The effects of survey protocol on detection probabilities and site occupancy estimates of summer breeding anurans

Recent declines in amphibian populations have created an urgent need for large-scale, long- term monitoring efforts and many anuran monitoring programs have been established that utilize calling surveys. Calling surveys can be effective monitoring tools; however, differences among survey protocols may bias survey results. Failure to take into account detection probabilities when monitoring anurans can lead to inaccurate inferences about site occupancy, since non-detections in survey data do not necessarily mean that a species is absent unless the probability of detection is 1. We used a likelihood-based method, in the form of the computer program PRESENCE, to estimate detection probabilities and site occupancy rates for summer-breeding anurans in the Western Piedmont of North Carolina. Using detection data from calling surveys, we evaluated how detectability and site occupancy for five anuran species were influenced by 1) time spent listening at each site, 2) number of surveys per site, and 3) sample- and site-specific covariates. We found considerable variation among species with regards to detection probability and site occupancy across survey duration and sampling occasion. Although 13% of all species detection occurred after 3 min, longer surveys did not significantly increase detectability of individual species. We found that detectability varied more with sampling occasion than with survey duration for each species. Covariates had differing effects on occupancy and detectability among individual species. Multiple surveys per site within a season are necessary to eliminate biased detection probabilities, but we found that 3- or 5-min surveys were adequate for detecting all species breeding at the time of the survey.