Roads are not the only determining factor in wildlife movement on the landscape, but due to the extensive distribution of the road network and variable sensitivities by certain species, their impact can be dramatic. Although it has been well documented that roads decrease habitat connectivity for wildlife due to animal-vehicle collisions and to habitat fragmentation, approaches for identifying connectivity across the landscape often do not fully examine the barrier effect of roads. We therefore modeled connectivity for a number of organisms, including the barrier effect of roads and also demonstrated the extent of that effect.
For the diversity of species used in Oregon's Connectivity And Mapping Project (OCAMP), wWe developed habitat permeability models with consideration for factors that may affect species movement on the landscape including type of vegetation cover, presence and proximity of water sources, proximity to development, road barriers, and more. We conducted extensive literature review and solicited feedback from species experts to identify impacts for a variety of taxa across their distribution in Oregon, USA. This information was used to create the resistance layer inputs for connectivity modeling.
In order to quantify the impact of roads on habitat connectivity, we experimentally removed road inputs to the models for three of the OCAMP species, the northern red-legged frog, coastal marten, and Pacific Slope Flycatcher whose range covers much of Western Oregon. These species were selected to represent a variety of movement types, habitat requirements, and variable sensitivities to road impacts. After removing roads from the connectivity model and reclassifying those areas to the adjacent land use, we rebuilt a habitat connectivity model with no roads. We then quantify the effect of roads via change detection mapping and using flow accumulation models, which aid in showing the impact of fragmentation as well as opportunities for connectivity mitigation.
While it is not realistic to expect the road network as we know it to disappear from the landscape, the ability to assess modeled wildlife habitat connectivity in the absence of existing widespread linear infrastructure allows for critical evaluation of where mitigation activities, such as wildlife crossing structures and fencing, may be most beneficial.