Evaluation of camera trap‐based abundance estimators for unmarked populations

Abstract

Estimates of species abundance are critical to understand population processes and to assess and select management actions. However, capturing and marking individuals for abundance estimation, while providing robust information, can be economically and logistically prohibitive, particularly for species with cryptic behavior. Camera traps can be used to collect data at temporal and spatial scales necessary for estimating abundance, but the use of camera traps comes with limitations when target species are not uniquely identifiable (i.e., “unmarked”). Abundance estimation is particularly useful in the management of invasive species, with herpetofauna being recognized as some of the most pervasive and detrimental invasive vertebrate species. However, the use of camera traps for these taxa presents additional challenges with relevancy across multiple taxa. It is often necessary to use lures to attract animals in order to obtain sufficient observations, yet lure attraction can influence species’ landscape use and potentially induce bias in abundance estimators. We investigated these challenges and assessed the feasibility of obtaining reliable abundance estimates using camera‐trapping data on a population of invasive brown treesnakes (Boiga irregularis) in Guam. Data were collected using camera traps in an enclosed area where snakes were subject to high‐intensity capture–recapture effort, resulting in presumed abundance of 116 snakes (density = 23/ha). We then applied spatial count, random encounter and staying time, space to event, and instantaneous sampling estimators to photo‐capture data to estimate abundance and compared estimates to our presumed abundance. We found that all estimators for unmarked populations performed poorly, with inaccurate or imprecise abundance estimates that limit their usefulness for management in this system. We further investigated the sensitivity of these estimators to the use of lures (i.e., violating the assumption that animal behavior is unchanged by sampling) and camera density in a simulation study. Increasing the effective distances of a lure (i.e., lure attraction) and camera density both resulted in biased abundance estimates. Each estimator rarely recovered truth or suffered from convergence issues. Our results indicate that, when limited to unmarked estimators and the use of lures, camera traps alone are unlikely to produce abundance estimates with utility for brown treesnake management.

Evaluating sources of bias in pedigree‐based estimates of breeding population size

“…This may pose a challenge in systems in which N S is unknown, when juveniles are difficult to sample, or current marker panels have limited information content for resolving pedigrees. However, in situations with such high uncertainty, pedigree‐based estimates of N S may still be more informative and logistically feasible than other approaches, particularly when detectability is too low to meet assumptions of traditional modeling frameworks (Amburgey et al, 2021). The strength of the pedigree‐based approach lies not only in the use of juvenile genotypes, which may be easier to obtain than data on the adult population, but also in the ability to use simulation analysis to objectively evaluate how critical uncertainties influence the magnitude and direction of bias in $$$ \hat{N} $$$ S .…”

“…This generally requires individuals to be physically observed at least once, with many estimators requiring repeated capture events. Although the assumption of detectability is generally not difficult to meet for most populations, it can be preclusive for studies of threatened and endangered species, species with complex life histories, and populations located in habitats that are difficult or hazardous to sample (Amburgey et al, 2021; Luikart et al, 2010). In such scenarios, population size estimates may be too uncertain to be of practical use for conservation (Couturier et al, 2013).…”

Occurrence and Abundance of an Apex Predator and a Sympatric Mesopredator in Rural Areas of the Coastal Range of Southern Chile

“…Camera trapping of unmarked species can be challenging, as it is difficult to use capture-recapture methods when assessing their relative abundances and could have biased inference estimating abundances [85,86]. In our study, the two carnivore species were unmarked, thus we assumed equal detectability and potential bias, as camera traps cannot record all animal presences in an area [87].…”

Animal reactivity to camera traps and its effects on abundance estimate using distance sampling in the Taï National Park, Côte d’Ivoire

“…Unbiased estimates of species density and abundance are critical to understand population evolutionary processes in order to assess and select conservation management actions ( Rowcliffe et al, 2011 ; Crum, Neyman & Gowan, 2021 ; Amburgey et al, 2021 ).…”