Roads are a major source of disturbance causing habitat destruction, fragmentation, and degradation by producing barrier effects, light and noise, chemical pollution, and wildlife deaths through vehicle collisions. While the impact of roads has been tested for many animals, less research has been done on flying animals, like birds and bats. Even though it seems like they can just fly over roads, this disturbance still fragments their habitats. Flying animals might not choose to fly over roads, or when they do, collide with vehicles if they don’t fly high enough.
Bats perform important daily migrations between breeding and feeding grounds. They follow similar routes each day for their migrations, returning to the same areas each night to roost. If roads intersect these routes, traffic at night can cause collisions with vehicles and restricts access to important habitats.
In the European Union, new developments must take measures to ensure that they do not impact bat populations. However, it is not known which measures would be effective at mitigating the effects of disturbances on bats, and too often untested measures are implemented. Bat overpasses, called gantries, are one such strategy. In a series of two studies out this year, a team of scientists set out to test the efficacy of these methods.
In their first study they tested three different styles of bat overpasses and six habitat types. To collect data on bat movement across roads, they developed an innovative technique to measure crossings by recording the sound of their calls. They placed recorders at different locations on both sides of the road, and from before sunrise until after sunset they recorded calls and counted the number of crossings.
They found that bats did not cross roads randomly, but followed predictable daily migration routes. When bat overpasses were located along these routes bats used them, but not at a higher rate than surrounding habitats. The most effective overpasses had forested habitat on both sides and avoided open areas like agricultural lands. The study did not collect bat crossing data before the overpasses were constructed, so they were unable to determine if they were effective or just used because they were already created along migration routes.
The authors suggest that before roads are put in, bat overpasses should be monitored to avoid fragmentation to habitats. Data collection should be done before the road is put in and then long-term monitoring done after. This was their approach for their second study. Using similar methods, the scientists monitored bat crossings before and after a bat overpass was constructed. They found that the number of bat crossings increased significantly after the installation of the bat overpass. They concluded that when overpasses are constructed along bat migration routes, and in the right habitat types, they can be an effective method to restore bat habitat connectivity.
Claireau, F., Bas, Y., Puechmaille, S.J., Julien, J.‐F., Allegrini, B., and Kerbiriou, C. 2019. Bat overpasses: An insufficient solution to restore habitat connectivity across roads. Journal of Applied Ecology, 56: 573– 584.
Claireau, F., Bas, Y., Julien, J.F., Machon, N., Allegrini, B., Puechmaille, S.J. and Kerbiriou, C. 2019. Bat overpasses as an alternative solution to restore habitat connectivity in the context of road requalification. Ecological Engineering, 131: 34-38.