Patterns of nestedness and species association in a pond-dwelling amphibian fauna

The pattern of nested species subsets has been reported for many taxa in naturally or anthropogenically fragmented habitats. Of continued interest to ecologists is whether nestedness is mure frequently and strongly associated with either selective extinction or selective colonization. We studied patterns of amphibian incidence at 118 ponds in southwestern Ontario from 1992 to 1994. Our objectives were to determine if individual species, groups of species, or the entire fauna was nested, and at what spatial scales, to examine patterns of species association, and to evaluate causal hypotheses. The entire fauna was significantly nested at the geographic, region and sub-region scales. Most species were highly nested, but differences occurred among regions for some species. Species grouped by their requirement for woodland habitat or susceptibility to fish predation showed a high degree of nestedness. Historic deforestation and introduction of predatory fish are the likely mechanisms of extinction in this fauna. Species grouped as ''good dispersers'' were less nested than ''poor dispersers''. Species incidence was positively correlated with potential dispersal abilities. Our results suggest the importance of both selective extinction and selective colonization in contributing to the degree of nestedness in this fauna. Nested patterns may be the rule for faunas having high turnover on mainlands. Species poor sites were dominated by the same assemblage of three species, suggesting that single large reserves are preferable to several small reserves for conservation of temperate zone pond-dwelling amphibian assemblages.