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Marine Ecology Progress Series

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MEPS 199:271-280 (2000)  -  doi:10.3354/meps199271

Limited effects of a keystone species: trends of sea otters and kelp forests at the Semichi Islands, Alaska

Brenda Konar*

U.S. Geological Survey, University of California, Santa Cruz, California, USA
*Present address: School of Fisheries and Ocean Sciences, University of Alaska Fairbanks, PO Box 757220, Fairbanks, Alaska 99775-7220, USA. E-mail:

ABSTRACT: Sea otters are well known as a keystone species because of their ability to transform sea urchin-dominated communities into kelp-dominated communities by preying on sea urchins and thus reducing the intensity of herbivory. After being locally extinct for more than a century, sea otters re-colonized the Semichi Islands in the Aleutian Archipelago, Alaska in the early 1990s. Here, otter populations increased to about 400 individuals by 1994, but rapidly declined to about 100 by 1997. Roughly 7 yr after initial otter re-colonization, there were only marginal changes in sea urchin biomass, mean maximum test size, and kelp density. These small changes may be the first steps in the cascading effects on community structure typically found with the invasion of a keystone species. However, no wholesale change in community structure occurred following re-colonization and growth of the sea otter population. Instead, this study describes a transition state and identifies factors such as keystone species density and residence time that can be important in dictating the degree to which otter effects are manifested.

KEY WORDS: Community structure · Trophic interactions · Urchin barrens · Enhydra lutris · Strongylocentrotus polyacanthus · Alaria fistulosa

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