Camera-based badger density estimation using the REM, CT-DS, and SMR

Accurate and precise assessment of population density plays a critical
role in effective wildlife management, but reliable estimates are often
difficult to obtain. Camera traps have emerged as valuable non-invasive
tools for studying elusive species, offering cost-effective solutions for
both marked and unmarked populations. We evaluated the
consistency of badger (Meles meles) density estimates obtained from the
random encounter model (REM) and camera trap distance sampling (CT-DS)
with independent estimates from spatial mark-resight (SMR) models and
quantified the bias in CT-DS arising from animals reacting to camera
traps. Six camera trap surveys were conducted in Cornwall, UK, in 2019 and
2021, and data were used to estimate badger density using the REM and
CT-DS. Four sites were included in a badger vaccination research project,
providing an opportunity to mark badgers with uniquely identifiable fur
clips to facilitate resighting within an SMR framework. We found
consistency in the density estimates across all methods, but the results
had wide confidence intervals. Density estimates derived from CT-DS tended
to be higher than those from the REM and were sensitive to the exclusion
of reactive sequences, resulting in a two-fold decrease in the estimated
density in one case. The REM tended to be the most precise method;
however, where badger density was low, precision was low using all
methods. Our findings suggest animal density can be assessed from camera
traps in the absence of individual identification; however, it is
important to account for reactive behaviours, especially where such
behaviour is prevalent. In these circumstances, we recommend utilising the
REM which offers a clear methodology for addressing bias arising from
reactive sequences. In addition, we emphasise the need for improved
precision to ensure the effectiveness of these methods in the context of
wildlife management. We offer practical considerations to facilitate the
broader application of these methods, drawing upon the example of disease
control through badger vaccination.