Role of Sea Otters in Structuring Nearshore Communities

Raft of sea otters and Alaria kelp off Pt. Carolus in Glacier Bay National Park and Preserve

Sea otters provide one of the best documented examples of top-down forcing effects on the structure and function of nearshore marine ecosystems in the North Pacific Ocean. Much of our knowledge of the role of sea otters as a source of community variation resulted from the spatial and temporal pattern of sea otter population recovery since their near extirpation about 100 years ago. During most of the early 20th century sea otters were absent from large portions of their habitat in the north Pacific. During the absence of sea otters, many of their prey populations responded to reduced predation through increased densities and sizes. Since the middle of the 20th century sea otter populations have been recovering previous habitats, due to natural dispersal and translocations. Following the recovery of sea otters, scientists have continued to provide descriptions of nearshore marine communities and have been able to contrast those communities before and after the sea otters return. At least three distinct approaches have proven valuable in understanding the effects of sea otters. One is contrasting communities over time, before and after recolonization by sea otters. This approach, in concert with appropriate controls, provides an experimentally rigorous and powerful study design allowing inference to the cause of the observed changes in experimental areas. Another approach consists of contrasting different areas at the same time, those with, and those without the experimental treatment (in this case, sea otters). A third approach entails experimentally manipulating community attributes and observing community response, usually in both treatment and control areas. All these opportunities currently present themselves at various locations throughout the sea otters’ range.

One area of recent reoccupation is Glacier Bay in Southeast Alaska, where sea otters were absent until as recently as 1994, but currently number > 4000 individuals. We are using this situation in Glacier Bay as a laboratory to experimentally evaluate the role of sea otter in structuring coastal marine communities in a predominately soft sediment habitat. It is predictable that the density and sizes of preferred sea otter prey such as crabs, clams, and urchins will decline in response to otter predation. This will result in fewer opportunities for human harvest, but will also result in ecosystem level changes, as abundance and sizes of prey for other predators, such as octopus, sea stars, fishes, birds and mammals are modified. Sea otters will also modify benthic habitats through excavation of sediments required to extract burrowing infauna such as clams. Effects of sediment disturbance by foraging sea otters are not understood. As the recolonization by sea otters continues, it is also likely that dramatic changes will occur in the species composition, abundance and size class composition of many components of the nearshore marine ecosystem. Many of the changes will occur as a direct result of predation by sea otters; other changes will result from indirect or cascading effects of sea otter foraging, such as increasing kelp production and modified prey availability for other nearshore predators.

Links

Western Ecological Research Center - Sea Otter Studies

Selected references

Estes, J. A., M. T. Tinker and J. L. Bodkin. 2009. Using ecological function to develop recovery criteria for depleted species: Sea otters and kelp forests in the Aleutian Archipelago. Conservation Biology 24(3):852-860. doi: 10.1111/j.1523-1739.2009.01428.x

Estes, J. A., J. L. Bodkin, and M. Ben-David. 2008. Marine Otters. In W. F. Perrin, B. Wursig,, J. G. M. Thewissen and C. R. Crumly (eds) Encyclopedia of Marine Mammals, 2nd Edition. Academic Press. 1352 pp.

Bodkin, J. L. and S. L. Boudreau. 2007. Toward an integrated science plan for Glacier Bay National Park and Preserve: Results from a Workshop, 2004, in Piatt, J.F., and Gende, S.M., eds., Proceedings of the Fourth Glacier Bay Science Symposium, October 26–28, 2004. U.S. Geological Survey Scientific Investigations Report 2007-5047:217-218.

Bodkin, J. L., G. G. Esslinger, and D. H. Monson. 2004. Foraging depths of sea otters and implications to coastal marine communities. Marine Mammal Science 20(2):305-321. doi: 10.1111/j.1748-7692.2004.tb01159.x