Regional dynamics and the status of amphibians

Increasing attention is being devoted to the dynamics of interacting local populations of species, especially the role of changes in incidence that affect regional persistence. The status of species in a regional context may be determined more by metapopulation dynamics than by purely demographic birth and death processes. The contrast between local and regional views of species' persistence is illustrated today by discussion of the status of amphibians. The amphibian fauna may provide an important indicator of the impact of anthropogenic disturbance to wetland ecosystems. We assessed the status of 11 amphibian species in southwestern Ontario, Canada, by estimating species richness, changes in presence and absence, and incidence at 97 ponds from 1992 to 1994. We detected a significant reduction in amphibian species richness in one of three regions. This loss of diversity relative to the historical species complement is a consequence of the land use history. We observed surprisingly high turnover of species at ponds, with increased incidence varying from 0.07 to 0.29 species per pond per year, and decreased incidence ranging from 0.16 to 0.30 species per pond per year. The incidence of common species across years included both declines (leopard frog, Rana pipiens) and increases (American toad, Bufo americanus). For eight relatively rare species, losses exceeded gains between 1992 and 1993, but this pattern was reversed between 1993 and 1994. Understanding environmental factors that determine the status of species will require an expanded, large-scale view of groups of populations (metapopulations) and their spatial dynamics.