Direct and cascading effects of sea star wasting on rocky intertidal communities

Abstract

Predation is an important driver of rocky intertidal community structure. Top predators like sea stars often regulate competitive species like barnacles and mussels by preying on them, which maintains ecosystem diversity. Fluctuations of top predators can have cascading effects in terrestrial, freshwater, and marine environments, often resulting in the restructuring of communities and ecosystems into alternative states. Sea star wasting (SSW) was a large-scale disturbance, starting in 2013 in the East Pacific Ocean and ranging from Alaska to Mexico, that removed sea stars from the rocky intertidal. While recovery has been ongoing in many places, community-wide effects may persist in places where sea stars have not recovered, or where predation continues to be reduced. In Alaska, it remains unclear if sea stars have recovered and if SSW had direct effects on intertidal sea star prey, which are often mid-trophic organisms, and if there were cascading effects down the food web. I used a long-term monitoring dataset from Gulf Watch Alaska (2006-2022) from four monitoring regions (Katmai Park and Preserve (KATM), Kachemak Bay (KBAY), Kenai Fjords National Park (KEFJ), and western Prince William Sound (WPWS)) with multiple sites each to investigate motile invertebrate community structure changes in relation to SSW. Permutational multivariate analysis of variance tests were used to test changes in motile invertebrate community structure and were paired with linear mixed effects models and analysis of variance tests to track any cascading changes down the food web. My results suggest that SSW had significant but variable effects on motile invertebrate community structure across different regions in the Gulf of Alaska. Two regions (KATM and KEFJ) significantly changed and then returned to the pre-SSW structure after 2-3 years, one region (KBAY) significantly changed and has not returned to the pre-SSW structure, and one region (WPWS) remained unchanged. Other than motile invertebrates, I found no indication of trophic cascades despite the significant reduction of sea stars across all regions. My analyses also showed the recovery of sea stars in many of the monitoring regions. Results of this study suggest that rocky intertidal ecosystem resilience and stability in the Gulf of Alaska may be dependent on recruitment dynamics and predator redundancy. This resilience and stability may also vary among regions, with some regions being more susceptible to large-scale disturbances than others.