You might interpret roadkill as a sign that highways are bad for wildlife, but it's possible these carcasses actually indicate that roadsides are attractive habitats that can support a large number of individuals. That's one interpretation, anyway, of a new study investigating small mammal populations living along highways in central Spain.
The study was performed by a trio of Spanish researchers who counted the abundance of three small mammal species--wood mice, greater white-toothed shrews, and Algerian mice--living along a busy roadway in an area classified as a Site of Community Importance. Over a period of two years, the scientists logged an impressive 8,640 trap-nights, using bits of fried bread to entice animals into captivity. Traps were distributed in 50-m bands located at three distances from the road: 0-50 m away, 500-550 m away, and 1000-1050 m away. The researchers also performed habitat surveys in 5-m circles centered on each trap. They classified habitats by type (e.g., bare ground vs. woodland), and then measured the height of any vegetation located in the circle.
Over the course of the study, the research team captured a total of 1,004 individuals, the vast majority (94.4%) of which were wood mice. Because of this skew, several of the further analyses were only performed on wood mouse data. Contrary to what you might expect, more mammals were captured nearer the road (in the 0-50 m band) than farther away. This was true in both years of the study even though an apparent population crash caused overall mammal numbers to drop in the second year. One interesting result of this crash was that it highlighted just how common the animals were near the highway: During the first year of the study, the ratio of mammals captured in the 0-50 m band relative to the other two bands was 3:2; during the second year, it jumped to a whopping 9:2. Cumulatively, these results suggest that the road verge acts as a refuge for the animals.
This pattern held true regardless of vegetation characteristics, though mice were particularly common in areas with dense scrub cover and higher grass and trees--areas, in other words, where the animals felt relatively safe and sheltered. These microhabitat effects are some of the first documented in the context of road-effects studies; they are also the only patterns measured in woodland areas rather than extremely human-disturbed areas or in deserts. In the future, it should be interesting to collect similar data in other types of habitats in order to see whether road effects vary across the landscape.
As for the current study, the authors believe that the most parsimonious explanation of their results is that predation pressures are lower near roads, thus allowing local populations to become more dense. Support for this is provided by the high proportion of roadside juveniles: This age class is one of the most susceptible to predators, so the commonness of young mice in verges indicates that less predation is occurring there. A road-mediated increase in survivorship could have beneficial cascading effects on the entire ecosystem--for example, by increasing the dispersal of seeds or the consumption of pest insects. The authors further suggest that we might even think about deliberately attracting small mammals to verges by manipulating the density and height of vegetation near roads.
That, of course, still leaves the issue of roadkill, which will probably always be a negative side effect of roadways--even if it is not one that puts populations at risk of local extinction. As previous studies have suggested, it might be a good idea to consider mitigation techniques such as reduced speed limits, wider shoulders, and even fences in order to add further protection to roadside dwellers.
Ruiz-Capillas, Pablo, Mata Cristina, and Juan E. Malo. 2013. Road verges are refuges for small mammal populations in extensively managed Mediterranean landscapes. Biological Conservation 158:223-229.