Isotopic and genetic insights into the persistence of the northern fur seal (Callorhinus ursinus) (Invited)

What factors allow some species to survive in the face of climate change, disease, or anthropogenic disturbance? How do species shift their geographic distributions in the face of such challenges? These pressing questions in ecology and conservation biology are difficult to answer when looking solely at modern populations or the recent historical record. We explore these questions through analysis of DNA and the isotopic composition of modern and ancient northern fur seals (NFS, Callorhinus ursinus). The NFS is an eared seal (otariid) that ranges along the north Pacific, where it breeds on offshore islands; by far the largest modern rookeries are on the Pribilof Islands in the Bering Sea. The species shows a high degree of philopatry, and females feed while nursing, wean pups at 4 months, and spend the rest of the year foraging far offshore further south. Archaelogical study reveals that Holocene NFS had numerous breeding colonies from the Channel Islands to the Aleutians. Temperate latitude colonies collapsed in the late Holocene in response to hunting pressures and perhaps, environmental change. The species has recolonized parts of its former range since the 1960s. Despite facing similar threats, other marine mammals have failed to rebound (e.g., Guadalupe fur seals) or have exceptionally low genetic diversity indicating recent and prolonged bottlenecks (e.g., northern elephant seals). Isotopic analyses of sub-fossil growth series indicate that extirpated mid-latitude colonies weaned much later (≥12 months), like all other otariid species that breed at temperate latitudes. As a result, females were tied to rookery sites year-round and had a much-reduced migratory range relative to modern NFS females breeding in the Bering Sea, a result also supported by isotopic analyses. Serial coalescent simulations of ancient and modern DNA reveals that exceptionally high migration rates and Arctic refugia provided resilience to NFS. These traits allowed the species to maintain high levels of genetic diversity even as it lost breeding colonies across its geographic range. Thus the behavioral differences suggested by isotopic analysis result from phenotypic plasticity rather than local adaptation, providing further survival strategies for the species. The detailed paleontological record available for northern fur seals offers an excellent opportunity to examine the genetic and behavioral diversity of a species within and among populations through time. We conclude that general species traits (panmixia, behavioral plasticity, broad geographic range, a secure refuge) can provide long-term resilience to overexploitation, climate change, and other stressors. For example, similar traits may have allowed southern elephant seals to colonize the Antarctic mainland during the middle Holocene, only to collapse back to sub-Antarctic islands with the onset of icy conditions in the latest Holocene. Overall, such knowledge will help to predict future ecosystem changes and to identify species that are most likely to benefit from management interventions such as assisted migration and translocation.