Recovery of keystone predation magnifies community niche space by expanding trophic level diversity and shifting basal resources

While predators can drive abrupt and profound changes in food web components, what is less well known and more difficult to quantify is how predators influence entire ecosystem organization and function. Sea otter (Enhydra lutris) recovery on temperate reefs is known to trigger regime shifts, converting urchin-dominated reefs to kelp-dominated reefs, yet the impacts of this keystone species on entire ecosystem dynamics remain less understood. We used stable isotope analysis and a Bayesian modeling approach to study the effects of sea otter recovery on the trophic niche space of a rocky reef species assemblage. Examination of community-wide niche metrics revealed an increase in the overall community niche space with increasing otter occupation time, driven by an increased number of trophic levels and greater uncertainty in basal resource diversity. At the species-level, the trophic positions of several reef associated fish increased with sea otter occupation time, suggesting that these species are feeding on higher trophic level prey. Concurrently, decreased trophic positions and distinct shifts in dietary carbon signatures of sea urchins suggest that sea otter recovery also drove an expansion at the base of the food web. These results contribute to our understanding of the broad, far-reaching role of keystone predation in reshaping entire ecosystems.