OVERVIEW

The textbook description of amphibian community structure has been based on keystone predation.  This concept relies on some sort of gradient over which the intensity of predation varies, encouraging reduction or elimination of dominant competitors in some locations and allowing dominant competitors to exclude inferior competitors in others. 

Our long-term research in Michigan (since 1988) and Connecticut (since 1996), largely conflicts with this view [abstract].  Instead of assemblages of strong interactors, it appears that larval amphibians in natural populations typically do not compete intensely and there is little evidence that predators result in exclusion from nonpermanent wetlands.  In the language of metacommunity theory, it appears much more like these amphibians may exist in layers of single species metapopulations.

Nevertheless, amphibian distributions are strongly patterned.  Where do these patterns come from?  The permanence of a wetland and its light and thermal environment appear to be key variables.  Repeated cohort loss from drying can lead to local extinction of a species.  The time constraint imposed by drying can be enhanced when vegetation grows up around wetlands.  Shading leads to lower temperatures and slower developmental rates.  Vegetation also evapotranspires.  In spite of any contrary intuitions, a given wetland heavily shaded by forest vegetation will dry faster than the same which have little vegetation near the basin.  The synergistic action of these factors can cause populations to go extinct.  Similarly, it appears that disturbances that remove vegetation and alter hydrology can promote population establishment.  The likelihood of establishment can be related to the proximity of source populations and the structure of the intervening terrestrial environment.

This research is important because, as with rocky intertidal systems, further research has shown that "post-settlement" mechanisms initially used to describe community structure now need to be revised and expanded.  For amphibians, linkages among breeding ponds and the dynamics of the terrestrial ecosystems that surround breeding ponds have demonstrable impacts on amphibians that may often trump the impacts of species interactions initially used as a basis for understanding amphibian communities.