No full-text available
To read the full-text of this research,
you can request a copy directly from the authors.
Patterns in Body Size, Population Dynamics, and Regional Distribution of Bracken Herbivores
Abstract
A number of previously reported qualitative patterns, relating local population dynamics (average population sizes and fluctuations in population size of component species in 2 adjacent habitats over 7 yr), body size of each species, regional distribution (at 216 sites throughout the British Isles), and feeding specificity are analyzed using Pteridium aquilinum insect data. Several predicted patterns are confirmed, despite the relatively small number of species considered (a maximum of 21). Species of small body size are more widely distributed nationally, have higher average population densities, and have populations that fluctuate more than those of large species. Hence, locally abundant populations and widely fluctuating populations are also more widespread nationally. Small species also tend to be host-plant specialists, whereas larger species are generalists, and small specialist species have local populations that are larger and more variable. -from Authors
... When considering relationships between body size and biological variables of interest, emphasis is often placed on consistent patterns among taxa and/or the predictive power of quantitative relationships (Gaston & Lawton 1988, Loder 1997. If similar relationships are detected in a wide range of species in a wide range of habitats they may be of general importance (Lawton 1999). ...
... Body size is central to ecological theory relating to both large-scale (biogeographic) and local-scale patterns of diversity, distribution and abundance. Greater species richness and greater maximum abundances in species with small body size are thought to be general characteristics of all organisms (Gaston & Lawton 1988. There is some evidence that these patterns also apply to coral reef fishes . ...
... Species richness and body size also appear to be intrinsically related, with greatest species richness found in small to intermediate size classes (Griffiths 1986, Morse 1988, Barlow 1994, Brown 1995, Siemann et al. 1996. It has also been suggested that habitat versatility within species is governed by body size and smaller species tend to exhibit greater habitat specificity than larger species (Wasserman & Mitter 1978, Gaston & Lawton 1988, Ziv 2000. However, the relationships among body size, abundance, species richness and versatility may be complex, as abundancediversity (Morse 1988, Siemann et al. 1996, abundance-habitat specificity (Griffiths 1986) and diversity-habitat specificity (Marzluff & Dial 1991, Ziv 2000 are intrinsically related to one another (Harvey & Lawton 1986). ...
... Practitioners in the emerging field of macroecology seek to understand the partitioning of physical space and ecological resources by species (Brown and Maurer 1989). In macroecological analyses, individual species function as replicates in searches for correlated patterns of geographical range size, body size, population density, trophic status, and intrinsic rate of increase (Damuth 1981;Brown and Maurer 1987;Gaston andLawton 1988a, 1988b;Brown and Maurer 1989;Lawton 1990). Analyses are typically carried out on very broad taxonomic and ecological groups, such as on the breeding birds of North America (Brown and Maurer 1987) or the herbivorous insects that feed on bracken (Gaston and Lawton 1988b). ...
... In macroecological analyses, individual species function as replicates in searches for correlated patterns of geographical range size, body size, population density, trophic status, and intrinsic rate of increase (Damuth 1981;Brown and Maurer 1987;Gaston andLawton 1988a, 1988b;Brown and Maurer 1989;Lawton 1990). Analyses are typically carried out on very broad taxonomic and ecological groups, such as on the breeding birds of North America (Brown and Maurer 1987) or the herbivorous insects that feed on bracken (Gaston and Lawton 1988b). ...
... A theoretical argument has also been advanced for a negative correlation between body size and geographical range size (Gaston 1988; Gaston andLawton 1988a, 1988b). The argument is based on the observation that the intrinsic rate of increase, r, decreases as a function of increasing body size - (Fenchel 1974;Southwood 1981;Gaston 1988). ...
Macroecologists interpret correlates of body size and geographical range size in an ecological context, but these patterns may also reflect historical or phylogenetic forces. We examined the relationship between range size and body size for a monophyletic group of 27 North American minnow species. Body size and range size were positively correlated, but both variables were also correlated with latitude. After controlling for effects of latitude, body size and range size were no longer correlated. The basal dichotomy of the cladogram defined an eastern and western clade; they differed in their geological and climatic histories and macroecological patterns. Within the western clade, only Bergmann's rule was confirmed. Within the eastern clade, both longitude and latitude of geographical range were positively correlated with body size. A simple measure of phylogeny was correlated with range size: species branching near the cladogram root had larger geographical ranges than species branching distally. After statistically removing the effects of latitude, longitude, and phylogeny, there was a significant positive correlation between body size and range size. Macroecological patterns are sensitive to phylogeny and speciation history, and they may be most informative for clades that occupy areas with a common climatic history.
... For many decades the relationship between body mass and abundance of animals has been a major subject of study (Elton, 1932(Elton, , 1933. There is a general trend for abundance to decrease with increasing body mass, among different taxa, such as insects and birds (Juanes, 1986;Gaston and Lawton, 1988;Blackburn et al., 1990;Nee et al., 1991;Santini et al., 2017). Studies on mammals from different regions of the Neotropics has also showed that density of tropical mammals decreases with increasing biomass, biomass increases with increasing body weight, and biomass is positively related to home range size (Milton and May, 1976;Clutton-Brock and Harvey, 1977;Eisenberg, 1980;Damuth, 1981;Peters and Wassenberg, 1983;Peters and Raelson, 1984;Robinson and Redford, 1989;Eisenberg, 1990;Santini el al., 2017). ...
... This also agrees with Lawton's (1989) hypothesis that, through pre-emptive competition (Schoener, 1983), larger species have access to a greater share of the resources, to the detriment of the smaller. Furthermore, generalists should also occur at higher densities than specialists, for they have access to more food items (Brown, 1984;Gaston and Lawton, 1988). ...
Assemblages of medium and large–sized mammals were studied in the Guyana shield of the Brazilian Amazonia. Diurnal and nocturnal line–transect samplings were carried out via the line–transect method in five different forest types along a 10–km transect, along which we also recorded habitat variables, such as tree species diversity, reproductive phenology, and residual fruit productivity. Group density was separately calculated for all mammalian species in the five forest types. Stepwise multiple regression analyses were performed to determine which habitat variables best predicted the mammalian species densities in the sampled forests. The sole determinants of mammalian densities in the forest types studied were basal area of each forest type, total number of tree species in each forest type, and tree reproductive phenology.
... Although little is known about large-scale patterns of spatial variation in abundance, recent studies suggest that. regional patterns of distribution and abundance are also influenced by the dynamics of local populations (Hanski 1982a(Hanski ,b, 1989Brown 1984, Gotelli and Simberloff 1987,' Gaston and Lawton 1988b. ...
... Although much is known about the factors that affect the distribution and abundance of organisms at a local scale, relatively little is known about largescale patterns of spatial variation in the abundance of species and about the particular ecological processes that affect rates of birth, survival and dispersal of local populations over the entire range of a species. In the last few years some ecologists have advocated the documentation of regional patterns of ecological structure as a fruitful approach to understanding the complexity of ecological communities (Brown 1981;Maurer 1987, 1989;Ricklefs 1987;Gaston and Lawton 1988b). One such pattern is the relationship 89 between regional distribution and average local abundance. ...
... Caracterizações mais finas incluem estimativas de abundância populacional e suas flutuações e a distribuição dos tamanhos de corpo dos membros da comunidade (Lawton 1991). Gaston & Lawton (1988) sugeriram que o tamanho do corpo é um componente crucial do padrão da comunidade e que há uma ligação forte entre esse componente e abundância, flutuação e distribuição regional das populações. A relação entre tamanho de corpo e abundância das espécies constitui-se objeto de vários estudos (Janzen 1973a, Gaston & Lawton 1988, Lawton 1989. ...
... Gaston & Lawton (1988) sugeriram que o tamanho do corpo é um componente crucial do padrão da comunidade e que há uma ligação forte entre esse componente e abundância, flutuação e distribuição regional das populações. A relação entre tamanho de corpo e abundância das espécies constitui-se objeto de vários estudos (Janzen 1973a, Gaston & Lawton 1988, Lawton 1989. ...
A diversidade de Coleoptera foi estudada em três fisionomias de Cerrado lato sensu em Brasília-DF: campo de murundum (1), campo sujo (2) e cerrado sensu stricto (3). Em cada área, os insetos foram coletados por varredura em 2000 m em cada tipo de vegetação. Foram coletados 1.044 espécimes de 155 espécies, distribuídos em 15 famílias. A abundância variou de um a 441 espécime/espécie, com a maioria das espécies representada por um único espécime. Obteve-se uma baixa similaridade faunística entre as áreas, com alta percentagem de espécies restritas a uma única área. O índice de diversidade (Shannon - H'=1,87; H'=4,16; H'=3,17) obtido para as áreas 1, 2 e 3, respectivamente, sugere que as espécies não estão homogeneamente distribuídas. Os valores obtidos para tamanho de corpo (1,0 a 11,0 mm) evidenciam que a maioria das espécies e espécimes foram menores que 5 mm. _______________________________________________________________________________ ABSTRACT Coleopteran diversity was studied in three physionomies of Cerrado lato sensu in Brasília-DF: campo de murundum (1), campo sujo (2) and cerrado sensu stricto (3). In each area, insects were collected by sweep sampling on 2,000 m in each vegetation type. A total of 1,044 specimens of 155 species, distributed in 15 families were collected. The abundance varied from 1 to 441 specimens/species, with most species represented by a single specimen. We found a low faunistic similarity between areas, with high percentage of species restricted to only one area. The diversity index (Shannon - H'=1,87; H'=4,16; H'=3,17) obtained for the areas 1, 2 and 3, respectively, suggest that the species are not homogeneously distributed. The values obtained for body size (1,0 to 11,0 mm) showed most species and specimens were smaller than 5 mm.
... An inverse relationship between body size and abundance is expected as a function of energetic constraints [13] in both terrestrial [14] and aquatic [15] assemblages/ecosystems. Furthermore, macroecological studies have demonstrated a relationship between body size and geographic range [16,17]. The expectation is that larger sized individuals are more capable of long range movements and thus, exhibit increased range sizes. ...
... However, the utility of body size and geographic range as model predictors to describe long term population dynamics is understudied. Conceptually, small bodied species are expected to exhibit greater population variation as a result of higher intrinsic rates of increase r [17]. Similarly, species with larger geographic ranges are expected to be generalists for environmental niches [18] and more likely to exhibit stable populations. ...
We combine evolutionary biology and community ecology to test whether two species traits, body size and geographic range, explain long term variation in local scale freshwater stream fish assemblages. Body size and geographic range are expected to influence several aspects of fish ecology, via relationships with niche breadth, dispersal, and abundance. These traits are expected to scale inversely with niche breadth or current abundance, and to scale directly with dispersal potential. However, their utility to explain long term temporal patterns in local scale abundance is not known. Comparative methods employing an existing molecular phylogeny were used to incorporate evolutionary relatedness in a test for covariation of body size and geographic range with long term (1983 - 2010) local scale population variation of fishes in West Fork White River (Indiana, USA). The Bayesian model incorporating phylogenetic uncertainty and correlated predictors indicated that neither body size nor geographic range explained significant variation in population fluctuations over a 28 year period. Phylogenetic signal data indicated that body size and geographic range were less similar among taxa than expected if trait evolution followed a purely random walk. We interpret this as evidence that local scale population variation may be influenced less by species-level traits such as body size or geographic range, and instead may be influenced more strongly by a taxon's local scale habitat and biotic assemblages.
