences among types of hard substrates were small, certain species were uniquely associated with specific substrates with wood and granite performing best. While there was no establishment of epibenthic shellfish reefs within 1.5 years, we conclude that introducing hard substrates effectively enhances ecological complexity in soft‐bottom coastal ecosystems within that timeframe. We recommend the use of substrates with a historical ecological relevance in the system for scaling up introduction of substrates for ecological restoration.
The rapid reorganization of global biodiversity has triggered an intense research effort to understand its consequences for ecosystem functioning. However, efforts to monitor biodiversity change and evaluate the outcomes for ecosystem states and processes are currently poorly aligned. While most monitoring programs evaluate ecosystem status by reporting measures of taxonomic diversity, it is not the number of species but rather the exhibited traits of these species that regulate function. Trait-based approaches assume that trait diversity and variability relate to changes in functions across environmental gradients, but this relationship remains to be explored for most marine benthic ecosystems. Using macrozoobenthic communities from the Dutch Wadden Sea as a model, we compiled information on traits related to animal-sediment relationships. This trait information was then combined with species’ abundance data from a 19 years-long database to calculate different taxonomic and functional metrics that reflect macrozoobenthic diversity, function, and community structure. Finally, we compared how these taxonomic and functional metrics change along with sediment texture gradients. Our analyses showed that the structure of macrozoobenthic communities and various diversity metrics all changed with sediment gradients. The observed changes in the communities’ species composition were associated with directional shifts in the relative presence of specific functional traits with increasing sediment grain size, from communities dominated by small body size, deposit-feeding, and short life span to communities characterized by large to medium body size, suspension-feeding, and long life span. We observed limited functional redundancy and high sensitivity of functional trait-based measures to changes in the community composition along sediment gradients. Our findings suggest that a trait-based approach provides valuable information about the ecological function of marine macrozoobenthic species complementary to traditional biodiversity measures (e.g., species richness, Simpson diversity, etc.). Hence, these measures may be used to characterize changes in ecosystem functioning in time and space using traditional monitoring datasets.
Oyster populations within the coastal bays of Virginia have greatly declined, mainly due to overharvesting and disease, and past restoration efforts have largely focused on increasing their populations. Current restoration goals have now expanded to simultaneously procure the wider ecosystem services oysters can offer, including shoreline protection and ecosystem diversification. However, tradeoffs exist in designing artificial reefs because it is unlikely one design will optimize all services. This study compares the services provided by reef designs varying in elevation and width located adjacent to an intertidal marsh within a coastal bay of VA, USA. We quantified wave attenuation to determine potential coastal protection of the adjacent marsh, and changes to sediment composition and infaunal communities before and after reef construction for 3 years. After construction, we also quantified oyster size and population density to compare high and low elevation reef designs. High elevation reefs were more effective at attenuating waves and fostering oyster growth compared to low elevation reefs. Oysters atop high elevation reefs were on average approximately twice as dense and 20% larger than those on low elevation designs. Reef width had a minimal effect on oyster population density; densities on high and low reefs were similar for designs with one or three rows. The presence of oyster reefs also increased infaunal diversity and sediment organic matter. Our results indicate that artificial reef design can differentially affect the services provided through restoration, and elevation is especially important to consider when designing for oyster population enhancement and coastal protection.
Oyster aquaculture is one of several methods for the restoration of Delaware Inland Bays; however, little is known about its potential impacts on the benthic community of the bays. In this study, water quality parameters were measured and polychaetes were collected from 24 sampling locations at Rehoboth, Indian River, and Little Assawoman Bays from July to October 2016 and 2017. We aimed to assess the impact of Eastern oyster farming under different stocking densities (50 and 250 oysters/gear) and distances away from the sites where the off-bottom gears are implemented (under gears, one meter, and five meters away). No significant impact was detected on polychaetes’ abundance and richness in regard to the presence of oyster gears. The number of polychaetes and species richness was significantly higher in Little Assawoman Bay in comparison to the Indian River and Rehoboth Bays. Results showed that the Ulva lactuca bloom that happened in 2016 could negatively impact the low abundance and richness observed in the polychaetes community. Similarly, the values of polychaetes abundance and species richness did not change significantly in samples that were taken far from the oyster gears. Dominant polychaetes families were Capitellidae and Glyceridae contributing to more than 70% of polychaetes’ number of individuals. Our results help to understand the role of oyster aquaculture in restoring the viability in the natural habitat of the Delaware Inland Bays.
A survey of the adjacent waters east of Nantucket, Massachusetts provided an opportunity to evaluate the epibiota attached to cobbles, rocks, boulders, and Atlantic surfclam shell in a region of high tidal current velocity and sand scour where burial, exhumation, and scour may limit epibiont coverage on exposed, and thus otherwise highly preferred for attachment, substrates. Such conditions may confute the expectation that substrate complexity always adds significantly to ecosystem value by expanding the range of habitat options and consequently increasing species richness and trophic linkages. Sedimentary particles potentially providing good attachment substrate for erect sessile epibiota included surfclam shells, abundant at many locations, cobbles, nearly ubiquitous, rocks, routinely encountered, and occasional boulders. The attached epibiota fell into three categories based on their biases for particle types. Some preferred the largest particles or evidence of their occupation was best preserved on these particles: these included sponges, mussels, and barnacles and their scars. Some preferred intermediate and smaller terrigenous particles; these included tunicates and encrusting bryozoans. Some preferred surfclam shells, namely the slipper shells and erect hydroids. Slow-growing attached epibionts were exceedingly rare and all soft-bodied attached epibionts were rare. Only barnacles and their taphonomic scars and hydroids were common. The frequency of barnacle scars relative to intact barnacles suggests sediment scour under a high-flow regime. Mussels were rarely attached to larger sedimentary particles such as cobbles and rocks, though commonly occurring locally as mussel beds on sand and pebble, further supporting the ephemerality of exposed unscoured attachment sites. The absence of attached epibionts demonstrates that edaphic processes minimize the importance of cobbles, rocks, boulders, and shells in community structure in some subtidal high-energy regimes, defying expectations from their contribution to substrate complexity. Their apparent contribution to habitat complexity belies their resultant much more minor role in determining community composition, thereby limiting their ecosystem value.
