Occupancy estimates of spring breeding amphibians in a landscape fragmented by agriculture

Background/Question/Methods

Research efforts in amphibian conservation have increased over the past decade due to the sensitivity of amphibians to various anthropogenic disturbances. We undertook a two-year study to examine the rates of pond occupancy for spring breeding amphibians in a fragmented agricultural landscape. Using program PRESENCE 3.0, we investigated the relationship between amphibian occupancy and environmental variables using an information-theoretic approach.  In 2008 and 2009, we sampled 57 ponds using double-throated collapsible minnow traps for four nights each during the early spring breeding season. We used a two-step process to address a priori hypotheses on amphibian occupancy of breeding ponds. First, we modeled sampling covariates we predicted would affect detection probabilities (precipitation, average temperature, maximum temperature, time of survey, or constant), while holding site occupancy constant. Second, we tested the a priori occupancy models simultaneously with the best model for detection probability incorporated. We were able to model occupancy for seven of eight spring pond-breeding amphibians that occur in the region: Ambystoma jeffersonianum (Jefferson Salamander), A. texanum (Small-mouthed Salamander), A. tigrinum (Tiger Salamander), Pseudacris crucifer (Spring Peeper), P. triseriata (Western Chorus Frog), Rana sphenocephala (Southern Leopard Frog), and Rana sylvatica (Wood Frog). 

Results/Conclusions

The presence of predatory fish was significant in determining the likelihood of occupancy for all species except R. spenocephala and, though non-significant, there is a noticeable negative relationship between the presence of fish and the probability of occupancy by R. sphenocephala. Occupancy rates for P. crucifer, P. triseriata, and R. sphenocephala significantly increased with distance to row-crop agriculture. Occupancy rates increased with increasing canopy cover for A. jeffersonianum and A. texanum and there was an apparent trend for increased occupancy rates by R. sylvatica with increasing canopy cover. Our results indicate that anthropogenic disturbances such as fish stocking and agricultural fragmentation of the landscape significantly affect amphibian occupancy rates. Further, our results provide insight into the environmental variables important in managing for spring breeding amphibians by means such as pond and habitat restoration and/or creation.