Do small mammals avoid roads because of the traffic?

Summary

• 1
  Roads can act as barriers to animal movement, which may reduce population persistence by reducing recolonization of empty habitats and limiting immigration. Appropriate mitigation of this barrier effect (e.g. seasonal road closures, location and design of wildlife over- or underpasses) depends upon whether the animals avoid the road itself or the traffic on the road. Empirical studies of road avoidance to date do not generally differentiate between these.
• 2
  We conducted short- and long-distance translocations and trapping studies of white-footed mice (Peromyscus leucopus) and eastern chipmunks (Tamias striatus) near two-lane paved roads, which differed widely in traffic amount, from 47 to 15 433 vehicles per day.
• 3
  In the trapping study (13 sites) only five animals moved across a road, in comparison to 36 animals that moved the same distance without an intervening road (P < 0·0001). In the short-distance translocations (15 sites), 51% of the small mammals that were translocated across roads returned, in comparison to a return rate of 77% of animals that were translocated a similar distance with no intervening road (P = 0·009).
• 4
  In the long-distance translocation study (24 sites) we found that each intervening road reduced the probability of successful return by about 50%.
• 5
  We found no significant effects of traffic amount on return rates in either the short-distance or the long-distance translocations studies.
• 6
  Small mammal densities were not lower near roads and we found no evidence for a decrease in density near roads with increasing traffic amount.
• 7
  Synthesis and applications. Our results suggest that small mammals avoid the road itself, and not emissions such as noise from the traffic on the roads. Our results imply that the barrier effect of roads on these species cannot be mitigated by measures aimed at reducing traffic amount; other measures such as wildlife passages would be needed.