Beta diversity, the compositional variation among communities, is often associated with environmental gradients. Other drivers of beta diversity include stochastic processes, priority effects, predation, or competitive exclusion. Temporal turnover may also explain differences in faunal composition between fossil assemblages. To assess the drivers of beta diversity in reef-associated soft-bottom environments, we investigate community patterns in a Middle to Late Triassic reef basin assemblage from the Cassian Formation in the Dolomites, Northern Italy, and compare results with a Recent reef basin assemblage from the Northern Bay of Safaga, Red Sea, Egypt. We evaluate beta diversity with regard to age, water depth, and spatial distance, and compare the results with a null model to evaluate the stochasticity of these differences. Using pairwise proportional dissimilarity, we find very high beta diversity for the Cassian Formation (0.91 ± 0.02) and slightly lower beta diversity for the Bay of Safaga (0.89 ± 0.04). Null models show that stochasticity only plays a minor role in determining faunal differences. Spatial distance is also irrelevant. Contrary to expectations, there is no tendency of beta diversity to decrease with water depth. Although water depth has frequently been found to be a key factor in determining beta diversity, we find that it is not the major driver in these reef-associated soft-bottom environments. We postulate that priority effects and the biotic structuring of the sediment may be key determinants of beta diversity.
A survey of the region eastward of Nantucket provided an opportunity to examine the cold temperate–boreal boundary along the high‐energy Great South Channel. Here described are the benthic macroinvertebrate community types encountered, with a focus on the influence of climate change on the range boundaries of the benthic biomass dominants and the potential existence of transient multiple stable states. The survey identified three primary community types. The shallowest sites were occupied by a surfclam‐dominated community, comprising an abundance of large (≥150 mm) surfclams, and a few common attached epibiota primarily attached to exposed surfclam shell. Two communities exist at intermediate depths, one dominated by submarket and small market‐size surfclams (<150 mm) and the other, created by mussel mats and their attendant epibiota, crabs, sea urchins, and other mobile epifauna. Mussels are a foundational species, establishing a hard‐bottom terrain conducive to these other denizens in soft‐bottom habitat. Cobbles were nearly ubiquitous, rocks were routinely recovered, and boulders were encountered occasionally. Slow growing attached epibionts were exceedingly rare and mobile epifauna were not obviously associated with these large sedimentary particles; nor were the surfclam or mussel communities. The frequency of barnacle scars suggests sediment scour under the high‐flow regime characteristic of the surveyed region, which voids the habitat potential of these sedimentary particles. The abundance of surfclam shell indicates that surfclams have inhabited the shoaler depths for an extended time; limited shell at deeper sites supports the inference from the absence of large animals that these sites are relatively newly colonized and represent further evidence of an offshore shift in range brought on by increasing bottom water temperatures. The dichotomous nature of the two primary community types at mid‐depths suggests that these two communities represent multiple stable states brought on by the interaction of an invading cold temperate species with the receding boreal fauna resulting in a transient intermingling of species, which, however, structure the habitat into exclusionary stable states rather than overlapping in a co‐occurrence ecotone.
In the past, ecological theories ignored positive interactions between organisms,
maybe due to the considerable attention given to competitive interactions. Positive
interactions as facilitation are very important aspects of ecosystems, determining
cascade effects and influencing ecosystem services. The Stress Gradient Hypothesis
suggests that a rise in environmental stress will correspond with an increase of
positive interactions.
We are living in a period in which atmosphere composition and climate conditions
are changing at a high rate, where the oceans are exploited by the overpopulated
lands. Our sea is undergoing stresses from various source. Hence, it might be right to
suppose, following the Stress Gradient Hypothesis, that since the beginning of these
adverse conditions there was an increase in positive interactions.
A valid method to verify this hypothesis might be a systematic review of the
literature and a meta-analysis.
The first part of this essay is an introduction to the general matters described above
and the aims of the essay. The second part is about the definition of a protocol for the
systematic review of the literature concerning positive interactions.
Ecosystem engineering research has recently demonstrated the fundamental importance of non-trophic interactions for food-web structure. Particularly, by creating benign conditions in stressful environments, ecosystem engineers create hot beds of elevated levels of recruitment, growth, and survival of associated organisms; this should fuel food webs and promote production on the ecosystem scale. However, there is still limited empirical evidence of the influence of non-trophic interactions on the classical food-web processes that determine energy transfer, that is, consumer–resource interactions. On the basis of a biomanipulation experiment covering 600 m2 of an intertidal flat, we show that ecosystem engineers influence resource uptake efficiency and the accumulation of algae following nutrient enrichment in a soft-sediment food web. Nutrient additions increased chlorophyll a concentrations in the sediment by 90%, but only in plots where we also introduced high densities (2000 per m2) of a burrowing bivalve, the common cockle Cerastoderma edule. The artificial cockle beds increased the nutrient uptake efficiency of the biofilm and promoted sediment accumulation, which suggests that the cockles facilitated the sediment-living algae by increasing sediment stability. This indicates that ecological interactions, rather than the availability of nutrients per se, set the limits for production in this coastal ecosystem. Our results emphasize the need to include facilitation theory and recognize that positive interactions between species are key to understand, manage, and restore ecosystems under human influence.
The effective use of ecosystem engineers in biodiversity conservation is contingent on an understanding of those factors that influence the magnitude and direction of their effects. At patch scales, effects of ecosystem engineers on associated communities can range from positive to negative according to how the ecosystem engineer modifies environmental conditions. In a metaanalysis of 68 empirical studies, we assessed how, for a widespread group of ecosystem engineers- the marine habitat-forming bivalves-bivalve taxon, density, habitat, tidal elevation and latitude, as well as habit, or lifestyle, of associated taxa, influences the magnitude and direction of their effect on associated invertebrates. Overall, marine bivalves had a positive effect on both species abundance and species density, but effect sizes varied considerably according to bivalve traits and environmental setting. Oysters enhanced invertebrate abundance to a greater extent than either mussels or pinnids, perhaps because of the greater habitat heterogeneity they provide. Nevertheless, the effect of mussels on associated communities was generally more responsive to spatial variation in engineer traits and environmental context than the effect of oysters or pinnids. Positive effects of mussels on associated species abundance decreased at high mussel densities, were greater at subtidal than mid-low intertidal elevations and differed among faunal habits depending on habitat setting. Knowledge of those conditions under which positive effects of bivalves on associated biodiversity is greatest will help in identifying which species of ecosystem engineer, at which sites, should be prioritised for conservation and restoration, where the goal is enhancement of biodiversity.
