Quantifying effects of roads on amphibians that breed in vernal ponds

Many amphibians spend most of their time feeding and resting in terrestrial habitats but migrate to fishless vernal ponds for breeding and larval development.  These biphasic species may be particularly vulnerable to roads near breeding ponds because they can disrupt migration corridors.  We sampled 171 vernal ponds over a 3-yr period in the Delaware Water Gap National Recreation Area for the presence of three amphibian species (Ambystoma maculatum, A. opacum, and Rana sylvatica).  We developed a graphical approach within a GIS environment to quantify several important aspects of roads within 500-m buffers around ponds including road density and road directionality (number of compass bearings intersected by roads).  We used logistic regression to determine the relative importance of derived road statistics on the probability of occurrence for each species.  We found that road density within buffers negatively affected all three species, but directionality was particularly important for wood frogs (Rana sylvatica). We also found that the distance of roads from pond boundaries was important in some cases.