Differential use of highway underpasses by bats

- Roads can form barriers to movement for many species, but wildlife crossing structures may reduce these impacts
- Use of underpasses likely varies among species depending on their movement behaviour.
- We studied whether the flight patterns of insectivorous bats influenced their use of open-span bridges and box-culverts.
- Bats that fly close to vegetation crossed the highway more under bridges than under culverts or by going over the road.
- Bats that fly in open areas were not likely to use a crossing structure but crossed the highway over the road.

Roads can form barriers to movement for many species, and may reduce the ability of individuals to access foraging and breeding habitat. The impacts of roads on terrestrial fauna has been well studied, however little is known of the impact of roads on insectivorous bats. Wildlife crossing structures (e.g. fauna underpasses) may reduce the barrier impacts of roads and improve connectivity across roads. Use of underpasses by wildlife likely varies among species depending on their movement behaviour. In this study, we investigated whether the flight patterns of insectivorous bats influenced their use of underpasses. We monitored bat activity under and above 6 open-span bridges, 6 box culverts and 6 unmitigated sites along a major highway in Australia. We used Poisson regression models within a Bayesian framework of inference to compare the activity of 12 bat species (grouped into three guilds based on their flight patterns: clutter-adapted, edge-adapted, and open-adapted species) under the structures, over the road above the structures, above unmitigated segments of the highway, and in the vegetation adjacent to the roads. Bats were less active above the road than they were in the surrounding vegetation or under bridges. Two of the three guilds (i.e. seven species) crossed the highway more under bridges than they did under culverts or by going over the road, which suggests that bridges may reduce barrier effects of the road better than culverts. Installing bridges instead of culverts may better reduce the impacts of roads on multiple insectivorous bats species with a single structure type.