Detectability of amphibians in Pacific northwest forests

Forest management strategies may have significant effects on populations of forest-dependent species. In the US Pacific Northwest, studies of the effects of alternative forest management practices on amphibians are clouded by the uncertain detectability relative to species and survey method. Concern over recent declines in amphibian populations has highlighted a need for improved detectability for monitoring. In a mark-recapture study in western Oregon, we examined amphibian detectability among three headwater forest assemblages: instream, bank-dwelling, and upslope forest amphibians. Instream and stream-bank habitats along 50-meter reaches of four first-order headwater streams were hand sampled during the 2007 spring season, and animals were marked using alphanumeric fluorescent tags. In the adjoining upslope forest habitat, artificial cover boards were used to monitor the terrestrial amphibian assemblage from spring 2006 to spring 2007. Instream, bank and upslope detection probabilities and relative abundances were calculated for each of dominant amphibian species detected including coastal giant salamander (Dicamptodon tenebrosus), ensatina (Ensatina eschscholtzii), Dunn’s salamander (Plethodon dunni), western red-backed salamander (Plethodon vehiculum), and southern torrent salamander (Rhyacotriton variegatus). Detection probability varied interspecifically as well as among assemblages. Upslope salamander detections varied considerably. For several species, low mobility likely contributed to relatively higher capture probabilities. Our results suggest current hand-sampling methods may be sufficient to monitor headwater stream and bank amphibian populations, but cover boards may require multiple surveys to improve estimates of amphibian numbers.