This paper investigates the changes in the numbers of waders using the Dutch Wadden Sea since 1990, when intertidal stocks of cockles Cerastoderma edule and mussels Mytilus edulis reached an all-time low due to overfishing. Following the collapse of the intertidal shellfish beds, these beds were better protected and shellfish stocks increased again. However, this recovery was restricted to the eastern part of the Dutch Wadden Sea, whereas stocks of shellfish showed no trend in the western part, or even declined, as did Baltic tellin Macoma balthica (Fig. 2). As expected, trends of shellfish-eating birds differed between the western and the eastern parts of the Dutch Wadden Sea (Tab. 1, Fig. 4-6). Red Knot Calidris canutus and Common Eider Somateria mollissima declined in the western Dutch Wadden Sea and increased in the eastern part. Herring Gull Larus argentatus declined in both areas, but the decline was more marked in the western part. Surprisingly, Oystercatcher Haematopus ostralegus strongly declined in both parts of the Dutch Wadden Sea. The absence of a clear difference be-tween the trends in the two parts may be linked to the extreme longevity and site faithfulness of this species, as well as hand cockle fishery. Some of the bird species that do not feed on shellfish but on prey living on or in shellfish beds also showed a decline in the western and an increase in the eastern Dutch Wadden Sea. This was especially clear for Turnstones Arenaria interpret. Species feeding predominantly on worms increased in both parts, but the trend was more marked for the western Dutch Wadden Sea. Whereas the trend of shellfish-eating species was reasonably explained by a change in food abundance (itself probably linked to both eutrophication and shellfish fishery), it is at present not clear what factors are primary responsible for the trends in wormeating species. Candidates include shellfish fishery, climate change and eutrophication. These hypotheses are not mutually exclusive and future research should focus on determining the relative importance of each of these hypotheses and how they interact.
Five new species of Cyclanthera are here described: C. cogniauxii from the Central Cordillera of Costa Rica; C. dieterleana from Durango, Mexico; C. dioscoreoides with a disjunct distribution in México, Mexico, and Suchitepéquez, Guatemala; C. entata from Chiapas, Mexico; and C. heiseri from Durango, Sinaloa, and Jalisco in Mexico.
The vegan package (https://cran.r-project.org/package=vegan) provides tools for descriptive community ecology. It has most basic functions of diversity analysis, community ordination and dissimilarity analysis. Most of its multivariate tools can be used for other data types as well. Together with the labdsv package, the vegan package provides most standard tools of descriptive community analysis. Package ade4 provides an alternative comprehensive package, and several other packages complement vegan and provide tools for deeper analysis in specific fields. Package BiodiversityR provides a Graphical User Interface (GUI) for a large subset of vegan functionality. The vegan package is developed at GitHub (https://github.com/vegandevs/vegan/). GitHub provides up-to-date information and forums for bug reports. Most important changes in vegan documents can be read with news(package="vegan") and vignettes can be browsed with browseVignettes("vegan"). The vignettes include a vegan FAQ, discussion on design decisions, short introduction to ordination and discussion on iversity methods. To see the preferable citation of the package, type citation("vegan").
Ecosystem engineers are organisms that directly or indirectly modulate the availability of resources to other species, by causing physical state changes in biotic or abiotic materials. In so doing they modify, maintain and create habitats. Autogenic engineers (e.g. corals, or trees) change the environment via their own physical structures (i.e. their living and dead tissues). Allogenic engineers (e.g. woodpeckers, beavers) change the environment by transforming living or non-living materials from one physical state to another, via mechanical or other means. The direct provision of resources to other species, in the form of living or dead tissues is not engineering. Organisms act as engineers when they modulate the supply of a resource or resources other than themselves. We recognise and define five types of engineering and provide examples. Humans are allogenic engineers par excellence, and also mimic the behaviour of autogenic engineers, for example by constructing glasshouses. We explore related concepts including the notions of extended phenotypes and keystone species. Some (but not all) products of ecosystem engineering are extended phenotypes. Many (perhaps most) impacts of keystone species include not only trophic effects, but also engineers and engineering. Engineers differ in their impacts. The biggest effects are attributable to species with large per capita impacts, living at high densities, over large areas for a long time, giving rise to structures that persist for millennia and that modulate many resource flows (e.g. mima mounds created by fossorial rodents). The ephemeral nests constructed by small, passerine birds lie at the opposite end of this continuum. We provide a tentative research agenda for an exploration of the phenomenon of organisms as ecosystem engineers, and suggest that all habitats on earth support, and are influenced by, ecosystem engineers.
Ecosystem engineers can strongly modify habitat structure and resource availability across space. In theory, this should alter the spatial distributions of trophically interacting species. In this article, we empirically investigated the importance of spatially extended habitat modification by reef-building bivalves in explaining the distribution of four avian predators and their benthic prey in the Wadden Sea—one of the world’s largest intertidal soft-sediment ecosystems. We applied Structural Equation Modeling to identify important direct and indirect interactions between the different components of the system. We found strong spatial gradients in sediment properties into the surrounding area of mixed blue mussel (Mytilus edulis) and Pacific oyster (Crassostrea gigas) reefs, indicating large-scale (100s of m) engineering effects. The benthic community was significantly affected by these gradients, with the abundance of several important invertebrate prey species increasing with sediment organic matter and decreasing with distance to the reefs. Distance from the reef, sediment properties, and benthic food abundance simultaneously explained significant parts of the distribution of oystercatchers (Haematopus ostralegus), Eurasian curlews (Numenius arquata), and bar-tailed godwits (Limosa lapponica). The distribution of black-headed gulls (Chroicocephalus ridibundus)—a versatile species with many diet options—appeared unaffected by the reefs. These results suggest that intertidal reef builders can affect consumer-resource dynamics far beyond their own boundaries, emphasizing their importance in intertidal soft-bottom ecosystems like the Wadden Sea.
The importance of positive effects of ecosystem engineers on associated communities is predicted to increase with environmental stress. However, incorporating such non-trophic interactions into ecological theory is not trivial because facilitation of associated species is conditional on both the type of engineer and the type of abiotic stress. We tested the influence of two allogenic ecosystem engineers (lugworms, Arenicola marina L. and cockles, Cerastoderma edule L.) on the main primary producers (microphytobenthos) of the tidal flats, under different abiotic stresses controlled by reefs of blue mussels (Mytilus edulis L.). We added 25,000 cockles or 2,000 lugworms to 5 × 5 m plots, both in a muddy site with high sedimentation rates located coastward of a mussel bed, and in a sandy site without mussels and characterized by high hydrodynamic stress. After a year, cockles increased algal biomass in the sandy area, but not in the mussel bed site, where high values were measured in all plots. However, lugworms did not affect algal biomass in any of the sites. Field measurements suggest that cockles outweighed negative effects of water currents in the site without mussels by locally increasing sediment stability, whereas mussels overruled the effects of cockles in the wake of the reefs through hydrodynamic stress alleviation and/or biodeposition. Our results suggest that non-trophic interactions by ecosystem engineering bivalves control primary production of intertidal areas, and that the sediment-stabilizing effect of cockles plays a crucial role where the overruling effects of mussel beds are not present.
