Move it or lose it: Predicted effects of crossing structures and population density on Mojave desert tortoise genetic connectivity

Background/Question/Methods

Linear barriers can reduce wildlife movements across formerly contiguous landscapes, impacting genetic connectivity. Crossing structures may improve gene flow by allowing safe passage, but population density might also influence connectivity. The Mojave desert tortoise (Gopherus agassizii) was federally protected in 1989 due to population declines largely related to habitat loss. The tortoise has continued to decline and anthropogenic disturbance has increased across its range in the Mojave Desert, southwest US. While crossing structures have received limited attention, population density has not been considered in tortoise connectivity research. We assessed the role of crossing structures and population density in genetic connectivity using individually-based spatially explicit forward-in-time simulations. We constructed resistance surfaces with a range of barriers to movement (Euclidean, absolute barrier, barrier with crossing structures) and under a range of population densities (low, moderate, high). We also constructed a resistance surface of tortoise habitat with anthropogenic disturbance along the Nevada/California border using variable population density. Simulations were run for 200 non-overlapping generations using 20 microsatellite loci from empirical data. We predicted genetic diversity and differentiation in time-series, using outcomes averaged across 30 replicates. Population genetic structure was examined using a Bayesian approach and a spatial principal components analysis.

Results/Conclusions

Our results showed that gene flow was predicted to improve across a linear barrier with crossing structures, provided population density was not low. Low density was projected to result in population declines of over 75% and a loss in genetic diversity of over 35% regardless of landscape surface. Genetic signals of isolation were detectable earlier in time at low density, likely because genetic drift increased sensitivity. The simulation of current anthropogenic disturbance predicted population declines of 15% and a loss in genetic diversity of 5%. Genetic differentiation more than doubled during the simulation period with clusters estimated to increase from one to five. Our results indicate that crossing structures can improve tortoise genetic connectivity when population density is moderate or high. Additionally, genetic patterns may be detectable as early as five generations, and are predicted to be seen within 40 generations, following habitat disturbance.