Habitat fragmentation and the distribution of amphibians: patch and landscape correlates in farmland

Abstract

We studied the effects of agriculturally induced fragmentation of forests and wetlands on amphibian assemblages and their distribution in a landscape of the midwestern United States. Potential breeding pools and upland areas in 30 forest patches of various sizes and degrees of isolation were intensively sampled for amphibians during April through August 1996 and March through August 1997 in Indiana. Species presence was documented using pitfall traps, anuran vocalization surveys, and cover-board sampling for adults and minnow traps and dip nets for larvae. Amphibian, anuran, and salamander assemblages were nonrandomly distributed across the landscape. American toads (Bufo americanus) and gray treefrogs (Hyla versicolor) were ubiquitous, whereas the distributions of several other species were ordered in a predictable manner. Logistic regression was used to develop predictive models of probabilities of occurrence for species in response to forest and wetland patch and landscape variables. Occurrence of redback salamanders (Plethodon cinereus) was positively associated with the area of a forest patch. Occurrence of ranid frogs was positively associated with proximity of wetlands for three of four species, and occurrences of smallmouth salamanders (Ambystoma texanum), spring peepers (Pseudacris crucifer), and western chorus frogs (Pseudacris triseriata) were related to the degree of wetland permanency. Multiple linear regression revealed that species richness was greatest for wetlands with intermediate degrees of permanency. The observed nonrandom distribution exhibited by several amphibians suggests that they respond to landscape-level attributes. Moreover, species differed substantially in the nature of their responses to fragmentation, consistent with differences in their life history and ecology.