... However, body size of herbivores has not been studied in the context of land use change, thus it is unknown whether landscape composition affects them, and subsequently whether this affects crop damage. Gaston & Lawton [14] found that body size of small insect herbivores can vary more widely than large herbivore species in the same ecosystem, and that their populations are more variable. Additionally, smaller insect herbivores may be less able to move across landscapes with fewer resources [15]. ...
Land use change affects both pollinator and herbivore populations with consequences for crop production. Recent evidence also shows that land use change affects insect traits, with intraspecific body size of pollinators changing across landscape gradients. However, the consequences on crop production of trait changes in different plant interactors have not been well-studied. We hypothesized that changes in body size of key species can be enough to affect crop productivity, and therefore looked at how the field-realistic variation in body size of both an important pollinator, Bombus impatiens (Cresson), and a key pest herbivore, Lygus lineolaris (Palisot), can affect fruit size and damage in strawberry. First, we determined if pests vary in body size along land use gradients as prior studies have documented for pollinators; and second, we tested under controlled conditions how the individual and combined changes in size of an important pollinator and a key herbivore pest affect strawberry fruit production. The key herbivore pest was smaller in landscapes with more natural and semi-natural habitat, confirming that herbivore functional traits can vary along a land use gradient. Additionally, herbivore size, and not pollinator size, marginally affected fruit production—with plants exposed to larger pests producing smaller fruits. Our findings suggest that land use changes at the landscape level affect crop production not just through changes in the species diversity of insect communities that interact with the plant, but also through changes in body size traits.
... Moreover, these ephemeral energy sources favour species with smaller bodies (Schoener and Janzen, 1968). A habitat with a uctuating energy source is more easily exploited by widespread, dispersive individuals capable of withstanding large variations in population size, and such species are more likely to be characterised by small body size (Gaston and Lawton, 1988). ...
Wooded pastures combine trees and pastures in an integrated land resulting from traditional silvopastoral practices. With their sparse tree cover, wooded pastures are expected to represent an ecotone between open area pastures and forests with potentially high species diversity, although this remains to be tested for animal groups including ground beetles. In this study, we aimed to characterise and compare species communities, diversity indices, biomass and ecological traits of ground beetles in wooded pastures, forests and non-wooded pastures. Pitfall traps were set up in 29 study sites located in the Swiss Jura mountains. Ground beetle communities in wooded pastures largely encompass those in open pastures and forests, although some species are found only in forests or open areas. Wooded pastures and open pastures have an equivalent species diversity level, which is significantly higher than the one in forests. Ground beetle diversity is positively correlated with the existence and surface of Biodiversity Promotion Areas. Areas with high tree cover (70–100%) favour brachypterous and hygrophilic species, whereas areas with reduced tree cover (0–20%) favour xerophilic and winged species. Ground beetles' size and biomass increase with tree cover. Wooded pastures are an important ecotone, ensuring a gradual change of land use systems between open areas and forests, where a wide range of species from both land use systems are found. This semi-natural habitat plays an important function in the conservation of ground beetles.
... Ammunét et al. (2010) showed that density effects differed between two geometrid moths; larvae of a larger moth, Epirrita autumnata, and a smaller moth, Operophtera brumata, were negatively affected by both con-and heterospecific larval densities, and the former experienced greater negative impacts on pupal mass than the latter. Because larger herbivores generally require more food for development than smaller herbivores (Brown & Maurer, 1986;Gaston & Lawton, 1988), E. autumnata may have experienced greater negative density effects than O. brumata because of stronger food limitation in E. autumnata given its greater food demand, which is consistent with the results of our study. ...
Recent studies on insect interactions on plants have revealed that herbivorous insects indirectly interact with each other through changes in plant traits following herbivory. However, less attention has been given to plant biomass relative to plant quality in relation to indirect interactions among herbivores. We explored the extent to which the larval food demand of two specialist butterflies (Sericinus montela and Atrophaneura alcinous) explains their interaction on a host plant, Aristolochia debilis. A laboratory experiment showed that plant mass consumption by A. alcinous larvae was 2.6 times greater than that by S. montela. We predicted that A. alcinous, which requires more food, is more vulnerable to food shortages than S. montela. In a cage experiment, an asymmetric interspecific interaction was detected between the two specialist butterflies; S. montela larval density significantly decreased the survival and prolonged the development time of A. alcinous, but A. alcinous density affected neither the survival nor the development time of S. montela. The prediction based on the food requirement was partly supported by the fact that increasing A. alcinous density likely caused a food shortage, which more negatively affected A. alcinous survival than S. montela survival. Conversely, increasing the density of S. montela did not reduce the remaining food quantity, suggesting that the negative effect of S. montela density on A. alcinous was unlikely to be due to food shortage. Although aristolochic acid I, a defensive chemical specific to Aristolochia plants, did not influence the food consumption or growth of either butterfly larva, unmeasured attributes of plant quality may have mediated an indirect interaction between the two butterflies. Consequently, our study suggests that not only the quality but also the quantity of plants should be considered to fully understand the characteristics, such as symmetry, of interspecific interactions among herbivorous insects on the same host plant.
... Nahrung and Swain (2015) found that insects that had become successful invaders were generally those with smaller body size, more generations per year, lower incidence of diapause, longer flight season, and with close host association. Body size is also hypothesized to be mechanistically related to the increased ability to exploit resources, higher rates of intrinsic growth, quick establishment (Gaston and Lawton, 1988;Williamson and Fitter, 1996;Forys and FIGURE 3 Relationships between niche similarity and breadth ratio for all species. Breadth ratio is the log e transformed ratio of the breadth of native niche to that of the invaded niche. ...
Coleoptera are key elements of terrestrial trophic interactions and generate significant economic and ecological benefits, but their representatives also represent severe pest species. Understanding how invasive species operate is indispensable to identify and anticipate potential invasion areas. However, few studies have explored niche dynamics and drivers of invasions in this group. Here we examined niche dynamics across 54 invasive beetle species native to Europe and assessed whether factors such as human influence index, feeding habits, body size, and niche breadth are associated with the degree of invasion. The realized niches had low similarity in invasive and native ranges (i.e., invaded areas are climatically dissimilar to native ranges). This included a high degree of niche expansion in invaded areas but also environments occupied in the native ranges but unoccupied in the invasive range (unfilling), suggesting that altered species–climate relationships during invasion processes are common. Niche expansions showed positive association with small native niche breadth sizes and movements from highly disturbed native areas to less disturbed invaded ranges; unfilling was associated with invaded niche breadth size and frequency of species occurrence. Both were related to dissimilar realized climatic niches in invaded ranges. Colonization of invaded areas might be triggered by low quality resources in native areas. Unfilling levels might be related to the year of introduction and loss of biotic constraints present in their native distribution, leading to the use of different climatic spaces in the invasive areas. This idea is reinforced by larger invasive climatic niche breadth. Our results provide insight into patterns of invasive species, and initial holistic exploration towards the understanding of invasive species dynamics.
... In contrast to fast organisms, slow organisms, with longer lives, buffer noise and thus maintain high stability. Consistent with this, empirical research has found negative correlations between population variability (one measure of stability) and both body size and generation time, and positive correlations between population variability and growth rates across multiple taxa and kingdoms [13][14][15][16]. Taken altogether, larger and slower organisms appear capable of buffering noise better than small and fast organisms. ...
Nature is replete with variation in the body sizes, reproductive output and generation times of species that produce life-history responses known to vary from small and fast to large and slow. Although researchers recognize that life-history speed likely dictates fundamental processes in consumer-resource interactions like productivity and stability, theoretical work remains incomplete in this critical area. Here, we examine the role of life-history speed on consumer-resource interactions by using a well-used mathematical approach that manipulates the speed of the consumer's growth rate in a consumer-resource interaction. Importantly, this approach holds the isocline geometry intact, allowing us to assess the impacts of altered life-history speed on stability (coefficient of variation, CV) without changing the underlying qualitative dynamics. Although slowing life history can be initially stabilizing, we find that in stochastic settings slowing ultimately drives highly destabilizing population disappearances, especially under reddened noise. Our results suggest that human-driven reddening of noise may decrease species stability because the autocorrelation of red noise enlarges the period and magnitude of perturbations, overwhelming a species' natural compensatory responses via a ratchet-like effect. This ratchet-like effect then pushes species' population dynamics far away from equilibria, which can lead to precipitous local extinction.
... Generalist insect herbivores tend to be larger than specialists, and larger herbivores tend to have a wider geographical range (e.g. Gaston and Lawton, 1988). Thus, a wide geographical range of many species of insect herbivore would tend to support hypothesis 2. ...
This book is a comprehensive and detailed synthesis of the natural and human history of forests in the Guiana Shield, which is an ancient geological region located in northeastern South America (constitutes all or most of Brazil, Colombia, French Guiana, Guyana, Suriname and Venezuela), and the forces that have combined to shape their unique place in the modern tropical world. Chapters cover geology, climate, hydrology, soils, nutrient cycling, plant-animal interactions, archaeology, colonization and land use history, plant distributions and life history attributes, forest dynamics and conservation and management of flora and fauna.
... Moreover, our results highlight the importance of the species body size and reproductiongrowth trade-off in controlling population sizes and show that the relationship between population stability and fecundity is not direct, as it is mediated by SIBSM. Body size is a trait frequently correlated to longevity and slow growth (Gaston & Lawton, 1988;Morris et al., 2008;Owen & Gilbert, 1989) and our result corroborates with this expectation. However, our results also indicate that not only larger body sized but high investment in SIBSM over the gradient of species size may confer higher resistance to interannual environmental oscillations (species with such traits tended to have lower CV), and that it moderates the relationship between population stability and fecundity. ...
• Understanding the factors that regulate temporal changes in population size is a core aspiration in ecology given the importance of population stability on the maintenance of species interactions, effects on local communities, the stability of ecosystems, and for biodiversity conservation. Understanding temporal trends in population size can support management practices as this may indicate demographic resilience for exploited species. Theoretical studies have long suggested that life‐history traits regulate population stability, but empirical support remains limited, especially for species‐rich environments. Additionally, harvesting has been suggested as an important factor increasing the fluctuation in the number of individuals in populations.
• In this study, we analysed population stability of 70 Amazonian floodplain fish species in relation to life‐history traits and the degree of fishing pressure. Our data covered a long time scale and broad geographical range of the Amazon floodplain. For that, we compiled datasets of two monitoring programmes, one comprising data from a single lake for 15 years and a second dataset with information from three floodplain lakes sampled over 5 years. The resulting geographical range spanned one of the most fished areas in the upper Amazon River, between the municipalities of Coari and Manaus, in the Brazilian Amazon. Temporal stability was measured as the coefficient of variation in species abundance. Population life‐history traits and the degree of fishing pressure were estimated at the species level.
