Estimating abundance of cryptic but trappable animals using trapping point transects: a case study for Key Largo woodrats

Abstract

Summary1. Obtaining robust abundance or density estimates is problematic for many rare or cryptic species. We combine elements of capture–recapture and distance sampling, to develop a method called trapping point transects (TPT), and we applied this method to estimate the abundance of the endangered Key Largo woodrat (Neotoma floridana smalli).2. Trapping point transects requires two separate surveys to be held concurrently in space and time. In the main survey, the encounter rate (number of animals caught per trap per session) is measured. In the trial survey, animals whose locations are known prior to opening traps are used to estimate the detection function g(r) (the probability of capturing an animal given it is distance r from a trap when it is set), so the effective trapping area in the main survey can be estimated. It is assumed animals in the trial survey are a representative sample of all animals in the population. Individual heterogeneity in trappability is accommodated using random effects in g(r).3. Performance of two TPT estimators was assessed by simulation. Generally, when underlying capture probabilities were high [g(0) = 0·8] and between‐individual variation was small, modest survey effort (360 trap nights in the trial survey) generated little bias in estimated abundance (c. 5%). Uncertainty and relative bias in population estimates increased with decreasing capture probabilities and increasing between‐individual variation. Survey effort required to obtain unbiased estimates was also investigated.4. Given the challenges of working with cryptic, sparse or nocturnal species, we tested the validity of this method to estimate the abundance of the Key Largo woodrats between 2008 and 2011.5. Trapping point transects was found to be an effective monitoring method yielding annual estimates of the extant wild population of 693, 248, 78 and 256 animals, with CVs of 0·45, 0·55, 0·82 and 0·43, respectively. The TPT method could be adapted to a range of species that are otherwise very difficult to monitor.