Limited evidence for negative effects of highway widening on North American large mammals

Roads are one of the most severe threats to wildlife globally because of their pervasive and linear nature. Despite considerable attention afforded to road-effects on animals generally, little information is available concerning the specific impacts of road widening, also known as twinning or highway expansion. To address this gap, we monitored the abundance and distribution of large mammals adjacent to an 18-km section of highway in Ontario, Canada, that underwent expansion from a two-lane to a four-lane, divided highway, over 4 years. We expected to observe an initial decrease in large mammal track abundance and an increased distance from the highway, followed by a period of habituation during which track abundance and proximity to the highway would return to baseline levels. To quantify the effects of highway expansion, we monitored snow-tracks of three ungulates, two felids, and three canids on transects extending perpendicular and parallel to the highway. Our analyses revealed that proximity was species-specific and varied seasonally, but we found little evidence that the new four-lane highway alignment affected the movement ecology of large mammals in our study area. Limited increases in traffic volume could partially explain why we detected no change; however, traffic volume will rise. Although road widening is common for accommodating increased traffic volume, no studies have explicitly investigated its effects on animal movement ecology. Our findings suggest that increasing the footprint of existing roads, rather than building new roads, may be a suitable solution for accommodating increasing traffic with minimal impacts on adjacent large mammal communities.