Success and succession in species and ecosystem recoveries: kelp forest community dynamics following decades of sea otter re…

The recovery of predators has the potential to restore ecosystems and fundamentally alter the services they provide. One of the most iconic examples of this potential comes from keystone predation by sea otters in nearshore habitats of the Northeast Pacific. On the outer coast of Washington state, USA, a population of sea otters re-introduced in 1969-1970 exhibited a more than 10-fold increase during the 1980s and 1990s. This population increase led to a pronounced decline in sea otter prey, particularly kelp-grazing sea urchins. Here we show that the reduction in invertebrate prey coincided with an expansion of kelp beds coastwide until the turn of the century. However, while sea otter and kelp population growth rates were positively correlated prior to 2000, this association diminished substantially in the last 15+ years. A recent 2015 survey of benthic invertebrates shows that sea otter prey have continued to decline as the sea otter population has continued to expand, but kelp abundance is not closely related to these trends. However, variability in kelp abundance has declined in the most recent 15 years of the time series. Altogether these results suggest that initial nearshore community responses to sea otter population expansion follow predictably from trophic cascade theory, but now other factors may be as or more important in influencing community dynamics. Thus, the role of sea otter predation may be context-dependent with shifting environmental conditions strongly affecting their utility in ecosystem restoration.