• Population temporal stability had significant relationships with three life‐history traits: maximum body size, fecundity, somatic investment before sexual maturation (SIBSM), and the interaction of fecundity and SIBSM. Species with small body size, high fecundity, and low SIBSM displayed low stability; the opposite happened to species that invest highly in somatic tissue before the first reproduction and have large body size. Fishing pressure had no significant contribution to explaining population stability. However, the sampling technique employed and the set of species considered in the study do not represent main targets of fisheries.
• Here we stress the importance of life‐history traits in controlling an essential part of the population size variation in a complex and species‐rich fish assemblage in the Amazon floodplains. Our results highlight the importance of the trade‐off between growth and reproduction in controlling population stability and complement explanations on how life‐history functional traits underlie differences in population dynamics over time. Our results contribute to theoretical development and can be used to support fisheries and biological conservation management strategies. Specifically, our results point to the possibility of inferring demographic resilience based on life‐history information in the absence of high‐quality population data.
... This promoted the description of early spatial patterns of geographic ranges such as the one showing that smallranged species are much more common than large-ranged ones and that species are not evenly spread through all the potential range (Rapoport 1975, Anderson 1977. Then, these geographic range patterns were rapidly associated with diverse hypotheses: habitat availability (Cody et al. 1975, Anderson andKoopman 1981), density dependent factors (Rosenzweig 1978), abundance (Bock andRicklefs 1983, Brown 1984), life history (Reaka 1980), and body size (Reaka 1980, Gaston andLawton 1988). ...
Understanding the geographic distribution of species across space and time is one of the long-standing challenges in ecology and evolution. Among the major components of species distribution, the species' geographic range size has been studied across several taxonomic groups and has been related to multiple ecological and evolutionary factors. The geographic range size of species is also of paramount importance in conservation strategies because it consistently emerges as a key correlate of extinction risk, where species occupying smaller geographic ranges are assumed to have a higher risk of extinction. Results concerning these fundamental and applied aspects of geographic range size have largely neglected freshwater fish, commonly focusing on the usual vertebrate groups (e.g. mammals, birds). However, freshwater fish, the most diverse vertebrate group, can provide novel insights about the geographic range size determinants and threats because of the unique dendritic shape and reduced amount of their habitat (i.e. river networks) compared to other terrestrial and marine ecosystems. In this PhD work, we analyzed for the first time the global patterns of geographic range size in freshwater fish species and tested previous hypotheses proposed to explain the variation of geographic range size in other taxonomic groups. Our findings showed that current and historical connectivity are the most important factors driving the geographic range size of freshwater fishes, contrasting with the main determinants reported for terrestrial and marine taxa. From an applied point of view, we focused on the usually observed macroecological relationship between the species' geographic range size and body size. This relationship would allow estimating the minimum geographic range size needed by species for long-term persistence. Based on ecological theory of species temporal fluctuations of abundances, we provide a mechanistic validation of this relationship, supporting its use to identify vulnerable species and their changes in extinction risk through reduced geographic ranges induced by anthropogenic factors. Using a tropical river basin as a case study, we used this macroecological relationship to quantify changes in species extinction risk due to the fragmentation of their ranges caused by hydropower development. The results and the data compiled in this thesis represent useful information to guide and inform conservation in freshwater fish and give the opportunity to continue filling theoretical gaps.
... In island ecosystems with finite resources and a lack of interspecific competition, large body size is no longer advantageous. Furthermore, as body size is negatively correlated with population density, higher population density of smaller individuals decreases extinction risk (Brown and Maurer, 1986;Gaston and Lawton, 1988;Damuth, 1993). The phenotypes of large herbivores on islands are shaped by intraspecific competition to efficiently extract available resources. ...
Island dwarfing is a paraphyletic adaptation across numerous mammalian genera. From mammoths to foxes, extreme body size reduction is shared by diverse organisms that migrate to an island environment. Because it largely occurs owing to ecological variables, not phylogenetic ones, skeletal characters in a dwarfed taxon compared with its ancestor may appear abnormal. As a result, allometric patterns between body size and morphological traits may differ for an island dwarf compared with its ancestor. The diminutive Late Pleistocene hominin, Homo floresiensis, displays a unique character suite that is outside of the normal range of variation for any extinct or extant hominin species. To better explain these as ecological traits due to island dwarfing, this research looks at how dwarfing on islands influences limb scaling and proportions in an organism in a similar ecological niche as H. floresiensis. Here, I analyze absolute limb lengths and static allometry of limb lengths regressed on predicted body mass of dwarfed island foxes and their nondwarfed relatives. Dwarfed island foxes have significantly smaller intercepts but steeper slopes of all limb elements regressed on predicted body mass than the mainland gray fox. These allometric alterations produce limbs in the island fox that are significantly shorter than predicted for a nondwarfed gray fox of similar body mass. In addition, the humerofemoral, intermembral, and brachial indices are significantly different. These results provide a novel model for understanding skeletal variation of island endemic forms. Unique body size and proportions of H. floresiensis are plausible as ecological adaptations and likely not examples of symplesiomorphies with Australopithecus sp. Caution should be exerted when comparing an island dwarf with a closely related species as deviations from allometric expectations may be common.
... In arthropod communities both, pests and their antagonist's complexes, are considered as entities, in which their constituent species produce patterns in abundance and distribution (Gaston and Lawton 1988, Holt et al. 1997, Lawton et al. 1998. Dominance structure, evenness, and community composition can be used to assess the anthropogenic impact on natural enemy populations but they are not capable to recognize the changes in functional structure, species interaction, and ecosystem functioning and essential services in functioning of ecosystem. ...
Der Begriff nachhaltige landwirtschaftliche Produktion zielt auf zukunftsfähige ressourcenschonende Wirtschaftsweise ab, die in der Gesellschaft zunehmend Akzeptanz findet. Wie sich Nachhaltigkeit gestaltet ist regional und zeitlich variabel und wird von politischen, sozialen und ökonomischen Themen beeinflusst. Die Landwirtschaft ist unter anderem durch die Nutzung von Pflanzenschutz und chemischen Pflanzenschutzmitteln in der Lage, dauerhaft stabile Erträge zu sichern. Der intensive Einsatz von nicht-nachhaltigen Praktiken kann jedoch zu verheerenden Auswirkungen auf die Vielfalt und die Fülle von nützlichen Arthropoden führen. Eine nachhaltige Bewirtschaftung kann dagegen natürliche Gegenspieler von Schaderregern fördern, aber die vom Gesetzgeber verursachten Interventionen, wie das breit angelegte Spektrum von Pestiziden, können die Selbstkontrolle von natürlichen Gegenspielern als Regulatoren verzerren bzw. negativ beeinträchtigen. Die konventionelle Landwirtschaft kann Lebensräume, ökologische Strukturen, Nahrungsnetze und funktionelle Biodiversität zerstören. Die Folgen sind Emigration, der Verlust von Arten und vereinfachte interspezifische Wechselwirkungen. Die Umsetzung von Maßnahmen zur nachhaltigen landwirtschaftlichen Produktion bedürfen geeigneter Parameter zur Abschätzung des Status quo und des Erfolgs der durchgeführten Maßnahmen. Diese Parameter sind nicht immer vorhanden und müssen erarbeitet werden. Diese Arbeit hatte das Ziel, anhand von Erhebungen des Artenaufkommens der Wickler (Tortricidae) und ihrer Larvalparasitoide in Apfelanlagen mit unterschiedlicher Bewirtschaftungsintensität und der davon abgeleiteten ökologischen Indices geeignete Parameter zu erarbeiten. Diese sollten die Auswirkung der Benutzungsintensität auf die funktionelle Biodiversität wiederspiegeln und geeignet sein, die Umsetzung von Nachhaltigkeitsmaßnahmen zu bewerten. Um den Einfluss konventioneller bzw. ökologischer Bewirtschaftung auf Wirbellosengesellschaften zu untersuchen, wurde eine Vergleichsstudie durchgeführt, die Nahrungsnetze der Larval-parasitoide, Biodiversitäts Indizes und Parasitierungsraten in Apfelanlagen mit vier verschiede-nen Bewirtschaftungsweisen erfasst. Die Probenahmen erfolgten 2011-2015 in Baden-Württemberg. Nach Intensität der Pflanzenschutzmittelanwendung wurden die Anlagen in die Kategorien Bewirtschaftet (ökologische und integrierte Bewirtschaftung) und Streuobst eingeteilt. Sie lagen in Denzlingen, Emmendingen, Goldener Grund, Versuchsstation Hohenheim, Ilsfeld, Neuhausen, Plieningen, Rommelshausen, Scharnhausen und am Bodensee. Die Probenahmen bestanden im Sammeln der Raupen (Tortricidae und Gelechiidae) mit Fallen aus Wellpappe und durch Zufallsfunde. Im Labor wurden die daraus schlüpfenden adulten Parasitoide taxonomisch bestimmt. In 7923 Larven fanden sich 324 Individuen von Parasitoiden aus drei Unterfamilien der Braconidae, Ichneumonidae und Perilampidae. Die größte Vielfalt, Häufigkeit und gleichmäßige Verteilung an Larvalparasitoiden fand sich auf Streuobstwiesen (z.B. in Plieningen), die keine oder nur minimale Pestizid Anwendungen erhielten. Die Interaktionsmuster der Nahrungsnetze (Verknüpfungs-grad) im Streuobst wiesen die meisten trophischen Links auf, verglichen mit anders bewirtschafteten Anlagen unter denen die kommerziellen (konventionellen) die geringste Biodiversität an Nutzarthropoden beherbergten. Ihre prozentualen Anteile wurden ebenfalls erhoben, um die Ähnlichkeit der Larvalparasitoid-Gesellschaften unter verschiedenen Bewirtschaftungen darzustellen. Es stellte sich heraus, dass Anlagen mit gleicher Bewirtschaftung ähnliche Parasitoiden-Gesellschaften aufweisen. Vier Parasitoiden-Arten erwiesen sich als positiv dichteabhängig von ihren Wirtsarten, während die anderen Arten entweder nicht dichteabhängig reagierten oder in zu geringen Zahlen auftraten, um eine Korrelation zu berechnen. Um Informationen über Bewirtschaftung im Apfelanbau, Bedingungen für den Pflanzenschutz, Schädlingsbefall und Haupthindernisse für die Förderung nachhaltiger Anbaumethoden im Iran zu erhalten und zu analysieren wurden im Juli 2014 mittels Fragebogen 39 Apfelanbauer aus Ost-Aserbeidschan, Fars, Isfahan, Teheran und West-Aserbeidschan befragt. Die Bewirtschaftung der Anlagen stand meist unter Aufsicht der Apfelanbauer. Bauern aus Isfahan litten unter mangelhaftem Ausbau der Straßen was ihnen den Zugang zu Märkten für den Absatz ihrer Produkte erschwerte. Die räumliche Entfernung zu Fachleuten beeinflusste die Intensität des Pflanzenschutzmittel Einsatzes durch die Bauern. Konventioneller Anbau überwog in allen Provinzen; Zugang zu biologischen Pflanzenschutzmitteln war weitgehend auf Teheran beschränkt. Insgesamt 29 Pestizide wurden gegen Obstschädlinge im Iran eingesetzt. Im regionalen Maßstab wurden die höchsten Schäden durch Unkräuter verursacht, auf der Ebene der Provinzen durch Schädlinge. Ausbrüche des Sekundär Schädlings Tetranychus urticae waren ein Anzeichen für menschliche Störfaktoren in der Landwirtschaft des Iran. Die Provinz Teheran verfügte über mehrere Apfelsorten während andere Provinzen eine geringe Vielfalt aufwiesen. Unter den Apfelproduzenten erfolgte die Sortenauswahl vorrangig nach Kriterien der Vermarktbarkeit.
