How many years of acoustic monitoring are needed to accommodate for anuran species turnover and detection?

Abstract

Anuran species are threatened by a range of anthropogenically induced disturbances, highlighting the need for robust and reliable monitoring programs to detect trends in species occupancy. Anuran species experience not only differences in seasonal distributions of breeding activity, but also interannual variations in habitat occupancy which are dependent on stochastic environmental processes. Acoustic calling surveys are a widely used and reportedly reliable technique for detecting a range of anuran species in Southern Ontario. Short-term applications of this technique may be challenged to detect species which are temporarily absent from suitable habitats due to natural extirpation processes. In this paper, I analyzed anuran occupancy data from 67 long-term acoustic monitoring sites across Southern Ontario to determine the relationship between accumulated species occupancy and number of monitoring years. Data was taken from Birds Canada’s Marsh Monitoring Protocol database for amphibians and filtered to extract sites which were monitored for at least 8 years and recorded at least 4 anuran species. Species accumulation curves were created using EstimateS. Results suggest that there may be detectability issues for some species such as the pickerel frog, wood frog, and western chorus frog. Interannual detection rates and species accumulation curves point to the need for long-term (> 5 years) monitoring programs to reliably detect on average more than 90% of anuran species occupying a monitoring site. They also reflect well-documented reports of high species turnover in anuran metapopulations, suggesting that extirpation-recolonization processes are widespread and frequent. Monitoring programs should address this issue by lengthening monitoring programs, particularly those that aim to document changes to species occupancy before and after major habitat disturbances.