Density from pellet groups: Comparing methods for estimating dung persistence time

Abstract

AbstractEffective wildlife management often relies on estimates of animal density, and cue counting is a viable estimation strategy. A key component of density estimation from dung, a form of cue counting, is estimation of the persistence time, , of dung piles. However, differences between observers on what constitutes a dung pile may alter subsequent density estimates. Additionally, many researchers studying white‐tailed deer (Odocoileus virginianus) have substituted for the number of days between the date in which 98% of deciduous trees shed leaves in autumn and field sampling. To address these 2 concerns, we compared 3 methods for estimating of white‐tailed deer pellet groups: (1) a common modelling approach based on observations from a single observer (single‐observer method), (2) a method that accommodates interobserver variation on the status of dung during field surveys (interobserver method), and (3) the days elapsed since 98% of deciduous trees shed autumn leaves (leaf‐off method). We then applied these 3 estimates to distance‐sampling data on pellet groups from white‐tailed deer that we collected along transects during 3 sampling seasons from 2019–2021 in west‐central Indiana. We estimated habitat‐ and year‐specific deer densities. Persistence probability of pellet groups varied across habitats and years, positively with age and number of pellets, and negatively with precipitation and temperature. In several instances, we found strong or marginal differences between densities estimated using the leaf‐off method and the other 2 methods. The densities using the interobserver and single‐observer methods were similar, with the latter being larger by an average of 8.0% (SE = 1.71). The latter also yielded coefficients of variation (CV) that averaged 16.6% (SE = 4.8) larger, attributable to interobserver discrepancies in scoring dung persistence. Density estimates from the leaf‐off method were 32.6% (SE = 15.3) and 37.8% (SE = 13.0) less than the density estimates from the interobserver and single‐observer methods, respectively. We encourage future researchers estimating density using multiple observers and dung sampling techniques to incorporate interobserver variation. We advocate that biologists relying on dung‐based estimation of density for white‐tailed deer abandon the conventional leaf‐off method and adopt other modelling approaches.