The Wadden Sea is a shallow coastal region, with a large area of
sedimentary tidal flats that extends from The Netherlands to Denmark and
has been declared a site of international importance in the Dutch and
German parts (Ramsar status and UNESCO World Heritage Site). Benthic
macrofauna are central to the ecosystem functioning of this area, as
they recycle nutrients, decompose organic matter and are an important
food source for many secondary consumers, like fish and waterbirds. Due
to the environmental gradients characteristic of estuarine systems, it
is expected that changes in assemblage composition will be observed
across the physical and environmental gradients of the Wadden Sea.
First, we explored the spatial variation in assemblage composition of
benthic macrofauna across the intertidal part of the Dutch Wadden Sea
using 3 years of biomass data. Then, we identified the relative
importance of six environmental variables for explaining and predicting
changes in assemblage composition across the intertidal areas of the
Wadden Sea using generalised dissimilarity modelling (GDM). In
accordance with the environmental gradients across this system, the
biomass contributed by a few common species differed from west to east
and were distinct in the Dollard. In the west, bivalves Mya arenaria,
Cerastoderma edule and Ensis directus contributed a relatively large and
equal share of the total biomass, whereas C. edule contributed the sole
largest share of the total biomass towards the east. The polychaete
Alitta succinea became a large share of the total biomass in the upper
Ems and in the Dollard estuary, but contributed little elsewhere.
Similar to the observed differences in species composition, the spatial
patterns in assemblage composition, as predicted by the GDM models,
identified the Dollard as distinct and that the prevalence of assemblage
types in the west differed to the east. Median grain size, followed by
microphytobenthic biomass, and exposure time were the most important
variables describing differences in assemblage composition. That the
Wadden Sea forms a heterogeneous landscape where assemblage composition
varies across multiple gradients has repercussions for management and
monitoring.
Habitat-modifying species such as Mytilus edulis strongly impact both community structure and ecosystem functioning through positive or negative interactions with other species and by changing physical and biological conditions. A study of natural patches of mussels showed that C and N content of sediment was higher in mussel patches compared to the surrounding sand community. Species richness and biomass of associated macrofauna and -flora was enhanced even by the presence of single mussels and increased rapidly with patch size up to about 150 cm(2), while no change in sediment meiofauna was found. In order to separate the effects of structure and function of M edulis, a manipulative field experiment was performed with constructed patches of cleaned live mussels or intact empty shells. After 3 mo colonisation, the number of associated species in both treatments approached those in natural mussel patches, indicating that species richness was mainly due to physical structure. Abundance and biomass of associated flora and fauna were higher if live mussels were present because mussel biodeposition and nutrient regeneration supplied limiting resources and increased carrying capacity. Species composition was also affected. Complexity and biodiversity increased with time, especially if Fucus vesiculosus plants established themselves. Measurements of community metabolism showed that the associated community found in mussel patches depends on mussel biodeposition for 24 to 31% of its energy demand. Thus, the energy balance of the community is dependent on the function of live mussels.
Many criteria for statistical parameter estimation, such as maximum likelihood, are formulated as a nonlinear optimization problem. Automatic Differentiation Model Builder (ADMB) is a programming framework based on automatic differentiation, aimed at highly nonlinear models with a large number of parameters. The benefits of using AD are computational efficiency and high numerical accuracy, both crucial in many practical problems. We describe the basic components and the underlying philosophy of ADMB, with an emphasis on functionality found in no other statistical software. One example of such a feature is the generic implementation of Laplace approximation of high-dimensional integrals for use in latent variable models. We also review the literature in which ADMB has been used, and discuss future development of ADMB as an open source project. Overall, the main advantages of ADMB are flexibility, speed, precision, stability and built-in methods to quantify uncertainty.
Michael W. Beck, Kenneth L. Heck, Jr., Kenneth W. Able, Daniel L. Childers, David B. Eggleston, Bronwyn M. Gillanders, Benjamin Halpern, Cynthia G. Hays, Kaho Hoshino, Thomas J. Minello, Robert J. Orth, Peter F. Sheridan and Michael P. Weinstein
Self-facilitation through ecosystem engineering (i.e., organism modification of the abiotic environment) and consumer-resource interactions are both major determinants of spatial patchiness in ecosystems. However, interactive effects of these two mechanisms on spatial complexity have not been extensively studied. We investigated the mechanisms underlying a spatial mosaic of low-tide exposed hummocks and waterlogged hollows on an intertidal mudflat in the Wadden Sea dominated by the seagrass Zostera noltii. A combination of field measurements, an experiment and a spatially explicit model indicated that the mosaic resulted from localized sediment accretion by seagrass followed by selective waterfowl grazing. Hollows were bare in winter, but were rapidly colonized by seagrass during the growth season. Colonized hollows were heavily grazed by brent geese and widgeon in autumn, converting these patches to a bare state again and disrupting sediment accretion by seagrass. In contrast, hummocks were covered by seagrass throughout the year and were rarely grazed, most likely because the waterfowl were not able to employ their preferred but water requiring feeding strategy ('dabbling') here. Our study exemplifies that interactions between ecosystem engineering by a foundation species (seagrass) and consumption (waterfowl grazing) can increase spatial complexity at the landscape level.
Three successive years of recruitment failure, intensive fishery, and high rates of natural mortality eliminated all intertidal mussel beds and nearly all cockle beds from the Dutch Wadden Sea in the course of 1990. In late 1990 and early 1991, zoobenthic biomass and particularly bivalve stocks were unprecedentedly low. The following unusual events were noted:
Relatively high diatom and chlorophyll concentrations in the autumn of 1990 and an exceptionally early and dense spring bloom of diatoms in March 1991;
Exceptionally early onset of rapid growth in the bivalve Macoma balthica in March 1991;
Above‐average weights of the soft parts of bivalves in the 1990/91 winter;
Elevated mortality rates in bivalves that could serve as alternative prey for birds specialised on big bivalves (oystercatchers and eiders);
Signs of food shortage in such birds: high mortality rates and emigration to other feeding areas;
A rapid recovery of the benthos started in the summer of 1991, accelerated by an exceptionally high recruitment in some of the affected species.
It is concluded that the Wadden Sea ecosystem responded in an elastic way to the removal of two of its key species. The events are not seen as merely coincidental but are regarded as a chain of causes and effects.
