Inbreeding in red-cockaded woodpeckers: Effects of natal dispersal distance and territory location

Inbreeding depression constitutes a significant threat to the viability of small populations. In addition to small size and isolation of populations, short distance dispersal may elevate risk of inbreeding, but empirical evidence is scarce. Inbreeding depression has been demonstrated in the highly endangered red-cockaded woodpecker Picoides borealis. It has been suggested that conservation efforts to support extant populations should aim at spatially aggregating territories to enhance dispersal success. This however may aggravate inbreeding risk because distance between territories and hence dispersal distances become short. We analysed empirical data from a long-term study of the demography of the red-cockaded woodpecker and found that inbreeding risk varied inversely with natal dispersal distance of the mother. Using an individual-based, spatially explicit population model that incorporates simulations of environmental and demographic stochasticity and an empirically derived, species-specific estimate of inbreeding costs, we demonstrated that inbreeding depression significantly elevated extinction risk in this species. On the other hand, even though dispersal distances in populations with spatially aggregated territories were shorter and the proportion of inbred individuals was higher than in other populations of the same size, such populations were still more persistent. Despite the overall adverse effect of inbreeding depression on viability of red-cockaded woodpecker populations, lowering interterritorial distances can be viewed as a valuable conservation tool. Given the small size and isolated location of most extant red-cockaded woodpecker populations however, our findings suggest that inbreeding depression represents a significant threat to the survival of this species.