From the predictable to the unexpected: kelp forest and benthic invertebrate community dynamics following decades of sea otter expansion
The recovery of predators has the potential to restore ecosystems and fundamentally alter the services they provide. One iconic example of this is keystone predation by sea otters in the Northeast Pacific. Here, we combine spatial time series of sea otter abundance, canopy kelp area, and benthic invertebrate abundance from Washington State, USA, to examine the shifting consequences of sea otter reintroduction for kelp and kelp forest communities. We leverage the spatial variation in sea otter recovery to understand connections between sea otters and the kelp forest community. Sea otter increases created a pronounced decline in sea otter prey—particularly kelp-grazing sea urchins—and led to an expansion of canopy kelps from the late 1980s until roughly 2000. However, while sea otter and kelp population growth rates were positively correlated prior to 2002, this association disappeared over the last two decades. This disconnect occurred despite surveys showing that sea otter prey have continued to decline. Kelp area trends are decoupled from both sea otter and benthic invertebrate abundance at current densities. Variability in kelp abundance has declined in the most recent 15 years, as it has the synchrony in kelp abundance among sites. Together, these findings 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 utility of sea otter predation in ecosystem restoration must be considered within the context of complex and shifting environmental conditions.
KeywordsSea otters Sea urchins Kelp forests Top–down control Trophic cascades Predator–prey interactions Keystone predator Community ecology Spatial ecology Marine ecosystems Enhydra lutris Macrocystis pyrifera Nereocystis luetkeana
We thank H Jackson and G Galasso for piloting the research vessels during field work, and the United States Coast Guard station at Neah Bay for providing docking space. We thank Washington Department of Fish and Wildlife for their excellent sea otter surveys. C. Pfister provided thoughtful discussions, field assistance, and comments on the manuscript. J. Hale, K. Laidre, and two anonymous reviewers provided comments that improved the manuscript. Order of authorship was determined in part by efficiency of SCUBA-based navigation of Tatoosh Island while dodging menacing Steller sea lions. This study was supported by funding from the National Marine Fisheries Service, the Office of National Marine Sanctuaries, and the NOAA Integrated Ecosystem Assessment program.
Author contribution statement
AOS, CJH, JFS, KSA, BEF, KEF, NT, and GDW designed the surveys and performed field work. AOS, CJH, and JFS analyzed the data. AOS, CJH, and JFS wrote the manuscript; other authors provided editorial advice. BEF created Fig. 1.
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