... According to William and Kirk (1991), there was a correlation between insect size and host size. Body size has many ecological implications (Peters 1983), including those relating to an insect's distribution, abundance, and population variability (Gaston and Lawton 1988). ...
Tunisian table grape production has significantly increased since two decades due to vineyards regional
expansion and yield improvement. But, since several years, decline symptoms on Vitis vinifera have
been recorded in some areas. A study case of a vineyard in Naassen area (near to Tunis) was chosen to
investigate the disease origin and the physiological, biochemical, and histological modifications
associated with vine decline. The investigation revealed characteristic symptoms on leaves, old and
young shoots similar to decline symptoms of Grapevine Trunk Diseases. Based on cultural
characteristics, laboratory investigations revealed the presence of Phaeomoniella chlamydospora and
Phaeoacremonium spp., Diplodia seriata and Botryosphaeria dothidea, from root and shoot samples,
respectively. These fungi are known as the main pathogens responsible for the Esca, Black dead arm
and Excoriose. Molecular analysis confirmed the identification of Diplodia seriata. Beside
morphological alterations on leaves and shoots, symptomatic vines presented significant reductions of
30 and 20% in trunk diameter and bud break rate, respectively, and delayed spring growth compared to
healthy ones. Furthermore, roots and stems from declined vines contained 3 times more starch than
those from asymptomatic ones. Decline survey revealed a heterogeneous dispersion of symptoms in the
vineyard in accordance with water supply. The vines along the edge of vineyards are usually less
watered and show more decline symptoms. Decline dynamics in time and space scales have to be
considered in order to develop effective management strategies.
... Both the analyses of variance and the reduced major axis regressions suggest that the influence of feeding category on the size variation index is more important than that of body size per se. To some extent, this interaction between body size, range of body size and feeding ecology might be anticipated since polyphagous insect species are often (but not always) larger than specialists (e.g., Gaston & Lawton, 1988;Gaston & Reavey, 1989), and this is confirmed by our data. It is possible that the positive correlation between the number of individuals available for analysis and the size variation index may be the result of our measurements being unrepresentative of the body range within the populations studied (i.e., if our sample sizes were too small to represent the true variation in body size). ...
... According to William and Kirk (1991), there was a correlation between insect size and host size. Body size has many ecological implications (Peters 1983), including those relating to an insect's distribution, abundance, and population variability (Gaston and Lawton 1988). ...
Haouel-Hamdi, S., Titouhi, F., Boushih, E., Dhraief, M.Z., Amri, M., and Mediouni-Ben Jemâa, J. 2017. Population demographic traits and reproductive parameters of the seed beetle Callosobruchus maculatus infesting stored lentil and chickpea commodities. Tunisian Journal of Plant Protection 12: 67-81. This paper carried out first exhausted investigations on pest status of the cowpea weevil Callosobruchus maculatus on two food legumes namely chickpea (Amdoun 1 variety) and lentil (Ncir variety) during six months of storage. Data on populations' dynamic, demographic traits, reproductive parameters, juvenile and adult fitness, economic injury level (EIL) and damages (impact on germination and weight losses) were studied through this work. Results revealed that C. maculatus is a major pest on stored chickpea in Tunisia. Moreover, results indicated that reproductive parameters, the juvenile and adult fitness of C. maculatus exhibited great variations among hosts. In this respect, linear regression analysis demonstrated that hosts have significant effects on adult fitness. Results showed that host contributed respectively by 77% for body weight and 80% for body size. Chickpea was more suitable host compared to lentil, since the mortality rate of eggs and larvae and the generation duration means were higher in lentil. In addition, significant differences were observed in the Susceptibility Index of the two food legume hosts showing chickpea seeds as moderately susceptible to C. maculatus attacks while, lentil seeds were resistant. C. maculatus caused large reductions in seed germination (78% chickpea and 33% lentil for highest infestation level 80%) and seeds weight (45% for chickpea against 8% for lentil after 6 months of storage) of both hosts; the infestation levels and the weight losses were significantly different in the storage periods. Overall, this study provides reasons for farmers and traders to make a decision to take a control action against C. maculatus during storage. Moreover, this work pointed out the variability of economic injury levels with host legumes.
... Empirical evidence for the resource-use hypothesis (Fig. 1b) is weak and ambiguous ( Gaston et al. 1997b; positive relationships: e.g. Barger and Esch 2002, Kotze et al. 2003, Heino 2005, Faulks et al. 2015; no relationship: Gaston and Lawton 1988, Root and Cappuccino 1992, Gregory and Gaston 2000, McCreadie and Adler 2014negative relationships: Gaston et al. 1997b, Päivinen et al. 2005, Verberk et al. 2010). In our study, the degree of feeding specialization had no effect on either the local distribution or the mean abundance of a species. ...
The nearly universal positive relationship between the distribution and abundance of species has been explained by several hypotheses but hitherto no consensus has been reached. Here, we used monitoring data of 105 phytophagous true bug species (Heteroptera) from 150 grassland sites over six years, to test how (1) range-position, (2) resource-use, (3) resource-availability, (4) density-dependent habitat selection, (5) metapopulation dynamics and (6) habitat-dispersal affect the distribution-abundance relationship. For the use in a confirmatory path analysis, we constructed causal pathways representing the hypothesized relationships and tested them separately and in a combined analysis. Our results show that the distribution-abundance relationship in phytophagous true bugs is driven by habitat-availability. An increasing local density of the host-plants increases the distribution of the species in the landscape, which in turn increases their local abundance. Thereby habitat availability facilitates dispersal success. We conclude that local abundance of herbivores facing habitat destruction could decline owing to a decrease in population dynamics between sites at the landscape scale. Finally, our results underline the potential of confirmatory path analysis for testing competing hypotheses. This article is protected by copyright. All rights reserved.
... -Th e infl uence of defensive indices on species strength and selectiveness varied with seasonality. Th e dry season at the study site is dominated by generalist herbivores ( López-Carretero et al., 2014 ), which are likely to be less susceptible to the variation in resistance traits ( Gaston and Lawton, 1988 ;Redfearn and Pimm, 1988 ;Hughes, 2000 ;Barber and Marquis, 2011 ). Th e presence of these herbivores could explain why during the dry season the inverse relationship between selectiveness and the defensive index PC1 was observed. ...
Premise of the study: Plant–herbivore networks are highly specialized in their interactions, yet they are highly variable with regard to the relative importance of specific host species for herbivores. How host species traits determine specialization and species strength in this antagonistic network is still an unanswered question that we addressed in this study.
Methods: We assessed plant cover and anti-herbivore resistance traits to assess the extent to which they accounted for the variation in specialization and strength of interactions among species in a plant–herbivore network. We studied a tropical antagonistic network including a diverse herbivore–host plant assemblages in different habitat types and climatic seasons, including host plants with different life histories.
Key results: Particular combinations of leaf toughness, trichome density, and phenolic compounds influenced herbivore specialization and host species strength, but with a significant spatiotemporal variation among plant life histories. Conversely, plant–herbivore network variables were not influenced by plant cover.
Conclusions: Our study highlights the importance of species-specific resistance traits of plants to understand the ecological and evolutionary consequences of plant–herbivore interaction networks. The novelty of our research lies in the use of a trait-based approach to understand the variation observed in diverse plant–herbivore networks.
Key words: plant–herbivore interactions; antagonistic networks; plant resistance traits; host plant cover; specialization; species strength
... Surprisingly little attention has been paid to the links between different macroecological patterns, even when, as in the examples listed above, they share variables in common. A few studies have considered several patterns for a given species assemblage but, with some notable exceptions, they have done little to explore the shared mechanisms (Harvey & Lawton 1986;Brown & Maurer 1987;Gaston & Lawton 1988a, ‡Present address and correspondence: T.M. Blackburn, School of Biosciences, University of Birmingham, Edgbaston, Birmingham B15 2TT, UK. Tel: + 44 121414 5893 Fax: + 44 121414 5925. ...
1. Associated with the development of the held of macroecology has been the recognition and analysis of a number of different patterns in the large-scale abundance and distribution of species. The mechanistic bases of these patterns have usually been considered in isolation, yet the patterns are necessarily linked, as the same individual animals contribute to all of them. 2. Here, a model linking macroecological patterns in abundance, distributional extent and body mass is developed, based on how the finite amount of energy available to the species in a region is divided between them. The energy available to a species is assumed to support some quantity of biomass, which must then be allocated to either many small-bodied or fewer larger-bodied individuals. This identifies a necessary link between population size and body mass, which predicts when the variety of relationships between these variables in the published literature are expected to occur. 3. Although framed in terms of energy use by species, the model does not assume that energy per se is necessarily limiting populations. How individuals use space determines the form of relationships between population size and distributional extent, distributional extent and body mass, and population density and body mass. The model additionally allows a number of falsifiable predictions about the anatomy of macroecological patterns. 4. Support for the assumptions of the model is discussed.
... Generalist insect herbivores tend to be larger than specialists, and larger herbivores tend to have a wider geographical range (e.g. Gaston and Lawton, 1988). Thus, a wide geographical range of many species of insect herbivore would tend to support hypothesis 2. ...
... Definitions have been as coarse as a list of countries in which the species is found (Corbet and Hill, 1990) or the number of degrees of longitude and latitude where the species has been recorded (Reaka, 1980). Finer scale attempts have represented geographic range as the number of quadrats (of varying sizes) occupied by the species (Schoener, 1987;Ford, 1990), or the total number of localities at which the species has been recorded as occurring (Gaston and Lawton 1988). Arriving at standard approaches to quantify geographic range is important for understanding how the geographic range of a species reflects its relative vulnerability to extinction. ...
The Sungazer (Smaug giganteus) is an endemic lizard species that is threatened by habitat destruction and illegal harvesting, and as a result, is listed as ‘Vulnerable’ on the IUCN Red Data List. The species is restricted to the Highveld grasslands of South Africa, where over 40% of the area is used for crop monoculture, and much of the remainder has been transformed for human habitation and the construction of roads, dams, mines and power plants. This poses serious threats to the persistence of the species, as the Sungazer is a habitat specialist, and is strongly associated with pristine Themeda grassland. In addition, the species is illegally harvested from the wild for the traditional medicine, and pet trades. The rate at which these threats are removing habitat and affecting Sungazer populations is unknown, and the lack of such knowledge impedes effective conservation planning. This has prompted the call for research on the population ecology and life history of the species, so that the species can be managed.
Area of occupancy. A minimum convex hull was created around all QDGCs containing species occurrence records, and an Extent of Occurrence (EOO) of 5 833 800 ha was calculated. The distribution of the species (area of QDGCs and portions of QDGCs containing occurrence records that fall within Free State and Mpumalanga Provinces) was calculated as 3 819 600 ha. Of this area, 2 053 035 ha is currently natural. To assess the proportion of EOO and distribution actually occupied by Sungazers, I surveyed 120 random sites for Sungazer presence, and found 5 containing Sungazers (4.17%) within the EOO, and 4 (5.05%) within the distribution. This measure was used to calculate the Area of Occupancy (AOO), which was 103 678 ha.
