The structure and connectivity of an archipelagic population of black bears

Abstract

Understanding factors that influence the viability of populations is central to conservation biology. Small and isolated populations have elevated risk of extinction due to demographic and genetic stochasticity. The Apostle Islands National Lakeshore features a genetically unique and culturally important population of archipelagic black bears (Makwa; Ursus americanus). While dispersal is central to population viability, previous studies of this population did not sample the adjacent mainland black bear population on the Red Cliff Reservation (Gaa-miskwaabikaang). Therefore, we lack robust estimates of dispersal, gene flow and overall connectivity among the islands and with the mainland population. In partnership with Red Cliff Band of Lake Superior Chippewa and the National Park Service, we non-invasively collected black bear hair, and used 17 microsatellite markers to genotype 141 black bears. We then estimated genetic diversity, population structure, dispersal, and conducted a pedigree network analysis to identify areas of the archipelago important for connectivity and reproduction. We found evidence of a well-connected archipelagic bear population structured into five clusters and characterized by moderate dispersal between islands and mainland. We found that three of the islands are disproportionately important for genetically connecting the archipelago, but the islands were nevertheless reliant upon the mainland for gene flow and genetic diversity. The high connectivity between islands and the mainland demonstrates a potential metapopulation dynamic, where islands may serve as a reservoir of individuals for the mainland and the mainland supplying individuals likely important for maintaining genetic diversity of island populations. Given the importance of island–mainland connectivity, future tribal and federal collaboration will be important to maintain a genetically and demographically viable population of black bears.