Robust methods for estimating abundance of wetland-breeding amphibian species such as mark-recapture are often resource-intensive. This limits our ability to study the processes that influence species abundance. Alternatively, more efficient sampling methods, such as indices based on visual encounter surveys (e.g., egg masses; VES), may be biased by variability in detection probabilities and species biology (e.g., # of egg masses per female). We combine data sources (i.e., VES and capture-mark-recapture) to provide an efficient technique for monitoring wetland-breeding amphibians. Our study focuses on understanding factors determining local abundance of Spotted Salamanders, Ambystoma maculatum, in Pennsylvania. Our goals for this study were two-fold. First, we validate the use of visual encounter surveys to index adult population abundance for A. maculatum. To achieve this, we compared adult CMR abundance estimates and egg mass count data collected using double-observer VES. Second, we determine the abiotic and biotic characteristics that best explain the observed variation in our egg mass counts after accounting for imperfect detection.
Results/Conclusions
We found a strong relationship between estimated female abundance and estimated egg mass abundance (Beta[95% CI] = 1.06[0.92, 1.20]; P < 0.001; R2adj = 0.95) with an estimated ratio of 1 egg mass per adult per breeding effort. The estimated relationship between male abundance and estimated egg mass abundance (Beta = 1.25[0.91, 1.59]; P < 0.001; R2adj = 0.82) indicates an egg mass to adult male ratio of approximately 1.25:1. We next determined the factors that best explained variation in estimated A. maculatum egg mass abundance and consequently, adult abundance among sites. Our ‘best-fit’ model relating estimated egg mass density to abiotic and biotic site characteristics included effects for wetland hydroperiod (Beta= 0.534[0.418, 0.650]), density of Jefferson Salamanders, A. jeffersonianum (Beta = 0.046[0.038, 0.054]) and Wood Frogs, L. sylvaticus (Beta = 0.051[0.043, 0.059]) egg masses as well as quadratic effects of mean water temperature. We demonstrate how combining sampling approaches can provide efficient abundance estimates in wetland ecosystems. In particular, positive co-occurrence among species indicates shared habitat preferences that may enable us to predict the presence of difficult-to-detect species using only VES.