Population size. I recorded a mean burrow density (MBD) of 6.14 ± 0.87 burrows/ha for 80 sites across the distribution of the species. To estimate the number of burrows within the distribution, I multiplied the MBD by the AOO. I calculated 636 325 ± 90 282 burrows. Burrow occupancy data reported in the literature indicates that only 85.7% of burrows are occupied at a given time, and there is an average occupancy of 1.83 lizards/burrow in these burrows. When applied to the number of burrows calculated, a total figure of 998 247 ± 141 632 lizards is estimated to occupy a total of 545 490 ± 77 395 burrows. Population demographics data reported in the literature indicates that 61.2% of a population is made up of mature (sexually reproductive) individuals, and when applied to the total population size, total mature individual count is 610 927 ± 86 679 Sungazers.
Population decline. I visited 39 sites where Sungazer populations were reported in 1978, and found a population decline of 20.51% at these sites (0.59% decline/year). I assessed the change in land cover between 2001 and 2009 using geographic information systems (GIS) techniques and found a 13.3% decline in natural habitat across the distribution of the species over this time (1.48% decline/year). The loss of natural habitat was due primarily to an increase in cultivated areas.
Priority conservation areas. Five priority zones, representing the top 20% of optimal Sungazer habitat were identified using an ecological niche model. These zones are spread across the distribution, with sites situated in the west (Welkom), north centre (Vrede, Edenville), south east (Harrismith) and north east (Volksrust). In total, the priority zones cover 1.7% of the AOO, but are estimated to contain 3-4.4% of the total population based on the habitat quality. The population size estimated contained within these zones is four to five times the mean minimum viable population (MVP) estimated for vertebrate species.
Conclusion. I used my demographic measures to assess the conservation status of S. giganteus using Version 3.1. of the IUCN Categories and Criteria for conservation assessments. This assessment improves the precision of the measure of population reduction and includes geographic range for the species. My conservation assessment confirms the current listing of S. giganteus as ‘Vulnerable’ under criteria A2bcd and B2ab. I highlight the need for developing a protocol for translocations, a phylogeographic study to assess the landscape genetics of the species, an investigation of dispersal patterns and colonisation strategies.
... Natural primary producer communities typically comprise many species of various taxonomic levels with vastly different body sizes (Gaston and Lawton 1988). Body size is an important feature in many food web models because of its importance in numerous ecological interactions, including inter-species competition and prey selection by predators (Cohen et al. 1993;Williams and Martinez 2000). ...
... There is also some evidence to show that the relationship between body size and distribution range holds for a more restricted size range of related animals. Thus Gaston and Lawton (1988) showed that among insects there is a tendency for the smaller species to have wider distributions. (To be sure, there is also evidence that among birds, the largest species have a wider distribution range (Brown and Maurer 1987). ...
... For rainforest arboreal arthropods, several selection pressures may lead to a reduction of body size (see Basset & Kitching, 1991). One reason could be that small body size may actually increase dispersal abilities (Gaston & Lawton, 1988), and this factor certainly represents an important selective advantage in dynamic rainforest environments (Morse et al., 1988). However, in my study I did not find any differences in body length or forewing length for arctiid species related to succession, but the species found in the forest understorey had a broader frequency distribution of forewing length and body length. ...
... Moreover, the pioneer dune vegetation is subject to continual disturbance (burial under moving sand, strong winds with a high saline concentration, abrasion from blown sand, flooding and mechanical damage caused by tropical storms and hurricanes) [33], which makes host plants in these habitats highly unpredictable resource for herbivores [62]. In this context, both the unpredictable availability and low nutritional quality of the host plants in open habitats and present during the dry season can impose strong selective pressure on the herbivores [42], favoring species that are less specialized and less susceptible to variations in host resources [63], [64], [65]. In fact, in open vegetation habitats and during the dry season we found a predominance of herbivore species with relatively low host specificity. ...
Despite the dynamic nature of ecological interactions, most studies on species networks offer static representations of their structure, constraining our understanding of the ecological mechanisms involved in their spatio-temporal stability. This is the first study to evaluate plant-herbivore interaction networks on a small spatio-temporal scale. Specifically, we simultaneously assessed the effect of host plant availability, habitat complexity and seasonality on the structure of plant-herbivore networks in a coastal tropical ecosystem. Our results revealed that changes in the host plant community resulting from seasonality and habitat structure are reflected not only in the herbivore community, but also in the emergent properties (network parameters) of the plant-herbivore interaction network such as connectance, selectiveness and modularity. Habitat conditions and periods that are most stressful favored the presence of less selective and susceptible herbivore species, resulting in increased connectance within networks. In contrast, the high degree of selectivennes (i.e. interaction specialization) and modularity of the networks under less stressful conditions was promoted by the diversification in resource use by herbivores. By analyzing networks at a small spatio-temporal scale we identified the ecological factors structuring this network such as habitat complexity and seasonality. Our research offers new evidence on the role of abiotic and biotic factors in the variation of the properties of species interaction networks.
... In the context of ecological, ethological and physiological traits, and both within-and between-species comparisons, there has been evidence both for and against the 'jack-of-all-trades' hypothesis (e.g. Huey & Hertz, 1984;Jackson & Hallas, 1986;Gaston & Lawton, 1988;Whitlock, 1996). ...
Data for five closely related species of gammarid crustaceans are used to examine interspecific relationships between the breadth of fundamental tolerance or capacity and geographical range size. Gammarus duebeni is, almost without exception, the most tolerant species and that with the best physiological performance. Although there is some limited variation, the remaining species can be ranked broadly in the sequence G. zaddachi > G. salinus > G. oceanicus > G. locusta. The wide tolerance and high performance of G. duebeni is associated with the occupation of a wider range of environmental ‘types’ than any other of the species. In terms of geographical range size, the species can be ranked from most to least widespread in the sequence G. oceanicus > G. duebeni > G. zaddachi > G. salinus > G. locusta. This provides little support for Brown’s hypothesis, or the argument that the more widely distributed species within a taxonomic assemblage also tend to have the widest fundamental niches. However, if marine (G. oceanicus and G. locusta) and estuarine (G. duebeni, G. zaddachi, G. salinus) species are considered separately, then in each case the species with the largest geographical range is also the most tolerant/best performer. In this sense, the jack-of-all-trades is the master-of-all, rather than the master-of-none.
... Surprisingly little attention has been paid to the links between different macroecological patterns, even when, as in the examples listed above, they share variables in common. A few studies have considered several patterns for a given species assemblage but, with some notable exceptions, they have done little to explore the shared mechanisms (Harvey & Lawton 1986;Brown & Maurer 1987;Gaston & Lawton 1988a, ‡Present address and correspondence: T.M. Blackburn, School of Biosciences, University of Birmingham, Edgbaston, Birmingham B15 2TT, UK. Tel: + 44 121414 5893 Fax: + 44 121414 5925. ...
Associated with the development of the field of macroecology has been the recognition and analysis of a number of different patterns in the large‐scale abundance and distribution of species. The mechanistic bases of these patterns have usually been considered in isolation, yet the patterns are necessarily linked, as the same individual animals contribute to all of them.
Here, a model linking macroecological patterns in abundance, distributional extent and body mass is developed, based on how the finite amount of energy available to the species in a region is divided between them. The energy available to a species is assumed to support some quantity of biomass, which must then be allocated to either many small‐bodied or fewer larger‐bodied individuals. This identifies a necessary link between population size and body mass, which predicts when the variety of relationships between these variables in the published literature are expected to occur.
Although framed in terms of energy use by species, the model does not assume that energy per se is necessarily limiting populations. How individuals use space determines the form of relationships between population size and distributional extent, distributional extent and body mass, and population density and body mass. The model additionally allows a number of falsifiable predictions about the anatomy of macroecological patterns.
Support for the assumptions of the model is discussed.
The lepidopteran fauna on goldenrod, Solidago altissima, in central New York is extraordinarily diverse (63 species in 43 genera and 13 families, representing 61% of the total insect fauna on S. altissima), as determined through over four years of sampling at 43 sites. To best illustrate the functional structure of the community, caterpillars are divided into both feeding and sheltering guilds. We studied patterns in caterpillar size, feeding guild, sheltering guild, host range, phenology, abundance, and taxonomic relationships to gain insights about community assembly on this group of native caterpillars on a prominent native host species. Size distribution among caterpillar species is highly skewed toward the small end and does not differ between caterpillars with broad and narrow host ranges. There are relatively few species that feed on stems and rhizomes, suggesting challenges in adapting to those plant parts. Host range is a highly conserved trait in several lineages of goldenrod feeders (relatives of polyphagous species are polyphagous, and relatives of specialists are specialists). Polyphagous species and exposed-feeding species are usually rare; some specialist species are also chronically rare. There is a dearth of species in mid-summer, between the time that many leaf-folding and leaf-tying species complete development and the time that flower-feeding species appear. Species with high intimacy of association (gallmakers, stem borers, and leaf miners) tend to be specialists, as compared with exposed feeders, and they are necessarily small in size. Among exophages, size relations are constrained by phylogeny. Lineages have characteristic behaviors and means of association with the host; as a result, the structure of the fauna is determined by an interaction of ecological factors and evolved constraints in the lineages that have adapted to feed on Solidago.
In previous chapters, we addressed the mechanisms of host choice. This chapter deals with questions of the ultimate or functional bases of different host-choice patterns. What determines diet breadth and why are so many phytophagous insects relative specialists? The patterns of feeding illustrated in Chapter 1 indicate the degree to which narrow diets are common in different phytophagous insect groups. Yet, there are clearly advantages in being able to feed on many items; insect species with generalized feeding abilities can have a broad geographical range, many generations each year, and an almost certain availability of food at any time. Individuals may also take advantage of being able to select among foods to balance the nutrient intake. At face value, fitness should be greater for polyphagous species, but despite this, most species are specialists. Therefore, interest has focused on the reasons for specialization. It is generally thought that there are many factors involved in the evolution of, and maintenance of specialization, and that no one of them can be singled out as predominant. In addition, it seems likely that the underlying reasons may differ among insect groups.
Body size correlates with a large number of species traits, and these relationships have frequently been used to explain patterns in populations, communities, and ecosystems. However, diverging patterns occur, and there is a need for more data on different taxa at different scales. Using a large dataset of 155,418 individual beetles from 588 species collected over 13 years of sampling in Norway, we have explored whether body size predicts abundance, seasonality, and phenology in insects. Seasonality is estimated here by flight activity period length and phenology by peak activity. We develop several methods to estimate these traits from low-resolution sampling data. The relationship between abundance and body size was significant and as expected; the smaller species were more abundant. However, smaller species tended to fly for longer periods of the summer and peaked in midsummer, while larger species were restricted to shorter temporal windows. Further analysis of repeated sampling from a single location suggested that smaller species had increased flight period lengths in warmer years, but larger species showed the opposite pattern. The results 1) indicate that smaller species are likely to be disproportionately valuable in ecological interactions, and 2) provide potential insights into the traits influencing the vulnerability of some larger species to disturbances and climate change.
