Talk What are the effects of roadkills on mammal population’s persistence

Roads and associated traffic can negatively affect individual wildlife, populations and communities through additional mortality from wildlife-vehicle collisions. Road-kill rates are well documented for a wide range of terrestrial vertebrates, suggesting that endangered and common species face a new threat with the global road network expansion. Previous studies indicated that road-kill can account for up to 40% of the annual mortality of wild mammals (e.g. Eurasian badger Meles meles in Britain). However, the effects of wildlife-vehicle collisions on long-term population persistence are still undetermined.

The main goal of this study is to estimate the extinction risk of mammals due to wildlife-vehicle collisions across all continents. For each continent, we modelled the population viability analysis for all endangered species with observed road-kill rates and the five species with the highest observed road-kill rates that are Least Concerned by IUCN. We compiled road-kill rates from 199 peer–reviewed publications, reports and personal communications. We ran age-structured stochastic models to analyse the effect of road-kill rates on the populations of each selected species taking into account the life history traits (population density, age at first birth, interval between births, litter size, litter per year, month of recruitment, survival rates, life span). We used the study area reported in each publication to estimate the road and population density in order to calculate the proportion of the population removed annually through road-kills. The radius of the study area was based on the median daily movement of the individuals for species living in colonies (e.g. bats) and for the other species 5km and 50km for small (<1kg) and large-bodied species (> 1kg), respectively.

We obtained 1411 records of 406 different mammal species from 199 different sources (scientific, grey literature and unpublished data) across the globe.  A total of 111 species were selected to run the age-structured models (68 threatened species and 43 species with high road-kill rates). Among the compiled species we found that the three species with highest road-kill rates were: white-tailed deer Odocoileus virginianus (68 ind./km/year in USA), southern opossum Didelphis marsupialis (40 ind./km/year in Mexico), Long-tailed field mouse Aроdemus sylvaticus (35 ind./km/year in Russia). Apparently road-kills can remove a high proportion of the population for the following threatened species: maned wolf Chrysocyon brachyurus (36%), Eurasian otter Lutra lutra (9-26%), Little spotted cat Leopardus tigrinus (6-20%), Margay Leopardus wiedii (4-19%) and Iberian lynx Lynx pardinus (12%). Models show that road-kills can have strong impacts on the population density with a reduction of more than 50% for some populations of Barbastelle Barbastella barbastellus, maned wolf, Eurasian otter, raccoon Procyon lotor, roe deer Capreolus capreolus and tammar wallaby Macropus eugenii. Preliminary results show that the analysed species are not currently threatened to extinction except maned wolf that has 5% of extinction risk with the observed road-kill rate of 0.084 ind./km/year. This study highlights that although the extinction risk is null or very low, the reduction in population density on several species may make them more vulnerable to stochastic events and other threats.