Clarifying Assumptions Behind the Estimation of Animal Density From Camera Trap Rates

摘要

Foster and Harmsen (2012) presented a useful review of density estimation from camera trap data, concluding with an important call for increased rigor and transparency in study design. Although most of their review focuses on mark-recapture and mark-resight analyses, they also briefly critique a recent effort to estimate density of species that cannot be recognized individually, the random encounter model (REM; Rowcliffe et al. 2008). Here, we explain that this critique follows from a misrepresentation of sources of bias in the REM. First, Foster and Harmsen (2012) state that the REM assumes that animals move randomly and independently of one another. This assumption was mathematically convenient to derive a formula linking density with trap rate (Rowcliffe et al. 2008). However, Hutchinson and Waser (2007) demonstrated analytically that violations of this assumption do not bias expected contact rates as a function of density, and Rowcliffe et al. (2008) provided empirical evidence that field results are also unbiased in practice. Thus, the REM is not sensitive to nonrandom or non-independent movement of animals.