Small species (<100 mm total length) are a diverse and abundant component of coral-reef fish assemblages. However, difficulties identifying and estimating the abundance of small species has meant that they are often ignored or undersampled in community level studies. We use studies that sampled whole assemblages in order to examine size/diversity and size/abundance patterns within coral-reef fish assemblages. We then examine body size in relation to patterns of habitat use, species interactions (competition and predation) and life histories of coral-reef fishes. Species diversity typically peaks in small size classes (<100 mm) and there are many low diversity larger size classes. In some assemblages, diversity also declines among the smallest species (<50 mm). The high diversity of small coral-reef fishes may partly be attributed to the complex structure of coral reefs. However, we find no evidence that diversity/size distributions are controlled by increased availability of niches or greater living space for small fishes. Regional-scale processes also influence local diversity of coral-reef fishes. In particular, high rates of speciation among small species might contribute to the observed diversity/size distributions. Declines in abundance with increasing body size have been attributed to equal partitioning of energy among species of different body sizes ("energetic equivalence rule"). We find no support for the "energetic equivalence rule" in assemblages of coral-reef fishes. Although body size was generally a poor predictor of abundance for coral-reef fishes, the upper bound of the size/abundance distribution was very uniform among the assemblages we examined. This upper bound may prove useful for selecting species most likely to be resource-limited. Small species of coral-reef fishes are more closely associated with the reef matrix than their larger counterparts. We propose that this is largely the result of high predation risk for small species. It is generally believed that smaller species and smaller size classes within species are subject to higher mortality rates. However, further studies are required to test this assumption. The diversity and abundance of small reef fishes, and their restriction to specialized habitats may increase the likelihood of competitive interactions. There is ample evidence of intraspecific competition among individuals of similar size in small coral-reef fishes. There is less evidence of interspecific competition among coral-reef fishes. However, the outcome of interspecific interactions will, in part, depend on the relative size of species. Because body size can influence the intensity with which predation and competition act, consideration of population size-structure may help develop multifactorial models of population dynamics of coral-reef fishes.
The relationships between macro-ecological patterns and physiological investigations in insects, especially those dealing with respiratory metabolism, are assessed in an attempt to encourage the development of the interaction between macroecology and physiological ecology. First, we demonstrate that although physiological ecology has been explicitly concerned with a number of issues relating to species boundaries, many questions remain unanswered. We argue that there are essentially two ways in which the relationship between physiological tolerances and species range boundaries have been investigated. The correlational approach involves physiological inference, physiological prediction, isocline analyses and climatic matching, and has often been criticized for a lack of rigour, while the experimental approach seeks to examine experimentally the relationships between physiological variables and range edges. Second, we use the recent debate on processes underlying latitudinal patterns in body size to caution against the conflation of patterns and processes operating at intraspecific and interspecific levels, the dangers inherent in invoking single explanatory variables, and an undue focus on adaptationist (e.g. optimization) rather than nonadaptationist explanations or some combination of the two. We show that both positive and negative relationships between body size and latitude have been found at the intraspecific level and suggest that interactions between temperature-induced heterochrony, and the relationship between habitat durational stability, growing season length, and generation time can be used to explain these differences. Similar variation in documented patterns is demonstrated at the interspecific level, and the mechanisms usually proffered to explain such dines (especially the starvation/desiccation-resistance hypothesis) are discussed. Interactions between various environmental factors, such as host-plant quality, and their effects on size dines are also discussed. Third, we argue that respiratory metabolism, as a measure of ATP cost, and its spatiotemporal variation are critical to many explanations of macroecological patterns. Adaptive changes in metabolism reputedly involve both depression (stress resistance) and elevation of metabolic rate, although recent studies are increasingly calling these ideas into question. In particular, flow-through respirometry is revolutionizing results by allowing careful separation of resting (or standard) and active metabolic rates. These techniques have rarely been applied to studies of metabolic cold adaptation in insects, one of the most polemical adaptations ascribed to high-latitude and high-altitude species. We conclude by arguing that physiological investigations of species tolerances are important in the context of macroecology, especially species distributional patterns and the possible impact of climate change thereon. However, we caution that relationships between abiotic variables, species tolerances, and distributional ranges may be non-linear and subject to considerable modification by the presence of other species, and that many of the pressing questions posed by macroecology have not been addressed by insect physiologists. Nonetheless, we suggest that because an understanding of the dynamics of species distributions is of considerable importance, especially in the context of current conservation problems, insect physiological ecology has much future scope.
"Systemic management" describes a holistic, objective, and universally applicable form of management, providing a framework for addressing environmental challenges such as global warming, emergent diseases, deforestation, overpopulation, the extinction crisis, pollution, overfishing, and habitat destruction. Its goals are the consistently sustainable relationships between humans and ecosystems, between humans and other species, and between humans and the biosphere. This book presents a convincing argument that these goals, and the means to achieve them, can be inferred from empirical information. It describes how comparisons between humans and other species reveal patterns that can serve to guide management toward true sustainability, that is, ways that are empirically observed to work in natural systems by virtue of their emergence. It shows how this objective approach has not been possible in conventional management because sustainability is undermined by other human values. This book presents systemic management as a specialized process of pattern-based decision making that avoids the inconsistency, subjectivity, and error in current management practice. It clearly demonstrates how mimicking nature's empirical examples of sustainability can circumvent anthropocentric tendencies to overuse/misuse human values in management, and illustrates the best science for management (the science best suited for achieving sustainability) through examples of research that address specific management questions. It presents systemic management as reality-based management to replace the misdirected reductionism of conventional management with reductionism useful for directing human self-control.
This chapter discusses the factors that cause populations of certain species to outbreak which conversely keeps other species from outbreaking, have long interested population ecologists. Most investigations of the comparative advantages of aggregation and solitary living in insect herbivores have focused on the average performance of a female's offspring. The gregariousness of Microrhopala eggs and larvae only indirectly affects their vulnerability to natural enemies and does not influence how well they develop on their host plant. To examine the possibility that Exema larvae move around on the host plant to avoid damaged-induced changes, the behavior and performance of larvae on ramets were compared with simulated chewing damage to that of larvae on control ramets. Paper birch, Betula papyrifera Marsh, hosts many species of Lepidoptera, of which only a few are reported to cause damage that qualifies them as outbreak species. There exists a vast and varied literature on the effect of feeding alone, or in aggregations on survivorship and fecundity. Given the general lack of support for either enemy-based or plant-based causal links between aggregation and the tendency for outbreaks, the reasons why aggregation so consistently correlates with patterns of population dynamics remain elusive. Some may view space as the final frontier, but it is argued that it is at the crossroads of space and time that population biologists will meet their greatest challenge. Forging the link between spatial patterns and temporal dynamics has been and continues to be a major impasse for population biologists.
(1) Data on the abundance and seasonal phenology of herbivorous insects feeding upon bracken at Skipwith Common, in North Yorkshire in the years 1980-86 were used to determine the temporal predictability of this assemblage, both on an open site and on an adjacent woodland site. There were seventeen species on the open site, plus pooled data on two genera each with two species, and one genus with three species: twenty taxa in all. The woodland site held fifteen species, plus the same pooled genera: eighteen taxa in all. For simplicity we refer to all the taxa as species. (2) In both habitats taxonomic composition, and relative mean peak and total seasonal abundances per frond for each species are all conserved across years. In other words, rare species have remained rare, and common species common over the 7-year study period. On the open site, slightly less complete data allow us to extend these conclusions over 15 years. Species have also tended to enter the community in a predictable sequence each year, more so on the open site than in the woodland. (3) This assemblage conforms to the characteristics of one of the four theoretical ideal community types defined by Strong, Lawton & Southwood (1984) (type ii). Populations appear to be subject to density-dependent control, and therefore the community has a reasonably predictable structure, but it is not saturated with species and interspecific competition is unimportant.
Within a geographic assemblage, large-bodied species of macrolepidopteran moths tend, on average, to be less host-specific than small-bodied. Five possible explanations for this pattern are identified, based respectively on (i) phylogenetic relationships between species, (ii) latitudinal gradients in body size and feeding specificity, (iii) the relationship between range size and body size, (iv) larger body size as a buffer from environmental variation, and (v) the relationship between endophagous host associations and small body size. These mechanisms are tested using data for British macrolepidoptera and also evaluated using evidence from the literature at large. Although some of their assumptions are found to be justified, there is no significant support for any single mechanism. This lack of evidence for previously proposed mechanisms is discussed in the light of a recently proposed alternative explanation which combines theories of host quality and host defence mechanisms.
(1) We investigate problems associated with the measurement and interpretation of population density variability that have confounded most, if not all, previous studies of the subject. (2) The most commonly used measure of variability, S.D.(log[N + 1]), is subject to potentially very large bias, and other measures based on untransformed densities can be affected by the long-tailed frequency distributions of population data. (3) The fact that variability often depends on the mean density is usually ignored, yet it affects the generality of any statement based on a single estimate of variation. (4) When comparing variability between species, the generality of any conclusions is also limited by the choice of spatial and temporal scale of sampling. This potentially confounds many studies. (5) We discuss the patterns that emerge from considering spatial and temporal Taylor power plots, and suggest that combinations of such plots for individual species allow a classification according to markedly different types of dynamics; the underlying mechanisms of which are not well understood. (6) Finally, we consider ways of making meaningful comparisons of spatial and temporal variability across species.
This paper investigates whether body size-related constraints on home-range resource harvesting could lead to coexistence between interspecific competitors of different body size under conditions of complete niche overlap. With this objective, I analyzed the influence of body size on induced and sustainable resource limitation and its implications on the interaction between an individual and a fixed biomass of larger competitors in a four-dimensional space, consisting of two spatial dimensions describing competitor home range, resource availability, and time. It was shown that body size-related spatiotemporal constraints on home-range resource harvesting, trophic optimization, and relativity of resource availability determine absolute and relative amounts of unused home-range resources, restricting the influence of resource limitation induced by an individual to a definite size distance around its size. It was therefore concluded that: (1) when competition occurs asymmetrically with a superiority of large animals, size differences alone could allow coexistence, independently of any kind of resource partitioning; and (2) superiority of large animals should result from the resource density control that a larger competitor imposes on the smaller one whenever size differences for stable coexistence occur. Existing evidence of an inefficiency in home-range resource exploitation seems large enough to suggest a view of guilds based on a hierarchy of inclusive home ranges.
This paper investigates the relationships between competition strength on individuals. as measured by food absorption variations, and competitor body size. To this end I analysed the feeding behaviour of individuals of the freshwater isopod Proasellus coxalis under laboratory manipulation of competitor biomass, number of competitors into which biomass was partitioned (i.e., degree of biomass concentration) and time lag among resource utilizations (i.e., co-occurrence vs temporal separation of competitors). The study was carried out in the home range of single P. coxalis utilizing P-32-labelled resources. In order to manipulate biomass concentration, competitors used were smaller mayflies (Caenis sp. gr. macrura), conspecifics or larger gastropods (Planorbarius corneus and Lymnaea truncatula). Under co-occurrence conditions, food absorption of the target P. coxalis individual did not change significantly with increasing body size of a single competitor, but it was reduced to less than 5% with several small competitors. On the contrary, in conditions of temporal separation, individual absorption rate decreased significantly with the increase of the overall biomass of competitors which already exploited the resources, but it was not affected by biomass concentration. These patterns emphasize the role of spatio-temporal size-related constraints on home-range resource harvesting as a coexistence mechanism in detritivorous guilds.
