Roosting in road culverts: roost selection, roost availability and gene flow in a culvert roosting population of a trawling bat, the large-footed Myotis (Myotis macropus)

The large-footed Myotis (Myotis macropus) is Australia’s only a trawling bat and can be found in urban areas, roosting in concrete culverts under roads. However, little is known about the selection of these artificial sites and how much gene flow occurs between culvert roosts. We investigated culvert roost selection by M. macropus at two spatial scales and studied gene flow between culvert roosts in a large subtropical city in eastern Australia. We surveyed 365 concrete culverts, identified 23 roosts and collected wing tissue samples from 72 bats.

Using generalized additive models, we found the distribution of M. macropus roosts in concrete culverts can be predicted at a landscape level using the variables stream order, channel width, waterway density, and culvert height. Bats preferred culverts >1.2 m in height, and a preference for box culverts was detected although pipe culverts were also occupied. Predictive modelling identified that culvert roosts were a limited resource with only 5.5% of culverts identified as potential roosts.

We examined roost selection at the roost scale by comparing roost culverts to available culverts. Roost culverts differed significantly from available culverts and the primary difference was the availability of microhabitat (lift holes and crevices). Roost culverts had lift holes that had greater cavity dimensions than available culverts and crevices were only found at roost culverts. Culverts containing microhabitat were a limited resource in this urban landscape.
We used single nucleotide polymorphisms to study gene flow between culvert roosts in peri-urban and urban areas. Roosts in peri-urban areas had higher landscape connectivity between roosts compared to roosts in urban areas. Gene flow was moderate between peri-urban roosts and restricted between urban roosts. We found higher relatedness coefficients between individuals roosting in urban roosts compared to peri-urban roosts, indicating reduced gene flow between urban culvert roosts. These results reflect the greater connectivity and availability of roost sites in peri-urban environments and the geographic isolation and limited availability of urban culvert roosts in our study.

This study found that culvert roosts were limited at two spatial scales and that gene flow was restricted between urban culvert roosts. These findings suggest that disturbance to a culvert containing a roost in an urban area has the potential to be a significant impact to an urban population of M. macropus. Maintaining riparian connectivity in urban areas is vital to providing landscape and genetic connectivity for the viability of urban M. macropus populations.