Search for the Missing Sea Otters: An Ecological Detective Story

The data below are from a long-term and large-scale study of sea otters and kelp forest communities in southeast Alaska and the Aleutian Islands. Figure 5 and Tables 2 and 3 compare sea urchin and kelp abundances in areas with and without sea otters (see Figure 2 for a map showing the locations). Although sea otters formerly were found at all of these locations, they were exterminated from most of their range by hunting during the 19^th century. Amchitka and Adak Islands in the Aleutians were locations of some of the few remnant populations at the time otters were protected in the early 1900s. Sea otters were re-introduced to southeast Alaska in 1968-71. That population expanded into Surge Bay by the early 1970s and into Torch Bay in 1985.

Table 4 summarizes data from several studies on the diet of sea otters. What do these data tell you about the role of sea otters in their community? How do you think the sea otters are affecting these two groups of species? What effects do you expect sea otters to have on the rest of the kelp forest community?

Figure 5. Kelp density (individuals/0.25 m²) plotted against estimated sea urchin biomass (g/0.25 m²) for the Aleutian Islands and southeast Alaska (see Figure 2 for map). Points represent averages for sites at each location. Sea urchin biomass was estimated from samples of population density, size-frequency distribution, and the relationship between urchin diameter and wet mass. (From Estes, J.A., and D.O. Duggins. 1995. "Sea otters and kelp forests in Alaska: generality and variation in a community ecological paradigm." Ecological Monographs 65:75-100. Reproduced with permission of the Ecological Society of America.)

Table 2. Abundance and population characteristics of kelps and sea urchins at two locations in the Aleutians, Amchitka and Shemya Islands, in 1972 and 1987 (shown as means ± 1 standard error). The same four sites at Amchitka and two sites at Shemya were sampled in both years*. Sea otters were continuously abundant at Amchitka and absent from Shemya during the 15-yr period. (From Estes, J.A., and D.O. Duggins. 1995. "Sea otters and kelp forests in Alaska: generality and variation in a community ecological paradigm." Ecological Monographs 65:75-100.)

*The 1972 data were obtained from 10 haphazardly placed 0.25-m² quadrats/site, the 1987 data from 20 randomly placed 0.25-m² quadrats/site.

Table 3. Abundance and population characteristics of kelp and sea urchins at two locations in southeast Alaska, Torch Bay (1976-1978) and Surge Bay (1978 and 1988), shown as means ± 1 standard error. Sea otters were continuously absent at Torch Bay and present at Surge Bay during these time periods. (From Estes, J.A., and D.O. Duggins. 1995. Sea otters and kelp forests in Alaska: generality and variation in a community ecological paradigm. Ecological Monographs 65:75-100.)

*Primarily Alaria fistulosa and Nereocystis leutkeana.
**Primarily Laminaria groenlandica.

Table 4. Occurrence of prey items in sea otter stomachs and feces. (From Estes, J.A., N.S. Smith, and J.F. Palmisano. 1978. "Sea otter predation and community organization in the western Aleutian Islands, Alaska." Ecology 59:822-833.)

*Percent of total volume: carnivores 65 (including fish 62.2) and herbivores 35.

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