The definition of a species' range is difficult as the population density of a species generally decreases gradually from the core distribution area to the edge. Species' ranges and their boundaries are scale-dependent: they vary greatly within a species depending on the spatial scale used. I studied the ranges of 50 forest passerines in Finland based on atlas maps, which contain information of species' presence in 10 x 10 km squares. I combined the information of atlas maps at different scales of resolution (10 x 10 km, 20 x 20 km, 40 x 40 km, 80 x 80 km) within 160 x 160 km blocks. I calculated a fractal dimension, D, in a log-log regression analysis between the number of inhabited squares at different scales and scales of resolution. In this analysis, D varies between 1 and 2; the fractal value of 2 implies that all the squares are inhabited by a species. In a comparison of fractal dimensions in different migratory groups short-distance migrants had the largest values in Finland. I suggest that the differences in the ranges of migratory groups are due to the ability of species to survive in the harsh, climatic conditions. Habitat generalists had the widest ranges compared to species preferring either deciduous or coniferous forests. The use of fractal dimension based on observations made at different scales of resolution gives new insight into the spatial distribution patterns of animals and plants.
Our purpose in this paper is to determine how the degree of polyphagy of different herbivorous insect species affects their yearly population variability. We assembled data from three studies on herbivorous insects: on British aphids, British moths, and Canadian Macrolepidoptera. Within each data set, we compared estimates of population variability across species, and related these differences to estimates of the degree of polyphagy. The degree of polyphagy was negatively correlated or uncorrelated with population variability, i.e., highly polyphagous species have a weak tendency to be less variable than host specialists. This result lends some support to MacArthur's (1955) argument that polyphagous species may be less susceptible to fluctuating resource levels. Population variability in monophagous or oligophagous herbivorous insects may, in part, reflect variation in resource levels. However, we have not yet evaluated the possibility that the levels of predation and parasitism suffered may affect variability even more strongly.
Early studies suggested that simple ecosystems were less stable than
complex ones, but later studies came to the opposite conclusion.
Confusion arose because of the many different meanings of `complexity'
and `stability'. Most of the possible questions about the relationship
between stability-complexity have not been asked. Those that have yield
a variety of answers.
Distribution and local abundance of 62 landbird species were measured in winter across an elevational gradient in the Huachuca Mountains of southeastern Arizona. The number of 35-m radius plots and the number of habitats occupied by the species were positively correlated with their average abundances within occupied plots and habitats. Common species were no more conspicuous than rare ones, as measured by detectability on variable distance point counts. The same species that were most abundant locally also were most abundant on Christmas bird counts across Arizona and throughout the western United States. The positive correlation between distribution and abundance of winter landbirds appear to be neither an artifact of conspicuousness, nor a consequence of the geographic scale of comparison. Rather, it seems to be an intrinsic property of the species themselves, and one that has important ecological and evolutionary implications. An individualistic approach to avian ecology is indicated, emphasizing comparisons of rare and common species.
Pteridium aquilinum was studied at sites in northern England, and New Mexico. Bracken extrafloral nectaries secrete sugars and amino acids at low and variable rates. Nectar production increased rapidly to a peak in mid-June, then declined slowly to zero in late August. Seasonal patterns of ant activity on bracken fronds usually mirrored nectar availability. Spatially, ants were often, but not always, randomly distributed among fronds within a patch. The maximum potential predation rates which ants might impose on bracken herbivores were measured. Calculated attack rates indicated a high potential impact of ants on bracken herbivore populations. Other insect species visiting extrafloral nectaries on bracken include potential natural enemies (both predators and parasitoids); 'robbers' that steal nectar; and 'robber-herbivores', with adults that feed on nectaries and larvae that are bracken-specific herbivores. The hypothesis that the nectaries facilitate a mutualistic association with ants, which in turn defend the plant against insect herbivores, is discussed. The bracken-extrafloral nectary-ant association closely resembles many other ant-extrafloral nectary systems independently evolved by other plants, where ants are known to have a defensive role. Paradoxically, despite high attack rates on introduced caterpillars, available evidence suggests that the ants have no significant effect on bracken-specific herbivores. -from Authors
Experimentally excluded ants from one member of pairs of fronds within patches of Pteridium aquilinum at Skipwith Common, North Yorks. In addition, we compared the herbivore populations of bracken patches which had naturally different ant densities; this was done at 2 sites in northern England. Although ant activity on treatment fronds was significantly reduced, there was no effect on the total number of species per frond, nor on the population sizes of most species, compared to control fronds. Only one species, the hemipteran Monalocoris filicis was unequivocally more common on fronds from which ants were excluded. Between-patch comparisons showed no differences in total herbivore species richness on sites with markedly different number of ants. Differences between the populations of individual species revealed varying patterns which were difficult to relate to ant density. It is concluded that the effects of ant predation are feeble relative to other environmental or host-plant influences acting on M. filicis; for all other bracken herbivores, ant predation effects appear to be non-existent. Alternative hypotheses for the bracken nectary-ant association are discussed. We suggest that evolutionary adaptions amongst the herbivores, in response to ant predation, may now place most of the bracken insects in 'enemy-free space'. The association may be maintained by continuing mild selection if the ants are effective against non-adapted generalist species and potential colonists. -from Authors
(1) Communities of herbivorous insects feeding on the above-ground parts of bracken (Pteridium aquilinum (L.) Kuhn) were compared at matched open and woodland sites in the north of England (Skipwith Common) and the Sacramento Mountains, New Mexico (Sierra Blanca). (2) General supporting surveys of herbivores were also conducted throughout Britain and the south-western U.S.A. (Arizona and New Mexico). (3) Twenty-seven species of insects feed on the above-ground parts of bracken fronds in Britain, with another eight possibly or occasionally doing so. Only five species were found on bracken in New Mexico, with two more in Arizona. (4) This difference in faunal richness between Britain and the south-western U.S.A. is consistent with predictions based on standard species-area relationships, given the areas within which bracken grows in the two regions. (5) The small pool of species in Arizona and New Mexico means that local communities of bracken herbivores in this region are also species-poor. Compared with communities in Britain, bracken in New Mexico has a large number of apparently vacant niches, i.e. totally unutilized, or underutilized plant-parts. (6) Despite containing a markedly impoverished number of species, there is no clear evidence either for niche-expansion, or for density compensation in the New Mexico communities. Two possible exceptions to this generalization are enigmatic. (7) These results for bracken are probably representative of phytophagous insect communities in general, in that such communities appear rarely to be saturated with species, and are not structured to any major extent by interspecific competition.
Woodland Pteridium aquilinum emerges earlier, but unshaded bracken grows more rapidly, so that by mid-season both have attained a similar stage of development. Nevertheless, abundances of some herbivore species were correlated with date of emergence, regardless of habitat. The effects of shade were examined by removing woodland trees and artificially shading open areas. Again some species responded by changing their distributions according to their known preferences for light or shade. Feeding preferences, growth and mortality rates of a sawfly caterpillar, Strongylogaster lineata, were studied on open bracken, where it is common, and on woodland bracken where it seldom occurs; type of habitat proved to have no effect on caterpillar performance. The degree of shading of the host plant thus has pronounced, but variable effects on the distribution of bracken herbivores. -from Authors
Analysis, using Audubon Society Christmas bird counts, suggests that a positive correlation between geographical range and local abundance may be an inherent property of most groups of species. The North American winter bird assemblage is dominated by widespread species that have very large total population sizes, and that are relatively abundant locally as well as on average throughout their winter ranges. -P.J.Jarvis
Theory is equivocal about invasion success, implicating various combinations of r, a population's intrinsic rate of increase, K, its `carrying capacity', enemy-imposed death rates and the coefficient of variation in population numbers as determinants of establishment. Data for a wide variety of organisms, both vertebrates and invertebrates (including insects), accidentally or deliberately introduced by man into the British Isles, appear to show that the probability of establishment of an invader is positively correlated with body size. These data are consistent with the idea that the amplitude of population fluctuations is the main determinant of invasion success, but not with theoretically expected effects of r or K (because populations of small organisms generally fluctuate more than populations of large organisms, but both r and K decrease with increasing body size). However, data for various insect orders introduced into Britain show exactly the opposite trend, with probability of establishment decreasing with increasing body size, and hence possibly with decreasing r and/or K. Possible reasons for these contradictory results, including biases in the data, are discussed. Finally, data from a variety of sources, including insects released as biological control agents, show that enemies (parasitoids and predators) are often a cause of failure to establish particular species of introduced insects. These data add further to the uncertainty about the main determinants of invasion ability. The only clear conclusion to emerge from a combination of theoretical and empirical studies is that r alone is not obviously the main, or even an important determinant of invasion success.
Re-examines the relationship of body size and density in birds by using data from many species within different areas and over a large range of body weights, and classifies these data by feeding type and habitat to test whether the relationship depends on food resources.-from Author
Habitat is the template against which evolutionary pressures fashion the ecological strategy of a species; the instability-stability habitat spectrum gives rise to the r-K-selection continuum. Habitat stability for any animal is conveniently expressed by τ/H (τ = generation time and H = the length of time the habitat remains suitable for food harvesting). In animals whose habitats have a value of τ/H approaching unity, one generation will not affect the resources available to the next. The strategy for these habitats can allow overshooting of the equilibrium. In animals with permanent habitats (τ/H very small), overshooting, with the consequent overexploitation of resources, will be selected against. The logistic equation cannot represent a situation that involves overshooting. A more realistic approach arises from the consideration of the difference equation (2) which includes a time delay between density-dependence acting and the subsequent population change. Its three basic parameters are the equilibri...
The geographic range size of congeneric species of rodents and noctuid moths is positively correlated with the amplitude of fluctuation of their local populations. This result is explained by two complementary hypotheses. First, selective species extinction may preferentially "weed out" small-ranging species with large population fluctuations because they should be shorter-lived than comparably restricted species with stable population levels. Second, compared with allied species occupying small areas, widespread species may be selected at the individual level to have relatively opportunistic life histories enabling them to exploit relatively unstable resources and habitats. This two-part interpretation makes two testable predictions which are confirmed by numerous cited literature reports: (1) wide-ranging species should more often be found in disturbed, ephemeral, or strongly seasonal habitats than their narrowly distributed relatives, and (2) widespread species should have higher reproductive potentials than their restricted congenerics. Several ecological and evolutionary implications of these findings are discussed, including their possible bearing on the controversial problem of evolutionary stasis.
Eight species of Diptera belonging to the families Anthomyiidae, Cecidomyiidae and Agromyzidae utilize bracken in different ways and show preferences for different types of bracken habitat. The annual and local variation found in populations of these mining and gall-forming herbivores in different sites in Southeast England over a four-year period are described. Possible reasons for the differences in distribution patterns are discussed, in terms of the growth and phenology of the host plant, resource availability and natural enemies.
