Road mitigation structures: do they work for all species?

Many variables can affect the efficacy of road mitigation structures, and their usefulness can vary by species. Their use by wildlife can be affected by structural characteristics, such as the dimensions of an underpass crossing, as well as environmental characteristics, like the closest distance to native vegetation.

Species’ responses to road mitigation strategies can also change over time if they become habituated to the structures. Habituation can increase effectiveness in the case of structures meant to facilitate wildlife movement, because it means wildlife are more likely to use them. But for structures meant to deter road crossings, habituation can undermine their usefulness because wildlife are no longer deterred by the structures.

A new study examined how the structural and environmental characteristics of road mitigation structures, originally designed to reduce road mortality for the endangered ocelot in South Texas, impacted the use of these structures by non-target wildlife species. Researchers looked at how non-target species used wildlife crossing structures (primarily road underpasses), which are meant to facilitate safe passage across roads, and wildlife guards, which are meant to deter wildlife from interacting with roads.

Camera traps were used during and after construction to observe species’ usage of the structures over time. The structures also varied in their dimensions, such as height and width, allowing researchers to examine how these characteristics affected wildlife usage.

The camera trap data provided evidence that species response to the wildlife crossing structures and wildlife guards changed over time. In one example, a javelina did not cross the wildlife crossing structures until 4 months post-construction, although it approached the structure before then, indicating that there was a period of habituation. This has been seen for other species in other studies, and emphasizes how long-term monitoring is critical to understand how species will respond to road.

Several structural and environmental characteristics, including precipitation, presence of standing water, and dimension of structure, had species-specific effects on the usage of wildlife crossing structures and wildlife guards. For example, height impacted armadillos’ use of crossing structures; they are known to prefer bushy areas, and underpasses with lower heights may have provided a stand-in for cover. This emphasizes the extent to which species’ life history traits or habitat preferences may influence their likelihood of using road mitigation structures.

This study underlines the importance of using long-term data to examining species-specific responses to conservation practices. Roads are ubiquitous in many landscapes today; road mitigation structures can ameliorate some of the effects of roads on wildlife, like mortality and population isolation. Understanding how specific species respond to road mitigation structures, including their unique environmental and structural characteristics, will make their construction and maintenance more valuable in the long-term.

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