Conference Abstract
Long-term, large-scale studies of succession provide critical evidence for understanding how species assemble into communities and how they respond to disturbance events and stressors, including keystone species loss and changing environmental conditions. When stressors or disturbances are severe enough, they can trigger critical transitions from one community state to another in a process called a regime shift. From 2014-2016, rocky intertidal ecosystems in the northeast Pacific Ocean experienced extreme temperatures during a multiyear marine heatwave (MHW; known as “the blob”) and sharp population declines of the keystone predator Pisaster ochraceus due to sea star wasting disease (SSWD). In a 16-year succession experiment conducted at 13 field sites on four capes in Oregon and northern California, we quantified community structure in fixed plots before, during, and after the MHW onset and SSWD outbreak. We asked: (Q1) Where, when, and what kind of regime shifts occurred between community states? and (Q2) Which regime shifts were associated with the MHW and SSWD events? We used cluster analysis to classify experimental plots into community states and hidden Markov models (HMMs: state-space models with unobservable states) to identify regime shifts and evaluate the extent to which they were driven by the MHW and SSWD.