The services of ecological systems and the natural capital stocksthat produce them are critical to the functioning of the
Earth’s life-support system. They contribute to human welfare, both directly and indirectly, and therefore represent
part of the total economic value of the planet.We have estimated the current economic value of 17 ecosystem services
for 16 biomes, based on published studies and a few original calculations. For the entire biosphere, the value (most of
which is outside the market) is estimated to be in the range of US$16–54 trillion (1012) per year, with an average of
US$33trillion per year. Because of the nature of the uncertainties, thismust be considered a minimum estimate. Global
gross national product total is around US$18 trillion per year.
The stability of cohesive and non-cohesive sediments in a mixed intertidal habitat within the Ria Formosa tidal lagoon, Portugal, was examined during two field campaigns as part of the EU F-ECTS project. The cohesive strength meter Mk III was used to determine critical erosion shear stress (t c) within a variety of different intertidal habitats and substrata, including Spartina maritima fields and Zostera noltii beds. The best predictor(s) for t c were derived from a range of properties measured for the surface sediments (chlorophyll a; colloidal carbohydrate, water, organic content, % fraction o63 mm, and seabed elevation). Pigment biomarkers were used to identify the dominant algal groups within the surface phytobenthic assemblage. Strong, seasonally dependent relationships were found between t c and habitat type, chl a; colloidal carbohydrate and bed elevation. Typically, critical erosion thresholds decreased seawards, reflecting a change from biostabilisation by cyanobacteria in the upper intertidal areas, to biostabilisation by diatoms on the bare substrata of the channel edges. In the late summer/early autumn, cyanobacteria were the main sediment stabilisers, and colloidal carbohydrate was the best bio-dependent predictor of t c across the entire field area. In the late winter/early spring, cyanobacterial activity was lower, and sediment stabilisation by Enteromorpha clathrata was important; the best predictor of t c was bed elevation. The implications and use of proxies for sediment stability are discussed in terms of feedback and sedimentation processes operating across the intertidal area.
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The global decline in estuarine and coastal ecosystems (ECEs) is affecting a number of critical benefits, or ecosystem services. We review the main ecological services across a variety of ECEs, including marshes, mangroves, nearshore coral reefs, seagrass beds, and sand beaches and dunes. Where possible, we indicate estimates of the key economic values arising from these services, and discuss how the natural variability of ECEs impacts their benefits, the synergistic relationships of ECEs across seascapes, and management implications. Although reliable valuation estimates are beginning to emerge for the key services of some ECEs, such as coral reefs, salt marshes, and mangroves, many of the important benefits of seagrass beds and sand dunes and beaches have not been assessed properly. Even for coral reefs, marshes, and mangroves, important ecological services have yet to be valued reliably, such as cross-ecosystem nutrient transfer (coral reefs), erosion control (marshes), and pollution control (mangroves). An important issue for valuing certain ECE services, such as coastal protection and habitat–fishery linkages, is that the ecological functions underlying these services vary spatially and temporally. Allowing for the connectivity between ECE habitats also may have important implications for assessing the ecological functions underlying key ecosystems services, such coastal protection, control of erosion, and habitat–fishery linkages. Finally, we conclude by suggesting an action plan for protecting and/or enhancing the immediate and longer-term values of ECE services. Because the connectivity of ECEs across land–sea gradients also influences the provision of certain ecosystem services, management of the entire seascape will be necessary to preserve such synergistic effects. Other key elements of an action plan include further ecological and economic collaborative research on valuing ECE services, improving institutional and legal frameworks for management, controlling and regulating destructive economic activities, and developing ecological restoration options.
There is world‐wide concern about the effects of bottom‐dredging on benthic communities in soft sediments. In autumn 1988, almost a third of the 50‐km ² intertidal system around the island of Griend in the western Dutch Wadden Sea was suction‐dredged for edible cockles Cerastoderma edule and this study assessed subsequent effects. An adjacent area not directly touched by this fishery and an area from which the mussel Mytilus edulis beds were removed, served as reference areas.
Sediment characteristics, together with the total stock size and settlement densities of Cerastoderma , Baltic tellin Macoma balthica and soft‐shelled clam Mya arenaria , were documented during 11 successive autumns before (August–September 1988) and after (August–September 1989–98) the suction‐dredging event in fished and unfished areas. Four other areas in the Dutch Wadden Sea, where changes in densities of juvenile bivalves from 1992 to 1998 were measured, served as additional reference locations.
Between 1988 and 1994, median sediment grain size increased while silt was lost from sediments near Griend that were dredged for cockles. The initial sediment characteristics were re‐attained by 1996.
After the removal of all Mytilus and most Cerastoderma , the abundance of Macoma declined for 8 years. From 1989 to 1998, stocks of Cerastoderma , Macoma and Mytilus did not recover to the 1988 levels, with the loss of Cerastoderma and Macoma being most pronounced in the area dredged for cockles. Declines of bivalve stocks were caused by particularly low rates of settlement in fished areas until 1996, i.e. 8 years after the dredging.
A comparison of settlement in the short (1992–94) and medium term (1996–98) after cockle‐dredging in several fished and unfished areas spread over the entire Dutch Wadden Sea, showed a significant negative effect of dredging on subsequent settlement of Cerastoderma . Macoma also declined, but not significantly.
We conclude that suction‐dredging of Cerastoderma had long‐lasting negative effects on recruitment of bivalves, particularly the target species, in sandy parts of the Wadden Sea basin. Initially, sediment reworking by suction‐dredging (especially during autumn storms) probably caused losses of fine silts. Negative feedback processes appeared to follow that prevented the accumulation of fine‐grained sediments conducive to bivalve settlement.
Shallow soft-sediment systems are mostly dominated by species that, by strongly affecting sediment dynamics, modify their
local environment. Such ecosystem engineering species can have either sediment-stabilizing or sediment-destabilizing effects
on tidal flats. They interplay with abiotic forcing conditions (wind, tide, nutrient inputs) in driving the community structure
and generating spatial heterogeneity, determining the composition of different communities of associated species, and thereby
affecting the channelling of energy through different compartments in the food web. This suggests that, depending on local
species composition, tidal flats may have conspicuously different geomorphology and biological functions under similar external
conditions. Here we use a historical reconstruction of benthic production in the Wadden Sea to construct a framework for the
relationships between human impacts, ecosystem engineering and sediment dynamics. We propose that increased sediment disturbances
by human exploitation interfere with biological controls of sediment dynamics, and thereby have shifted the dominant compartments
of both primary and secondary production in the Wadden Sea, transforming the intertidal from an internally regulated and spatially
heterogeneous, to an externally regulated and spatially homogenous system. This framework contributes to the general understanding
of the interaction between biological and environmental control of ecosystem functioning, and suggests a general framework
for predicting effects of human impacts on soft-bottom ecosystems.
