IENE 2016

When roads divide wildlife populations, it prevents road crossing, leading to genetic consequences. The most studied of these consequences is genetic differentiation, but an additional effect can be a drop in genetic diversity. This can occur in areas nearest to roads, which we term a “genetic road-effect zone.” This can occur because individuals near roads have less accessible habitat and mates, essentially shrinking their gene pool. Therefore, the closer an individual is to the barrier, the more its gene pool is limited. Empirical studies have suggested that subpopulations nearer to roads have lower genetic diversity than clusters farther from roads. However, studies have yet to address the question of how roads affect genetic diversity across continuously distributed populations across heterogeneous landscapes, because they have focused on clumped populations or clumped sampling of continuous populations.

We tested the hypothesis that roads drive a decrease in genetic diversity using datasets on two kangaroo rat species, alpine newts, pronghorn antelope, roe deer, and simulations to assess the effect of roads on genetic diversity across landscapes with heterogeneous habitat quality. We analysed populations in which individuals were sampled continuously across their habitats, and we developed simple resistance surfaces for them, largely focused on road impacts. We used Mantel tests to select resistance surfaces with cost distance matrices that model genetic distances. We then used a spatial genetic diversity (sGD) approach to assess genetic diversity in Wright’s neighbourhoods across these resistant landscapes.

Our simulation results indicate that a drop in genetic diversity develops near roads when road avoidance or roadkill prevents crossing in > 50% of attempted migrants. Some empirical results fit this result, but others suggested that roads could have more complicated effects on genetic diversity; roads may even increase genetic diversity when road avoidance and roadkill are low. We describe the range of characteristics of species that may be most affected by roads, and mention road characteristics that are most likely to cause decreases in genetic diversity for wildlife populations. We suggest that the impacts of roads on genetic diversity could be significant in some taxa.