Wildlife crossings have benefited numerous species around the world, including in places such as Banff National Park and western Montana. Unfortunately, economic cost and regional politics often outweigh landscape connectivity needs when planning for highways, and wildlife crossings are usually an afterthought to construction. Even when they are included, one the major challenges that planners face is where to put them.

montana-usa-animal-bridge-wildlife-crossing-overpassDetermining the best location for a crossing depends on multiple factors, including target species’ range and mobility, and the need to maximize the number of species that benefit.  In their recent paper, Mimet et al. develop a new method for deciding where to locate over- and underpasses for wildlife crossings of highways.  They use graph theory, which treats suitable habitat patches as nodes that are linked by least-cost paths, to determine the best potential highway crossing locations in an area of the French Alps with dense highways.  They considered scenarios for determining locations both before a highway is built and after it is built, and had some helpful results and suggestions:

  • Highway impermeability had the greatest impact on highly mobile species, species requiring an extensive minimum area for population viability, and species living in more common land cover types.
  • Connectivity loss caused by the highway varied from 2 to 50%, depending on the species.  The best locations for crossings showed little gain in connectivity (10% or less) when crossing infrastructures were already in place, although there was a much larger gain in connectivity (up to 75% for  forest carnivore species) when locations were determined prior to the highway being built.
  • Allometric relationships can be a good way to standardize ecological parameters necessary for models used to determine the best locations, especially when information about dispersal distances or habitat area is not available for a species.
  • Using a nested analysis that models both shorter linkages (for intrapopulation connectivity) and longer linkages (for interpopulation connectivity) helps discriminate between local and regional impacts of highways and accounts for long-term population viability.

Resources

Mimet, A., C. Clauzel, J.-C. Foltête. 2016. Locating wildlife crossings for multispecies connectivity across linear infrastructures. Landscape Ecology. DOI: 10.1007/s10980-016-0373-y.

The road ahead: adaptive design for wildlife crossings under climate change (November 2015)