Key wordsbivalves-deposit feeders-internal regulation-human transformation-soft sediment food-webs-historic trends
During the years 1923–1926 Hagmeier & Kndler (1927) sampled the macrofauna of subtidal shallows and channels of the Wadden Sea close to the Island of Sylt (German Bight, North Sea). Reinvestigating this study area in 1980, a substantially altered faunal composition was recorded. An approach is made to quantify the comparison in terms of abundance, species richness and diversity of invertebrate taxa. Human interference is assumed to be responsible for the major changes. Natural oyster beds have been overexploited and the local population ofOstrea edulis has been driven to extinction. Subsequently, mussels(Mytilus edulis) spread in the entire region, promoted by shell fishery. Particularly barnacles and many polychaetes took advantage of the expansion of mussel banks which is substantiated by correlation analysis. Reefs of the colonial polychaeteSabellaria spinulosa stood in the way of shrimp trawling and became destroyed together with the associated fauna. A subtidalZostera marina bed was wiped out in 1934 by a natural epidemic disease but never succeeded in reestablishing itself. The associated fauna disappeared. Large epibenthic predators and scavengers (crabs, snails and starfish) survived all these changes. The total number of species remained approximately at the same level but molluscs experienced losses and polychaetes diversified. Overall abundance increased with a disproportionately large share of a few species(Mytilus edulis, Balanus crenatus, Cerastoderma edule, Scoloplos armiger). The subtidal fauna of the Wadden Sea proved to be vulnerable to human disturbance; thus, the present community can no longer be viewed as the outcome of entirely natural processes.
Bioturbating lugworms (Arenicola marina) were excluded from 400m² plots of intertidal sand which initiated sequences of direct and indirect changes in the structure
of the benthic community. The sessile, tube-building species Polydora cornuta and Lanice conchilega took advantage of the absence of lugworms and settled preferentially on lugworm exclusion plots. The protruding tubes provided
attachment for an ephemeral development of algal tufts (Berkeleya colonies and Enteromorpha thalli) which in turn enhanced settlement of the juvenile drifting clams Mya arenaria and Macoma balthica. This causal chain of enhanced bivalve settlement in the presence of above-ground structures, like animal tubes and algae,
on lugworm exclusion plots occurred in 2years at different tidal zones with different tube builders, algae and juvenile clams.
A significant response of L. conchilega in a year with relatively low lugworm abundances at the entire site suggests that not only the actual absence of large bioturbators
was responsible for the establishment of tube-dwelling species, but also a cumulative change of the sediment in exclusion
plots since the onset of the experiment. While the sediment on lugworm plots remained permeable, fine particles and organic
matter accumulated at exclusion plots. It is suggested that these differences in sediment characteristics were the product
of divergent benthic engineering by sediment destabilizing lugworms on control plots and sediment stabilizing species on exclusion
plots. Cumulative changes of the sedimentary habitat and cascading effects in the benthic community may explain the persistence
of patches that are dominated by either sediment stabilizing or destabilizing species in the assemblage mosaic of intertidal
sediments.
Interactions between organisms are a major determinant of the distribution and abundance of species. Ecology textbooks (e.g., Ricklefs 1984, Krebs 1985, Begon et al. 1990) summarise these important interactions as intra- and interspecific competition for abiotic and biotic resources, predation, parasitism and mutualism. Conspicuously lacking from the list of key processes in most text books is the role that many organisms play in the creation, modification and maintenance of habitats. These activities do not involve direct trophic interactions between species, but they are nevertheless important and common. The ecological literature is rich in examples of habitat modification by organisms, some of which have been extensively studied (e.g. Thayer 1979, Naiman et al. 1988).
An important ecological issue is developing an understanding of how patterns and processes vary with scale. We designed a field experiment to test how differences in the aerial extent of disturbance affected macrofaunal recolonization on a sandflat. Three different plot sizes (0.203 m2, 0.81 m2, and 3.24 m2) were dafaunted, and samples were collected to assess recovery over a 9-mo period. As the sandflat used for the experiment was prone to disturbance by wind-driven waves, we also measured changes in sediment bed height (an indicator of sediment stability) over the course of the experiment. Most common species revealed significant relationships between density and disturbance plot size. Scale-dependent recovery was also demonstrated by differences in species assemblage structure over the course of the experiment. Relative rates of colonization varied by ≈50% between large and small experimental plots. However, these differences were not related to specific species, particular functional groups, or potential modes of colonizations. The results revealed an unusually slow rate of faunal recovery following defaunation. Both increasing numbers of colonists, and density changes in ambient sediments made an important contribution to recovery. The relationship found between changes in sediment bed height and wind velocity indicated that wind-driven wave disturbance was an important factor influencing sediment instability. Sediment instability was higher in all experimental plots than in the ambient sediments, due to the initial removal of a dense spionid polychaete tube mat characteristically found at the study site. Sediment instability also increased with increasing plot size. Thus in this dynamic sandflat habitat, faunal emigration from recovering disturbed patches of sediment may significantly slow rates of recolonization. These results demonstrate that incorporating patch size, emigration, recovery time, and interactions between hydrodynamic conditions and habitat stability (particularly where colonists influence sediment stability) are crucial to generating a general understanding of recovery processes in sof-sediment habitats. While our results demonstrate the need for caution in scaling-up from small-scale studies, they do indicate that larger scale disturbances that destroy organisms with a role in maintaining habitat stability are likely to result in very slow recovery dynamics, particularly in wave-disturbed soft-sediment habitats.
Accelerated soil erosion is ubiquitous on human-modified hillslopes. A variety of erosion control products have been developed to reduce on-site soil resource degradation, and off-site transport of sediment and sediment-associated contaminants to receiving water bodies. However, limited quantitative data are available to assess erosion reduction effectiveness, and to establish the salient properties of the erosion control products. A replicated field-based rainfall simulation study was conducted to compare the runoff and erosion effectiveness of three coir (coconut) fiber rolled erosion control systems (RECSs) with a bare (control) treatment. Detailed temporal measurements of runoff and sediment transport were made during two phases of each experiment: (1) a 110-min application of rainfall via a rainfall simulator at 35mmh−1 after runoff initiation and (2) a 30-min period, at 3 times the flow rate of phase 1, applied via an overland flow generator. All coir treatments enhanced infiltration, delayed time to runoff generation, reduced intensity of rill incision, and reduced sediment output compared to bare treatments. More importantly, statistically significant differences were observed between coir RECSs of different architecture. For the two open weave coir systems tested, the most effective design had a higher mass per area, and less open space between the regularly aligned grid of fibers. The random fiber coir architecture was the most effective, having significantly lower runoff sediment concentrations, lower sediment yields, and a lower frequency of rill initiation. The differences in system architecture are examined in light of fundamental controls on runoff and erosion processes.
