Indirect effects of sea otter-driven trophic cascades vary across environmental and anthropogenic gradients
The near extinction of sea otters (Enhydra lutris) is one of the most dramatic examples of human-induced impacts to the structure and functioning of temperate nearshore marine ecosystems. Simultaneously, over-exploitation of commercial and recreational fisheries has contributed substantially to marine ecosystem change. In addition, context-dependent effects due to natural variation in abiotic conditions can drive differences in the manifestation of these impacts to coastal ecosystems. To understand the direct, indirect, and relative effects of these drivers on ecosystem service production, we performed a comprehensive assessment of nearshore rocky reef food web structure along the west coast of Vancouver Island and Haida Gwaii, British Columbia, Canada. In regions with varying levels of fishing pressure, sea otter occupation time, and oceanographic conditions, we performed subtidal and intertidal surveys of rocky reef habitats to quantify community structure, hook and line surveys to estimate relative fish abundance, and stomach content and stable isotope analyses to determine reef fish trophic niche space.
Sea otters exhibit strong top-down control of grazers, particularly sea urchins, and thus indirectly increase kelp population spatial occupancy and productivity. Surprisingly, despite order of magnitude differences in kelp forest size, we found that kelp-carbon signatures of reef fishes do not differ with sea otter occupation time. However, rocky reefs in areas with sea otters support higher biomass and diversity of reef fish prey. Concomitantly, large reef fish have higher biomass, occupy higher mean trophic positions and demonstrate narrower dietary niche space in areas with established sea otter populations. Our results also indicate that the impact of fishing on food web structure is moderated by the presence of sea otters and their indirect positive effects on fish productivity. Finally, context-dependent effects of sea otters are also evident, indicating that abiotic conditions, such as temperature and productivity, can constrain the indirect effects of trophic cascades. Ultimately, these analyses can help shape predictions of how climate change and trophic cascades, in concert with other drivers, affect the provisioning of services from coastal ecosystems.