Habitat characteristics are primary determinants of nearshore marine communities. However, biological drivers like predation can also be important for community composition. Sea otters (Enhydra lutris ssp.) are a salient example of a keystone species exerting top-down control on ecosystem community structure. The translocation and subsequent population growth and range expansion of the northern sea otter (Enhydra lutris kenyoni) in Washington State over the last five decades has created a spatio-temporal gradient in sea otter occupation time and density, and acts as a natural experiment to quantify how sea otter population status and habitat type influence sea otter diet. We collected focal observations of sea otters foraging at sites across the gradient in varying habitat types between 2010 and 2017. We quantified sea otter diet composition and diversity, and long-term rates of energy gain across the gradient. We calculated diet assemblage dissimilarity using Bray-Curtis distance and applied nonmetric multidimensional scaling (NMDS) to visualize relationships among diet composition, habitat type, and sea otter cumulative density. We fit a linear model to the Bray-Curtis distance matrix and conducted a permutation test with pseudo F-ratios (perMANOVA) to determine the influence of habitat type and sea otter cumulative density on diet composition.
Results/Conclusions
We found that sea otter diet diversity was positively correlated with cumulative sea otter density, while rate of energy gain was negatively correlated with cumulative density. per-MANOVA analysis indicated that site, sea otter cumulative density, and habitat type were all significant in explaining the dissimilarity in sea otter diet composition (P = 0.001, 0.017 and 0.03, respectively). Of the variance explained by the model (50%), site explained 75.1% of the variance in sea otter diet composition (r2 = 0.376), while habitat type explained 16.02% (r2 = 0.08) and sea otter cumulative density explained 9.04% (r2 = 0.045), indicating that habitat type explained 1.77 times more variance in sea otter diet composition than sea otter cumulative density. Our study highlights the importance of replicated studies of sea otter foraging in similar systems across a variety of geographic areas, and suggests that drivers of sea otter diet may vary between geographic regions. This research contributes to a better understanding of the effect of sea otters on the nearshore marine system, as well as provides a broader picture of sea otter population status in Washington State.