Polychaete tube lawns with high population densities are frequent in marine soft-bottom environments. The influence of single tubes on near-bed flow dynamics has been quite well studied, but the critical population density that separates sediment destabilising effects from stabilising effects remains uncertain. This article presents results obtained with artificial tubes in a recirculating flume at a current velocity of 5 cm s(-1). Four population densities were tested for their passive effects on the flow dynamics: expressed as percentage of the total surface area covered by tubes, they were 1.1, 2.0, 4.5 and 8.8%. Using a high-resolution 3-dimensional current sensor, horizontal and vertical flow velocity profiles were recorded within the artificial tube lawns. An important deceleration of the current velocity was observed at all population densities, ranging from 38.2% at the lowest population density to 83.8% at the highest. This deceleration, the shape of the vertical profiles, the calculated Reynolds stress values and the direct observation of sediment displacement led to the conclusion that the flow field is modified to gradually raise the effective level of the bottom towards the tube tips, resulting in skimming flow conditions at 8.8% surface coverage. Compared with field conditions, this is still a relatively low population density and thus means that many natural tube lawns have sediment stabilising effects, conditioning the substratum for further benthic succession.
Natural processes tend to vary over time and space, as well as between species. The ecosystem services these natural processes provide are therefore also highly variable. It is often assumed that ecosystem services are provided linearly (unvaryingly, at a steady rate), but natural processes are characterized by thresholds and limiting functions. In this paper, we describe the variability observed in wave attenuation provided by marshes, mangroves, seagrasses, and coral reefs and therefore also in coastal protection. We calculate the economic consequences of assuming coastal protection to be linear. We suggest that, in order to refine ecosystem-based management practices, it is essential that natural variability and cumulative effects be considered in the valuation of ecosystem services.
This investigation deals with the changes in macrofaunal communities in intertidal sandflats caused by the input of organic material via biodeposition produced by mussel beds. The area of investigation was the back barrier tidal flat system behind the East Frisian Island of Spiekeroog. Due to erosion by the ebb current a plume of biodeposits extended from an area of mussel beds (Mytilus edulis) towards the adjacent sandy sediments. This plume represented a gradient of decreasing contents of organic material. Five stations were installed along this gradient to analyse the effects of this organic input on the macrofaunal communities; a control station was placed on sandy sediments.
The macrofaunal communities differed along the transect. In the mussel patches oligochaetes dominated, whereas deposit-feeding polychaetes increased along the transect. These polychaetes were different from the communities of the sand flats. The changes in the macrofaunal communities discussed in the framework of the organic carbon contents of sediments and interspecific relationships.
After a cover of green algae during summer and after a cold winter, the mussels were almost completely eliminated from the area of investigation and reaggregated in another area of the flat. The layer of biodeposits eroded as well. After the decrease in contents of organic matter, the macrofaunal communities no longer followed the transitional distribution but showed a diffuse and seasonally influenced distribution.
The results indicate that biodeposits changed the quality of the sediment structure of the sand flats and that the amount of organic matter (TOC) is a main structuring factor for the macrofaunal communities.
A portable benthic annular flume for thein situmeasurement of suspension feeding (biodeposition rates) and erosion potential of undisturbed intertidal cohesive sediments with benthic biota is described. The experimental procedures used during deployment are also outlined. Results showing the changes in biodeposition rates, sediment resuspension, erosion rates and critical erosion velocities associated with large-scale (km) and small-scale (m) spatial differences are presented. Comparison between laboratory andin situmeasurements demonstrated marked differences in sediment erodability if the physical and biological properties of the sediment are not maintained.
Nonparametric multivariate analysis of ecological data using permutation tests has two main challenges: (1) to partition the variability in the data according to a complex design or model, as is often required in ecological experiments, and (2) to base the analysis on a multivariate distance measure (such as the semimetric Bray-Curtis measure) that is reasonable for ecological data sets. Previous nonparametric methods have succeeded in one or other of these areas, but not in both. A recent contribution to Ecological Monographs by Legendre and Anderson, called distance-based redundancy analysis (db-RDA), does achieve both. It does this by calculating principal coordinates and subsequently correcting for negative eigenvalues, if they are present, by adding a constant to squared distances. We show here that such a correction is not necessary. Partitioning can be achieved directly from the distance matrix itself, with no corrections and no eigenanalysis, even if the distance measure used is semimetric. An ecological example is given to show the differences in these statistical methods. Empirical simulations, based on parameters estimated from real ecological species abundance data, showed that db-RDA done on multifactorial designs (using the correction) does not have type 1 error consistent with the significance level chosen for the analysis (i.e., does not provide an exact test), whereas the direct method described and advocated here does.
To achieve better conservation and management of the NW-European Wadden Sea, about one hundred scientists from almost all institutes in and around the area have compiled the ecological literature and unpublished data in a series of handbooks and habitat maps.The procedure used for compiling the habitat maps of the Wadden Sea is presented. The overall 1 : 100 000 outline map shows the habitats of the littoral and supralittoral parts of the Wadden Sea, a shallow sedimentary coastal sea 450 km long and bordered by the southeastern North Sea. The map is a potential-biological map for benthic organisms, based on an integration of the environmental factors emersion period and sediment composition. Conspicuous communities of organisms have been added (mussel beds, seagrass beds, salt marshes).The map shows the overall geographical distribution of morphological forms and habitats. Based on this large-scale variation, the Wadden Sea has been subdivided into eight regions, which differ in respect of the importance of the barrier system, the shore of emerging flats, exposure to wave action, sediment composition, the distribution of seagrasses and salt marshes and the occurrence of islands within the area.
Effects of dredging on macrobenthic infauna were studied at several sites and water depths in the estuarine Dutch Wadden Sea. Dredging in tidal flat areas was found to have long-lasting effects: filling-in rates in such areas were extremey slow, sediment composition altered dramatically and recovery of benthos was virtually absent during the long period of filling-in. Such pits on tidal flats persisted for more than 15 years. In subtidal areas recovery of bottom structure as well as the inhabiting infaunal benthos proceeded rapidly, while the original situation may be expected to return. In tidal channels with strong currentts both filling in and recolonization took only about 1 to 3 years. In some subtidal areas with low tidal stream velocities, such as tidal watersheds, recovery proceeded less rapidly: it lasted 5 to 10 years.
