Abstract
Carcass persistence time and detectability are two main sources of uncertainty on roadkill surveys. In this study, we evaluate the influence of these uncertainties on roadkill surveys and estimates. To estimate carcass persistence time, three observers (including the driver) surveyed 114km by car on a monthly basis for two years, searching for wildlife-vehicle collisions (WVC). Each survey consisted of five consecutive days. To estimate carcass detectability, we randomly selected stretches of 500m to be also surveyed on foot by two other observers (total 292 walked stretches, 146 km walked). We expected that body size of the carcass, road type, presence of scavengers and weather conditions to be the main drivers influencing the carcass persistence times, but their relative importance was unknown. We also expected detectability to be highly dependent on body size. Overall, we recorded low median persistence times (one day) and low detectability (<10%) for all vertebrates. The results indicate that body size and landscape cover (as a surrogate of scavengers’ presence) are the major drivers of carcass persistence. Detectability was lower for animals with body mass less than 100g when compared to carcass with higher body mass. We estimated that our recorded mortality rates underestimated actual values of mortality by 2–10 fold. Although persistence times were similar to previous studies, the detectability rates here described are very different from previous studies. The results suggest that detectability is the main source of bias across WVC studies. Therefore, more than persistence times, studies should carefully account for differing detectability when comparing WVC studies.
Highlights
Roads and associated traffic promote a variety of negative effects on biodiversity, including habitat degradation and pollution, dispersal of exotic species, and barrier effects [1,2,3,4,5]
The results suggest that detectability is the main source of bias across wildlife-vehicle collisions (WVC) studies
With this study we aimed to evaluate the influence of carcass persistence time and detectability biases in quantifying roadkills
Summary
Roads and associated traffic promote a variety of negative effects on biodiversity, including habitat degradation and pollution, dispersal of exotic species, and barrier effects [1,2,3,4,5]. Virtually all species using road vicinities are negatively affected by WVC, from insects [10] to all terrestrial vertebrates [11,12,13,14,15] To avoid these negative effects, mitigation measures such as faunal passages and drift fencing [2,4,5,6] are generally applied at road sections with higher frequencies of roadkills[14]. Because these mitigation measures are often expensive, it is crucial that roadkill rates along the road network are properly quantified for a correct identification of most problematic road sections [16,17,18].Besides, correcting mortality estimates is very important to assess the effects of roadkills on population depletion. The use of such unbiased estimates has barely been used[16, 18, 19]
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