Carcass Data: When Fewer Carcasses are Not Better

Sandra Jacobson, US Forest Service
Shannon Crossen, ICF; Eric Abelson, US Forest Service
Topic Area
Terrestrial Wildlife and Ecosystem Interactions with Transportation

Roads are ubiquitous and are linked to a wide variety of ecological damage including habitat fragmentation and innumerable wildlife deaths caused by wildlife-vehicle collisions (WVCs). Mitigating effects of roads on wildlife is of interest to many stakeholders including Departments of Transportation, resource agencies, NGOs, and the public. WVC carcass data are commonly utilized to inform WVC prevention and to identify locations for wildlife connectivity mitigation (e.g., wildlife crossings). Traditionally, variables that are used when interpreting WVC carcass data include species population size, migratory or sedentary behaviors affecting movement patterns, and habitat features near WVC carcasses. However, wildlife behavior related to road barrier strength (i.e., traffic volume) is often neglected, or not appropriately understood, in WVC carcass data interpretation. Wildlife behavior may be substantially influenced by road traffic volume, resulting in a variety of potential risk avoidance behaviors and outcomes which are important to consider when interpreting WVC carcass data. For example, low density of WVC carcasses may indicate several possible scenarios. Low WVC carcass density can be a result of small population sizes, populations that are nearly extirpated due to a history of WVC mortalities (i.e. reducing the number of individuals near the road), high traffic-volume that has caused wholesale road avoidance, or other factors unrelated to movement needs across the road. Interpretation of WVC carcass data must therefore also consider the motivation of a species to attempt to cross a road at a given location. While populations such as migratory ungulates may experience high mortality on roads during spring and fall migration, other species may avoid crossing roads due to risk avoidance and, as a result, will not be represented in WVC carcass data. Said another way, few WVC carcasses for a species that avoids roads does not mean that the road isn't having a serious impact on that species. For road-avoiders, roads may be playing a large role in reducing access to essential resources such as habitat, mates, or other life requisites (rather than directly resulting in WVCs). Thus, species behavior and traffic volume are important influencing factors which should be considered when assessing the effects of roads on wildlife populations. These topics are addressed here and an inclusive framework for WVC carcass data interpretation is proposed. Such a framework aims to facilitate a comprehensive understanding of WVC carcass data which will facilitate improved approaches to WVC mitigation and improving wildlife connectivity across the world's roads.

Abstract Keywords
wildlife-vehicle collisions
traffic volume
Road Barrier