Increasing evidence shows that spatial interactions between sedentary organisms can structure communities and promote landscape complexity in many ecosystems. Here we tested the hypothesis that reef-forming mussels (Mytilus edulis L.), a dominant intertidal ecosystem engineer in the Wadden Sea, promote abundances of the burrowing bivalve Cerastoderma edule L. (cockle) in neighboring habitats at relatively long distances coastward from mussel beds. Field surveys within and around three mussel beds showed a peak in cockle densities at 50-100 m toward the coast from the mussel bed, while cockle abundances elsewhere in the study area were very low. Field transplantation of cockles showed higher survival of young cockles (2-3 years old) and increased spat fall coastward of the mussel bed compared to within the bed and to areas without mussels, whereas growth decreased within and coastward of the mussel bed. Our measurements suggest that the observed spatial patterns in cockle numbers resulted from (1) inhibition effects by the mussels close to the beds due to preemptive algal depletion and deteriorated sediment conditions and (2) facilitation effects by the mussels farther away from the beds due to reduction of wave energy. Our results imply that these spatial, scale-dependent interactions between reef-forming ecosystem engineers and surrounding communities of sedentary benthic organisms can be an important determinant of the large-scale community structure in intertidal ecosystems. Understanding this interplay between neighboring communities of sedentary species is therefore essential for effective conservation and restoration of soft-bottom intertidal communities.
BERLIN--
Edelgard Bulmahn has been a major force in German science and higher education since becoming research minister in 1998. She
has proposed an overhaul of Germany's university rules--seeking merit pay and "junior professorships" that would free young
scientists to pursue independent research--that has polarized the academic community. In a 9 April interview with
Science
in her Berlin office, Bulmahn discussed these and other topics in laying out her vision for German research.
Monocultures of mussels might alter the infaunal benthic community of adjacent and interstitial sediments through provision of a complex habitat, input of organically rich material and larval removal through filter feeding. At a site of commercial seabed mussel cultivation, we aimed to determine the effect of mussels on the infaunal community of an intertidal mudflat at different spatial scales and under different stocking strategies.
Mussels were laid at four different densities (2, 3, 5 and 7·5 kg m ⁻² ) on 400‐m ² plots in a 4 × 4 Latin square. Benthic samples were collected within and 10–100 m distant from the cultivation area c . 7 months prior to and 18 months after seeding the plots with blue mussels. Benthic community characteristics were related to initial seeding density and to the actual surface area of mussels associated with each set of samples collected within replicate plots.
The presence of mussels significantly changed the occurrence of some species of the infaunal community within the cultivated area. The infaunal communities supported fewer individuals and species than control treatments at all but the lowest mussel cover.
Species richness and the abundance of individuals per unit area also declined with increased area of mussel cover. The abundance of cirratulids and amphipods declined strongly with increasing mussel surface area.
Although the species composition and abundance of individual invertebrate species were altered by the presence of mussels, the distribution of individuals among species remained relatively unchanged.
Synthesis and applications . Overall, mussel beds changed the infaunal community, but the effects were localized (0–10 m) and not detectable at larger scales (10–100 m). Changes in benthic community composition could be reduced (but not eliminated) by lowering the stocking density of mussels to either 2 or 3 kg m ⁻² . Given the small edge effects associated with cultivated mussel beds, the use of larger mussel beds would be preferable to many smaller mussel beds.
1. Shellfish of marketable size can be harvested much more quickly and efficiently using mechanical methods such as tractor-powered harvesters and suction dredgers than by traditional methods. The adverse effects of such machines on non-target organisms need to be considered carefully before licensing such activities.
2. A tractor-towed cockle harvester was used to extract cockles from intertidal plots of muddy sand and clean sand in order to investigate the effects on other benthic invertebrates and their predators.
3. Harvesting resulted in the loss of a significant proportion of the most common invertebrates from both areas, ranging in the muddy sand from 31% of Scoloplos armiger (Polychaeta) (initial density 120 m−2) to 83% of Pygospio elegans (Polychaeta) (initial density 1850 m−2). Significant effects could not be detected in most populations with a density of less than 100 m−2.
4. Populations of Pygospio elegans and Hydrobia ulvae (Gastropoda) remained significantly depleted in the area of muddy sand for more than 100 days after harvesting, and Nephtys hombergi (Polychaeta), Scoloplos armiger and Bathyporeia pilosa (Amphipoda) for more than 50 days.
5. Invertebrate populations in clean sand with relatively few cockles Cerastoderma edule (Pelecypoda) recovered more quickly than those in muddy sand with a more structured community, which included several tube-dwelling species such as Pygospio elegans and Lanice conchilega (Polychaeta).
6. Bird feeding activity increased at first on the harvested areas, with gulls and waders taking advantage of invertebrates made available by harvesting. Subsequently, in the area of muddy sand, the level of bird activity declined compared with control areas. It remained significantly reduced in curlews Numenius arquata and gulls for more than 80 days after harvesting and in oystercatchers Haematopus ostralegus for more than 50 days.
7. It is concluded from this study that tractor dredging for cockles in high density areas causes a sufficiently large mortality of non-target invertebrates that harvesters should be excluded from areas of conservation importance for intertidal communities such as invertebrates, fish and birds.
Hypothesis-testing methods for multivariate data are needed to make rigorous probability statements about the effects of factors and their interactions in experiments. Analysis of variance is particularly powerful for the analysis of univariate data. The traditional multivariate analogues, however, are too stringent in their assumptions for most ecological multivariate data sets. Non-parametric methods, based on permutation tests, are preferable. This paper describes a new non-parametric method for multivariate analysis of variance, after McArdle and Anderson (in press). It is given here, with several applications in ecology, to provide an alternative and perhaps more intuitive formulation for ANOVA (based on sums of squared distances) to complement the description provided by McArdle and Anderson (in press) for the analysis of any linear model. It is an improvement on previous non-parametric methods because it allows a direct additive partitioning of variation for complex models. It does this while maintaining the flexibility and lack of formal assumptions of other non-parametric methods. The test-statistic is a multivariate analogue to Fisher’s F-ratio and is calculated directly from any symmetric distance or dissimilarity matrix. P-values are then obtained using permutations. Some examples of the method are given for tests involving several factors, including factorial and hierarchical (nested) designs and tests of interactions.