Passive acoustic methods for surveying visually cryptic frog species

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Passive acoustic survey methods have great potential for assessing cryptic species that are vocally active. The tiny Moss Frogs (genus Arthroleptella) that inhabit seepages on remote mountain tops in the Western Cape of South Africa are a case in point. Many species are restricted to individual mountains and most are on the IUCN Red List. Surveys are prohibitively expensive because each individual capture involves three to four person-hours of searching. However, males can be heard calling throughout the winter. Using a portable recorder, we gathered acoustic data from which abundance can be estimated and populations monitored. This involves estimation of both the spatial "footprint" of the acoustic array (i.e., what area it effectively surveys) and animal vocalization rates. Ideally both should be estimated as an integral part of the survey, rather than obtained from outside the survey. By combining recently developed mark-recapture methods that take explicit account of the spatial location of microphones, with data on acoustic signal strength and/or time-difference-of-arrival at different microphones, it is possible to do this. We describe the survey and estimation methods, which have many other potential applications. In the case of these frogs, acoustic survey is hundreds of times more efficient than methods involving physical capture.