| DISTRIBUTIONS OF POND-BREEDING ANURANS: AN OVERVIEW OF MECHANISMS |
DAVID K. SKELLY A1
A1 School of Forestry & Environmental Studies, Department of Ecology & Evolutionary, Biology, Yale University, 370 Prospect Street, New Haven, Connecticut 06520 USA
Concern over the fate of amphibians has led to increasing attention on the mechanisms that underly their distribution. Among pond-breeding anurans, surveys have revealed that most species are restricted to a minority of ponds, but distributions can shift over time. The mechanisms for these patterns fall into two broad classes: variation in environmental attributes among ponds, and the impact of spatial context surrounding ponds.
There is abundant evidence that embryonic and larval anurans are sensitive to variation in a large number of abiotic and biotic variables. In some cases these effects on performance have been linked to variation among natural ponds, and compelling evidence suggests that anuran distributions may be restricted by the action of local conditions. While these explanations are often discussed in terms of single variates (e.g., hydroperiod), field experiments suggest that elimination will often result from the joint action of multiple factors (e.g., low pH and high metal concentrations). While variation among ponds is usually considered in terms of the direct impacts on the survival, growth, and development of offspring, there is mounting evidence that adult behavior may regulate distributions via the selection of oviposition sites.
Spatial context for ponds can impact anuran distributions in distinct ways. Metapopulation models suggest that pond isolation may influence patterns of occupancy, a hypothesis supported by survey data. In addition, most pond-breeding anurans utilize terrestrial uplands as adults. The amount and quality of terrestrial habitats also can impact distributional patterns. Thus, via either mechanism, spatial context can impact distributional pattern in the absence of any impact due to between-pond variation.
Current evidence suggests that the restricted yet dynamic nature of anuran distributions results from factors operating within as well as surrounding breeding ponds. The dynamic natures of both ponds and surrounding uplands appear to have sizable impacts on distributional patterns. Recognition of this dynamism has critical importance for the development of effective conservation measures for anurans.
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