Abstract
The sea otter (Enhydra lutris) is a marine mammal hunted to near extinction during the 1800s. Despite their well‐known modern importance as a keystone species, we know little about historical sea otter ecology. Here, we characterize the ecological niche of ancient southern sea otters (E. lutris nereis) using δ13C analysis and δ15N analysis of bones recovered from archaeological sites spanning ~7,000 to 350 years before present (N = 112 individuals) at five regions along the coast of California. These data are compared with previously published data on modern animals (N = 165) and potential modern prey items. In addition, we analyze the δ15N of individual amino acids for 23 individuals to test for differences in sea otter trophic ecology through time. After correcting for tissue‐specific and temporal isotopic effects, we employ nonparametric statistics and Bayesian niche models to quantify differences among ancient and modern animals. We find ancient otters occupied a larger isotopic niche than nearly all modern localities; likely reflecting broader habitat and prey use in prefur trade populations. In addition, ancient sea otters at the most southerly sites occupied an isotopic niche that was more than twice as large as ancient otters from northerly regions. This likely reflects greater invertebrate prey diversity in southern California relative to northern California. Thus, we suggest the potential dietary niche of sea otters in southern California could be larger than in central and northern California. At two sites, Año Nuevo and Monterey Bay, ancient otters had significantly higher δ15N values than modern populations. Amino acid δ15N data indicated this resulted from shifting baseline isotope values, rather than a change in sea otter trophic ecology. Our results help in better understanding the contemporary ecological role of sea otters and exemplify the strength of combing zooarchaeological and biological information to provide baseline data for conservation efforts.
Highlights
The sea otter, Enhydra lutris (Figure 1), is a marine mammal found in coastal nearshore ecosystems along the North Pacific
We find that the majority of sampled ancient otter populations in California, ranging in age from 7,000 to 350 years before present, had a wider dietary niche than their modern counterparts
Ancient otters from San Miguel and San Nicolas Islands had the largest dietary niche space of all populations measured. This suggests that the current high diversity of invertebrate prey communities south of Point Conception relative to central California (Graham, Halpern, & Carr, 2008) has played a role in the past
Summary
The sea otter, Enhydra lutris (Figure 1), is a marine mammal found in coastal nearshore ecosystems along the North Pacific Across much of their range—Alaska, British Columbia, Washington, Oregon, and central California (Bodkin, 2015)—sea otters are considered a “keystone species” (Paine, 1969) disproportionately influencing ecosystem structure and function through indirect effects of predation (Estes & Palmisano, 1974; Hughes et al, 2016). We propose that industrial-scale exploitation of the marine environment by humans (McCauley et al 2015) may have led to a constriction of the ecological niche of modern sea otters relative to the past This should be true for the southern subspecies (Enhydra lutris nereis, Figure 1) which is currently found only in central California, an area with high human coastal densities and large-scale fisheries. Our results provide a framework for interpreting the contemporary ecological role of sea otters and for identifying potential areas of their historical niche that are underutilized and could be promoted with conservation efforts
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