Reductions in the dietary niche of southern sea otters (Enhydra lutris nereis) from the Holocene to the Anthropocene.

The sea otter (Enhydra lutris) is a marine mammal hunted to near
extinction during the 1800s. Despite their well-known 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 d13C and d15N 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 d15N
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; this likely reflects
broader habitat and prey use in pre-fur 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. The latter 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 d15N
values than modern populations. Amino acid d15N 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.