Eric Ward¹, Mark Scheuerell¹, and Steve Katz^2
1Northwest Fisheries Science Center and ²Washington State University
May 21, 2014

Kelp forest ecosystems (KFEs) occur globally along temperate-zone coast lines, and are some of the most diverse and productive ecosystems of the world. They also provide a wide array of ecosystem services that have sustained coastal human communities for millenia. However, harvest of apex predators and important grazers has had profound effects on KFE structure and services provided. Unfortunately, quantitative tools to quantify the impact of these stressors on ecosystem dynamics have been lacking. Here we present a novel method for analyzing temporal changes in (1) the strength of intra- and inter-species interactions within an ecosystem, and (2) the stability of the ecosystem itself. We applied a Bayesian time-varying vector autoregressive state-space (TVVARSS) model to a 32-year time series of KFE structure at San Nicolas Island, California (USA). Although sea otters were initially absent from the island, they were reintroduced in the late 1980s. We found that the resilience of the KFE has decreased over time concomitant with increased otter density, and that the KFE has also become more reactive to external perturbations such as stron El Niño events.