High‐density camera trap grid reveals lack of consistency in detection and capture rates across space and time

Abstract

Counts of independent photo events from camera traps are commonly used to make inference about species occupancy, the density of unmarked populations, and the relative abundance of species across time and space. These applications rest on the untested assumption that data collected from individual cameras are representative of the landscape location in which they are placed, and that nearby cameras would record similar data when any additional micro‐site differences are accounted for. We established a high‐density camera trapping grid (100 × 100 m; 27 cameras) in Virginia, USA, to explicitly test these assumptions, investigating variation in capture rates and detection probabilities for a range of terrestrial mammals during four 2‐month seasonal surveys. Despite controlling for numerous habitat and placement factors, we documented, across all 5 focal species, large ranges and coefficients of variation in both capture rate and detection probabilities, which were similar to those seen across 2 sets of independent forest sampling sites from a larger, more typical camera trap sampling design. We also documented a lack of spatial autocorrelation in capture rate at any distance. Measured local covariates relevant to the camera viewshed (stem density, camera height, log presence, effective detection distance [EDD], total dbh of oak trees) rarely explained any significant portion of observed variation in capture rates or detection probabilities across the grid. The influence of EDD, measured here for the first time for individual camera stations, was inconsistently important and varied in direction of effect depending on species and season. Our study indicates single‐camera stations may fail to sample animal presence and frequency of use in a robust and repeatable way, primarily resulting from the influence of both idiosyncrasies in animal movement and measured and unknown micro‐site characteristics. We recommend spatial replication within sites (e.g., small‐scale shifting of cameras or use of multiple stations) should be considered to minimize impacts of relevant micro‐site characteristics, some of which may be difficult to identify.

Effects of camera‐trap placement and number on detection of members of a mammalian assemblage

“…Our study confirms an increasing body of work showing that microsite characteristics, whether measurable with covariates or not, have a large influence on detection probabilities of species on CTs (Kolowski and Forrester 2017, Evans et al 2019, Kolowski et al 2021. Several species had different detection probabilities at microsites that were defined by different features (forest roads, wildlife trails, or cliff sides), likely due to differences among species in their microsite selection.…”

“…We also found contrasting results across species of the effects of visibility in front of the CT on detection probabilities. These are also likely related to differences in habitat selection and movement strategy among the species (Kolowski et al 2021) rather than differences in PIR-sensor sensitivity (Rowcliffe et al 2011, Hofmeester et al 2017. Movement of lynx, red foxes, mountain hares, and red squirrels, as predators and small prey, are more channeled toward sites with a short line of sight (many obstructions) compared to roe deer, which could explain the contrasting patterns we found between these species.…”

“…2019, Kolowski et al. 2021). Several species had different detection probabilities at microsites that were defined by different features (forest roads, wildlife trails, or cliff sides), likely due to differences among species in their microsite selection.…”

“…These are also likely related to differences in habitat selection and movement strategy among the species (Kolowski et al. 2021) rather than differences in PIR‐sensor sensitivity (Rowcliffe et al. 2011, Hofmeester et al.…”

Empirical evaluation of the spatial scale and detection process of camera trap surveys

“…Most practitioners presume their results are representative of large, vaguely-defined areas, but some studies suggest otherwise. For example Kays et al [19] found little spatial autocorrelation in detection rates past 25 m for five Central American mammal species and Kolowski et al [20] found no spatial autocorrelation at any scale, suggesting that cameras may represent small areas. While it might be safe to extend the inference of species presence directly in front of a camera trap to larger scales (e.g.…”