Large-scale field patterns are a fundamental source of inferences on processes responsible for variation in species richness among habitats. We examined species richness of larval amphibian communities in 37 ponds over seven years on the Univ. of Michigan's E. S. George Reserve. Ordination of the community incidence matrix indicated a strong major axis of variation in species associations that was correlated with pond hydroperiod, surface area and forest canopy cover. Communities were significantly nested with those species found in ponds with high canopy cover, small area and short hydroperiod being nested subsets of those found in ponds with contrasting characteristics. Presence of fish had strong negative effects on species richness; relaxation of this effect also was apparent when fish were extirpated from ponds by drought. We employed a model selection analysis to identify the most appropriate statistical model for predicting the long-term average species richness of these ponds from local abiotic and biotic (predator and competitor density) factors. A model including only the abiotic factors was overwhelmingly superior for the anurans; hierarchical partitioning indicated that area and canopy cover alone accounted for over 70% of the independent effects of predictor variables. The global model including both abiotic and biotic factors was the best supported model for the caudates, and correspondingly hierarchical partitioning suggested that area, hydroperiod, invertebrate predators and caudate biomass all accounted for 9 Á16% of the independent effects. Overall, biotic factors accounted for much less of the variation in species richness than abiotic factors. The patterns in larger, open-canopy ponds provided little evidence of competitive effects on species richness, though there were patterns consistent with competitive effects in small, closed-canopy ponds. The unusual temporal and spatial extent of these data enabled us to critically evaluate ideas regarding patterns in larval amphibian communities, and the effects of area, disturbance (hydroperiod) and productivity (canopy cover) on species richness of these communities. These results have important implications to the conservation of amphibian species richness in freshwater wetlands, which are among the most threatened ecosystems worldwide.Understanding processes responsible for variation in species richness among habitats is a fundamental goal of ecology, and large-scale field patterns are a fertile source for hypotheses concerning these processes. Such pattern analysis has a venerable history in ecology, in particular analyses of the distributions of both individual taxa and species richness along environmental gradients. These patterns provide inferences concerning the traits of species associated with performance differences along the gradients, and the factors affecting species coexistence and community assembly (Connell 1975, Ricklefs and Schluter 1993, Wellborn et al. 1996.Moreover, in contrast to the conduct of much of experimental ecology, suc...

Large-scale field patterns are a fundamental source of inferences on processes responsible for variation in species richness among habitats. We examined species richness of larval amphibian communities in 37 ponds over seven years on the Univ. of Michigan's E. S. George Reserve. Ordination of the community incidence matrix indicated a strong major axis of variation in species associations that was correlated with pond hydroperiod, surface area and forest canopy cover. Communities were significantly nested with those species found in ponds with high canopy cover, small area and short hydroperiod being nested subsets of those found in ponds with contrasting characteristics. Presence of fish had strong negative effects on species richness; relaxation of this effect also was apparent when fish were extirpated from ponds by drought. We employed a model selection analysis to identify the most appropriate statistical model for predicting the long-term average species richness of these ponds from local abiotic and biotic (predator and competitor density) factors. A model including only the abiotic factors was overwhelmingly superior for the anurans; hierarchical partitioning indicated that area and canopy cover alone accounted for over 70% of the independent effects of predictor variables. The global model including both abiotic and biotic factors was the best supported model for the caudates, and correspondingly hierarchical partitioning suggested that area, hydroperiod, invertebrate predators and caudate biomass all accounted for 9 Á16% of the independent effects. Overall, biotic factors accounted for much less of the variation in species richness than abiotic factors. The patterns in larger, open-canopy ponds provided little evidence of competitive effects on species richness, though there were patterns consistent with competitive effects in small, closed-canopy ponds. The unusual temporal and spatial extent of these data enabled us to critically evaluate ideas regarding patterns in larval amphibian communities, and the effects of area, disturbance (hydroperiod) and productivity (canopy cover) on species richness of these communities. These results have important implications to the conservation of amphibian species richness in freshwater wetlands, which are among the most threatened ecosystems worldwide.Understanding processes responsible for variation in species richness among habitats is a fundamental goal of ecology, and large-scale field patterns are a fertile source for hypotheses concerning these processes. Such pattern analysis has a venerable history in ecology, in particular analyses of the distributions of both individual taxa and species richness along environmental gradients. These patterns provide inferences concerning the traits of species associated with performance differences along the gradients, and the factors affecting species coexistence and community assembly (Connell 1975, Ricklefs and Schluter 1993, Wellborn et al. 1996.Moreover, in contrast to the conduct of much of experimental ecology, such...

