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 (VES) (e.g., egg masses), may be biased by variability in detection probabilities and species biology (e.g., no. 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 that determine local abundance of Spotted Salamanders, Ambystoma maculatum, in Pennsylvania. We first estimated abundance for a subset of wetlands using single-season, capture–mark–recapture data and then verified egg-mass counts collected from a wider network of wetlands as an unbiased index of abundance. We found a strong correlation between estimated adult abundance and estimated egg-mass abundance with an estimated ratio of one egg mass per adult per breeding effort. 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 included effects for wetland hydroperiod and quadratic effects of mean water temperature. We also report positive, but weak, association with two co-occurring amphibian species, Jefferson Salamanders, A. jeffersonianum and Wood Frogs, Lithobates sylvaticus. 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.
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Vol. 52 • No. 2