THE ABSENT NICHE: SEA OTTER ECOLOGY IN CALIFORNIA THROUGH THE HOLOCENE

Historic ecology is increasingly relevant and important, allowing scientists to quantify ecological shifts and establish baselines for conservation efforts. Sea otters are key members of nearshore communities, which are highly productive and threatened ecosystems. Here, we employ a historical perspective to examine sea otter ecology over the middle to late Holocene. We used stable carbon (δ¹³C) and nitrogen (δ¹⁵N) isotope analysis of ancient sea otter bone collagen from five archaeological sites in California–– San Nicolas Island (SNI), San Miguel Island (SMI), San Luis Obispo (SLO), Monterey Bay (MBY), and Año Nuevo (ANU)–– to quantify long-term changes in sea otter dietary ecology. We compared these data to δ¹³C and δ¹⁵N of modern sea otter whiskers from the same locations. We found ancient otters occupied much larger isotopic space overall, and likely had a higher degree of population dietary diversity than their modern counterparts. At all sites but MBY and ANU, a large proportion of ancient otters had high δ¹³C values, implying a strong reliance on kelp-based prey. Additionally, at SNI, SMI and SLO ancient otters had declining δ¹⁵N through time, indicating either loss of apex invertebrate consumers or competitive exclusion by other predators, such as humans. Our data suggest that historically kelp played a greater role as a food source for sea otter prey. Moreover, our results suggest anthropogenic habitat alteration may have begun in the middle Holocene in this system. In the future, we hope to expand our work to other sites in California, the Aleutian Islands and southeastern Alaska.