Optimising the cost of roadkill surveys based on an analysis of carcass persistence

Reliable estimates of wildlife mortality due to wildlife-vehicle collisions are key to understanding its impact on
wildlife populations and developing strategies to prevent or reduce collisions. Standardised approaches for
monitoring roadkill are needed to derive robust and unbiased estimates of mortality that are comparable across
different study systems and ecological contexts. When designing surveys, there is a trade-off between survey
frequency (and hence logistical effort and financial cost) and carcass detection. In this regard, carcass persistence
(the period a carcass remains detectable before being removed by decomposition or scavengers) is important; the
longer a carcass persists, the greater the likelihood it will be detected with lower survey effort by conducting
more infrequent surveys. Using multi-taxon carcass data collected over a month of repeated driven surveys,
combined with five covariates (species functional group, body weight, carcass position on road, carcass condition
[either flattened or not after impact], and rainfall prior to each survey), we explored the drivers of carcass
persistence with the overall aim of providing information to optimise the design of carcass surveys along linear
infrastructure. Our methodological approach included a survival analysis to determine carcass persistence, linear
regressions to test the effect of covariates, a subsampling analysis (using field data and a simulation exercise) to
assess how the proportion of carcasses detected changes according to survey frequency, and an analysis to
compare the costs of surveys based on study duration, transect length and survey frequency. Mean overall carcass
persistence was 2.7 days and was significantly correlated with position on road and within-functional group body
weight. There was no evidence for a significant effect of rainfall, while the effect of carcass condition was weakly
non-significant. The proportion of carcasses detected decreased sharply when survey intervals were longer than
three days. However, we showed that survey costs can be reduced by up to 80% by conducting non-daily surveys.
Expanding on the call for a standardised methodology for roadkill surveys, we propose that carcass persistence be
explicitly considered during survey design. By carefully considering the objectives of the survey and characteristics
of the focal taxa, researchers can substantially reduce logistical costs. In addition, we developed an R
Shiny web app that can be used by practitioners to compare survey costs across a variety of survey characteristics.
This web app will allow practitioners to easily assess the trade-off between carcass detection and logistical
effort.