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The growth of metacommunity ecology as a subdiscipline has increased interest in how processes at diVerent spatial scales structure communities. However, there is still a signiWcant knowledge gap with respect to relating the action of niche-and dispersal-assembly mecha nisms to observed species distributions across gradients. Surveys of the larval dragonXy community (Odonata: Ani soptera) in 57 lakes and ponds in southeast Michigan were used to evaluate hypotheses about the processes regulating community structure in this system. We considered the roles of both niche-and dispersal-assembly processes in determining patterns of species richness and composition across a habitat gradient involving changes in the extent of habitat permanence, canopy cover, area, and top predator type. We compared observed richness patterns and species distributions in this system to patterns predicted by four general community models: species sorting related to adap tive trade-oVs, a developmental constraints hypothesis, dis persal assembly, and a neutral community assemblage. Our results supported neither the developmental constraints nor the neutral-assemblage models. Observed patterns of rich ness and species distributions were consistent with patterns expected when adaptive tradeoVs and dispersal-assembly mechanisms aVect community structure. Adaptive trade oVs appeared to be important in limiting the distributions of species which segregate across the habitat gradient. How ever, dispersal was important in shaping the distributions of species that utilize habitats with a broad range of hydroperi ods and alternative top predator types. Our results also sug gest that the relative importance of these mechanisms may change across this habitat gradient and that a metacommu nity perspective which incorporates both niche-and dis persal-assembly processes is necessary to understand how communities are organized.

Ecologists have long realized that stable species richness values can mask rapid turnover in species composition. Because turnover occurs as a consequence of both local and regional processes, understanding the responsible factors provides insight on processes influencing community structure at different scales. Despite the insights to be gained from data on species turnover, they remain relatively uncommon. We present data on the interannual turnover in species composition of larval amphibian communities in 37 ponds over seven years. Species composition of a given pond community was highly dynamic; about half of the species that could be found breeding in a particular pond were actually present in a given year. All species participated in this community turnover, but to different degrees. Using a model selection approach, we show that a statistical model including local environmental factors (pond area, hydroperiod, and canopy cover) and pond connectivity on the landscape provided the best predictions of turnover. Averaged parameter estimates were significant for area, hydroperiod, and connectivity and these same variables were identified by hierarchical partitioning as having significant independent effects on turnover. Area and hydroperiod were negatively related to turnover, whereas connectivity was positively related to turnover. Additionally, the average fraction of years a species was present in a pond was positively correlated with average local population size, but even more strongly correlated with regional population size, suggesting both local and regional influences on turnover. Of the measured biotic factors (biomass of fish, invertebrate predators, anuran and caudate larvae), presence of fish was the only factor that significantly affected rates of turnover. Several mechanism could be responsible for changes in species composition (species extinctions, skipped breeding and movement of choruses), but extinctions appear to be the major cause of turnover. These results have important implications to understanding long-term persistence of species on landscapes and the causes of patterns in species richness on environmental gradients.Ecologists have a longstanding interest in the causes of patterns in species richness and composition of communities. Early observations suggested that, while species richness could be relatively stable in an environment, membership of the community could be dynamic. For example, theories of island biogeography and the effects of disturbance provided clear, early statements that a colonization/extinction dynamic may underlie relative stability in species richness across habitats that differ in characteristics such as area, isolation, or disturbance (MacArthur and Wilson 1967, Connell 1978). More recently, interest in the regional processes contributing to population persistence and the structure of ecological communities (Hanski 1999, Hubbell 2001 has further emphasized the importance of temporal turnover. This interest, however, also has highlighted the lack of long-term ...