(1) The dynamics of a predator-prey, or parasitoid-host, interaction are considered where the predator or parasitoid is a generalist whose population is buffered against changes in the particular prey being considered. (2) The interaction is then broadened to include, in addition, a specialist natural enemy, and three questions are examined within this framework. (i) Under what conditions can a specialist `invade' and persist in an existing generalist-prey interaction? (ii) How does the addition of the specialist natural enemy alter the prey's population dynamics? (c) How does the relative timing of specialist and generalist in the prey's life cycle affect the dynamics of the interaction? (3) The following conclusions emerge. (i) A specialist can invade and co-exist more easily if acting before the generalists in the prey's life cycle. (ii) A three-species stable system can readily exist where the prey-generalist interaction alone would be unstable or have no equilibrium at all. (iii) In some cases the establishment of a specialist leads to higher prey populations than existed previously with only the generalist acting. (iv) In some cases, a variety of alternative stable states are possible, either alternating between two-species and three-species states, or between different three-species states.
(1) The view that natural enemies are important in the regulation of temperate Lepidoptera populations has recently been challenged by Dempster (1983). This conclusion was based on examining twenty-four sets of life tables of Lepidoptera populations with more-or-less discrete generations, all but three of which failed to reveal natural enemies as density-dependent factors when analysed using the conventional technique of plotting some measure of proportionate mortality against population density per generation. (2) This conclusion is challenged here on the basis of how difficult it is to detect natural enemies as density-dependent factors, especially when they act together with other stochastic processes. The point is illustrated by analysing model populations in which the parasitoids are the only regulating factor. (3) The problem lies in attempting to detect density dependence that arises within a generation (e.g. by non-random parasitism between patches of different host density) from estimates of mean population size per generation. (4) The only real solution is to seek much more detailed information from within each generation, and thus to design life table sampling programmes that are stratified in the two dimensions of space and time.
We present data and analyses demonstrating that large species utilize a disproportionately large share of the resources within local ecosystems. Even though small species tend to have higher local population densities, these are not sufficient to compensate for their lower rates of energy use per individual. The relationship is very general; holding for example for birds, mammals, fish and plants. We suggest that several ecological advantages enable larger species and larger individuals within species to monopolize resources, and that the resulting selection pressures are responsible for the evolutionary trend towards increasing body size seen in many phyletic lineages. Our results contradict important studies1-4 that have concluded that species of small body size use at least as large a proportion of the resources within ecosystems as their larger relatives.
The types of social organisation displayed by the African antelope species have been assigned in this paper to five classes, distinguished largely by the strategies used by the reproductively active males in securing mating rights, and the effects of those strategies on other social castes. The paper attempts to show that these strategies are appropriate to each class because of the effects of other, ecological, aspects of their ways of life. The paper describes different feeding styles among antelope, in terms of selection of food items and coverage of home ranges. It argues that these feeding styles bear a relationship to maximum group size of feeding animals through the influence of dispersion of food items upon group cohesion. The feeding styles also bear a relationship to body size and to habitat choice, both of which influence the antelope species' antipredator behaviour. Thus feeding style is related to anti-predator behaviour which, in many species, influences minimum group size. Group size and the pattern of movement over the annual home range affect the likelihood of females being found in a given place at a given time, and it is this likelihood which, to a large extent, determines the kind of strategy a male must employ to achieve mating rights. The effects of the different strategies employed by males can be seen in such aspects of each species' biology as sexual dimorphism, adult sex ratio, and differential distribution of the sexes.
Although it is a commonplace that small animals are more abundant than large ones, few attempts have been made to quantify this and none for non-mammalian species. This study uses estimates of animal density and body mass culled from 12 journals published between 1961 and 1978 to test and extend Damuth's relationship between population density and body size of herbivorous mammals. In general, his analysis is supported, for density usually declines roughly as W
-0.75 and poikilotherms maintain higher densities than homeotherms. However the residual variation is higher than Damuth's regressions might suggest and significant differences exist among animal groups. In particular, birds maintain much lower, and aquatic invertebrates much higher abundances than a general curve for all species would suggest. Carnivores are significantly rarer than herbivores. These relationships may be used to compare the average relative contributions of species of different size to community structure and function. Such relations also provide a necessary basis both for more complete empirical analyses of the determinants of animal abundance and for the construction of more realistic conceptual models in theoretical ecology.
It has been suggested by Cohen and Newman (1985) that many of the patterns in published food webs can be derived from a stochastic model in which the species are arranged in a trophic hierarchy (the cascade model). We suggest that, if predators are larger than their prey, a trophic hierarchy can be generated on the basis of body size Empirical evidence from the literature shows that there is a positive relationship between predator and prey size for a range of invertebrates and that predators are usually larger than their prey. Using experimental data on an aquatic food web we show that body size can lead to the type of trophic hierarchy used in the cascade model, suggesting that many food web patterns may be a product of body size. This conclusion is discussed with respect to the limitations of the food web data and the relationship between static and dynamic models of web structure.
Forty communities were assembled through the controlled inoculation of algae, protozoans and a rotifer according to either of 2 distinct introduction schedules. These introduction schedules were constructed such that species reinvaded on average either every 6 or 8 w. Ten of the 20 beakers experiencing each invasion schedule contained 300 ml of media; the remaining beakers in each invasion category contained 100 ml of media. Species richness consistently increased throughout the initial 4 w and 7 w of the experiment for the low and high invasion rate beakers, respectively. The numbers of species in the low rate beakers were uncorrelated with time during the last half of the experiment; however, species richness gradually declined during this period in the high rate beakers. The extinction rate is shown to be disproportionately higher for large organisms in the small microcosms. Such selective extinction is consistent with the MacArthur-Wilson equilibrium model.
The relationship between fecundity and long-term adult fluctuations of 29 temperate noctuid moths was investigated and is explained in terms of the r- and K-selection strategies. The potential population growth rate per year (PGR) and the rate of return to equilibrium (sensu Pimm 1982) have been calculated and correlated with generation-to-generation fluctuations. A highly significant correlation was found between PGR and the coefficient of generation-to-generation variation. The r-strategy species possess the highest PGR, the highest generation-to-generation coefficient of variation and their larvae are polyphagous. Species with typical features of K-strategists exhibit the lowest PGR values and the lowest generation-to-generation coefficients of variation. Their larvae seem to be exclusively oligophagous.
Within species, population density tends to be greatest in the center of the range and to decline gradually toward the boundaries. This pattern holds over a range of spatial scales from steep environmental gradients within local regions to the entire geographic range. Exceptions include: 1) abrupt changes in abundance that usually correspond to sharp, discontinuous changes in single environmental variables; and 2) multimodal patterns of abundance caused by environmental patchiness. Among closely related, ecologically similar species spatial distribution is positively correlated with average abundance. A single general theory accounts for these observations and follows logically from 3 assumptions. 1) The abundance and distribution of each species are limited by the combination of physical and biotic environmental variables that determines the multidimensional niche. 2) Spatial variation in these environmental variables is somewhat stochastic but autocorrelated, so that nearby sites tend to have more similar environmental conditions than more distant ones. 3) Closely related, ecologically similar species differ in nor more than a very few niche dimensions. A more formal model can be developed that predicts that under these assumptions the distribution of population density over space should approximate a normal probability density distribution.-from Author
The species richness (diversity) of local plant and animal assemblages—biological communities—balances regional processes
of species formation and geographic dispersal, which add species to communities, against processes of predation, competitive
exclusion, adaptation, and stochastic variation, which may promote local extinction. During the past three decades, ecologists
have sought to explain differences in local diversity by the influence of the physical environment on local interactions among
species, interactions that are generally believed to limit the number of coexisting species. But diversity of the biological
community often fails to converge under similar physical conditions, and local diversity bears a demonstrable dependence upon
regional diversity. These observations suggest that regional and historical processes, as well as unique events and circumstances,
profoundly influence local community structure. Ecologists must broaden their concepts of community processes and incorporate
data from systematics, biogeography, and paleontology into analyses of ecological patterns and tests of community theory.
Systems of differential equations exhibiting complex periodic or chaotic behavior can sometimes be associated with one-dimensional mappings (difference equations) which encapsulate the properties of the attractor governing the full n-dimensional system. Recent advances make it possible to reconstruct the underlying attractor, and hence the one-dimensional map, from time-series data for a single state variable (species). This technique is illustrated with reference to a hypothetical system consisting of a single predator species and 2 species of victims. Trapping records for Canadian lynx Lynx canadensis suggest that the method may have application to real-world populations. -Author
A new concept is introduced to analyse species' regional distributions and to relate the pattern of distributions to niche relations. Several sets of data indicate that average local abundance is positively correlated with regional distribution, i.e. the fraction of patchily distributed population sites occupied by the species. This observation is not consistent with the assumptions of a model of regional distribution introduced by Levins. A corrected model is now presented, in which the probability of local extinction is a decreasing function of distribution, and a stochastic version of the new model is analysed. If stochastic variation in the rates of local extinction and/or colonization is sufficiently large, species tend to fall into two distinct types, termed the "core" and the "satellite" species. The former are regionally common and locally abundant, and relatively well spaced-out in niche space, while opposite attributes characterize satellite species. This dichotomy, if it exists, provides null hypotheses to test theories about community structure, and it may help to construct better structured theories. Testing the core-satellite hypothesis and its connection to the r-K theory and to Raunkiaer's "law of frequency" are discussed. /// Предлагается новая концепция дпя анализа регионального распределения видов и дпя сравнения характера распределения с соотношением ниш. Несколько серий данных показали, что величина средией локальной численности положительно коррелирует с региональньм распроистранением, то есть с относительньм количеством мозаисно расположенных видовых стаций, занятых данньм видом. Это наблюдение не соотвествует модели регионального распределения, предложенной Левинсом. Здесь предлагается исправленная модель, в которой обсуждается вероятность локального исчезновения вида, как уменьшающаяся функция распространения и стохастическая версия новой модели. Если стохастические колебания скоростей локального исчезновения и/или колонизации достаточного велики, проявляется тенденция разделения видов на два четких типа, называемых "основньм" и "сателлитньм". Первые обьины в своем регионе, локально многочисленны, а сателлитные виды характеризуются противоположньми признаками. Эта дихотомия если она существует, позволяет использвать нуль-гипотезу для проверки теории структупы сообщества и она может помочь в создании более совершенной теории. Обсуждаются результаты проверки гипотезы "основных-сателлитных" видов и ее связи с г-К теорией и биологическими спектрами Раункиера.
A central problem in ecology is to identify and understand patterns in
the distribution and abundance of species1,2. Here, we
analyse six patterns for insect populations and explore their
inter-relationships. For moths, aphids, carabid beetles, and insects
feeding on bracken, we examine links between local population abundance,
local population variability, regional distribution, and body size. We
show that population characteristics are correlated both theoretically
and empirically. Widespread species are generally locally abundant, and
have populations that fluctuate more than scarce, geographically
restricted species. Predicted effects of body size are less well
supported, although common, widespread, widely fluctuating species tend
to be small.
Biological control of insect pests is characterised by a persistent, strong reduction in the pest population following the introduction of a natural enemy. Analysis of mathematical models suggests that differential exploitation of patches of the pest in a spatially heterogeneous environment provides the most likely mechanism to account for known successes.
