Empirical and simulation evaluations of an abundance estimator using unmarked individuals of cryptic forest-dwelling taxa

Conservation and management programs use populations of sentinel taxa, such as stream-associated amphibians, as indicator species due to their perceived sensitivity to environmental change. Estimating population size with traditional tools such as mark–recapture estimators may be impractical for forest-dwelling stream-associated amphibians, many of which are cryptic and have low detection probabilities. In addition, sampling techniques can alter habitat conditions, particularly with repeat sampling, and compromise inferences about management impacts. We used simulated and empirical data and N-mixture models to estimate detection probabilities and abundances for two amphibian genera, giant Dicamptodon and torrent Rhyacotriton salamanders, sampled with a less-invasive approach than other methods. We surveyed forested headwater streams located in western Washington, USA, for salamanders 7 July–27 August 2008. We assessed model sensitivity to changes in animal abundance (5 and 15) and detection probability (0.05–0.5), as well as study design alternatives including number of sample plots (50–150) and number of sampling visits (2–4). We also evaluated the effects of stream temperature and stream order on detection probability using data collected from forested streams in Washington, USA. Precision of detection probability estimates improved as the number of plots and sampling occasions increased. Variability of estimated population sizes decreased with higher detection probability, although species abundance had little effect on precision of detection probability estimates. Detection probability estimated from empirical data ranged from 0.07 to 0.65 for giant salamanders and 0.06–0.67 for torrent salamanders. Giant salamander detection probability was positively associated with stream temperature regardless of stream order, and was higher in second- and third-order streams than first-order streams. Detection probability for torrent salamanders varied with stream temperature, order, and the interaction of those covariates, with detection increasing with temperature for second- and third-order streams but showing a flat or decreasing trend for first-order streams. N-mixture models, paired with careful consideration of study design alternatives, can produce robust estimates of abundance and obviate the traditional reliance on indices of relative abundance for many rare and sensitive taxa. Use of a less invasive technique for sampling stream-associated amphibians resulted in sufficient animal captures for suitable model performance. Our simulation results can inform study design and direct efficient allocation of field effort.

► We model amphibian detection probabilities and abundances.
► We assess model sensitivity to changes in parameters and design alternatives.
► Giant salamander detection probability was associated with stream temperature.
► Torrent salamander detection probability varied with stream temperature and order.
► Simulation results inform study design feasibility and allocation of field effort.