Sea Otters and Kelp Forests in Alaska: Generality and Variation in a Community Ecological Paradigm

• Published in: Ecological Monographs
• Main Authors: Estes, James A., Duggins, David O.
• Format: Article in Journal/Newspaper
• Language: English
• Published: Wiley 1995
• Subjects: Alaska; Aleutian Islands
• Online Access: http://dx.doi.org/10.2307/2937159; https://api.wiley.com/onlinelibrary/tdm/v1/articles/10.2307%2F2937159; https://esajournals.onlinelibrary.wiley.com/doi/pdf/10.2307/2937159

Multiscale patterns of spatial and temporal variation in density and population structure were used to evaluate the generality of a three—trophic—level cascade among sea otters (Enhydra lutris), invertebrate herbivores, and macroalgae in Alaska. The paradigm holds that where sea otters occur herbivores are rare and plants are abundant, whereas when sea otters are absent herbivores are relatively common and plants are rare. Spatial patterns were based on 20 randomly placed quadrats at 153 randomly selected sites distributed among five locations with and four locations without sea otters. Both sea urchin and kelp abundance differed significantly among locations with vs. without sea otters in the Aleutian Islands and southeast Alaska. There was little (Aleutian Islands) or no (southeast Alaska) overlap between sites with and without sea otters, in plots of kelp density against urchin biomass. Despite intersite variation in the abundance of kelps and herbivores, these analyses demonstrate that sea otter predation has a predictable and broadly generalizable influence on the structure of Alaskan kelp forests. The percent cover of algal turf and suspension feeder assemblages also differed significantly (although less dramatically) between locations with and without sea otters. Temporal variation in community structure was assessed over periods of from 3 to 15 yr at sites in the Aleutian Islands and southeast Alaska where sea otters were 1) continuously present, 2) continuously absent, or 3) becoming reestablished because of natural range expansion. Kelp and sea urchin abundance remained largely unchanged at most sites where sea otters were continuously present or absent, the one exception being at Torch Bay (southeast Alaska), where kelp abundance varied significantly through time and urchin abundance varied significantly among sites because of episodic and patchy disturbances. In contrast, kelp and sea urchin abundances changed significantly, and in the expected directions, at sites that were being recolonized by sea ...