Persistent genetic connectivity in caribou may buffer against inbreeding effects

The Mulchatna Caribou Herd (MCH) in southwest Alaska has undergone significant demographic fluctuations, with a 94% decline over the past three decades, reducing the population from an estimated 200,000 to 13,000 individuals. This decline and concurrent range contraction, coupled with radio-telemetry and global positioning system (GPS)-collar studies, revealed indications of herd substructure. Females showed fidelity to one of two spatially distinct calving aggregations (designated as east and west), an attribute typically used to define individual herds in Alaska, and to three breeding areas within the greater MCH range (designated east, central, and west). To assess the genetic consequences of the population decline and apparent spatial substructuring, we analyzed genotyping-by-sequencing data from 121 adult female caribou. We found no strong genetic differentiation between east, central, and west subgroups, suggesting that the herd currently functions as a single genetic population despite observed spatial structuring during breeding and calving. In addition, we found no significant levels of inbreeding, likely due to the previously large population size. Overall, our results support the idea that the movement of the MCH is best characterized by a spatially structured but genetically cohesive population, in which connectivity between migratory and nonmigratory groups occurs over a large geographic range.