Benthic community modification and recovery following intensive grazing byStrongylocentrotus droebachiensis

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Abstract

IntensiveStrongylocentrotus droebachiensis grazing of a previously sampled one-hectare plot provided an opportunity for quantitatively investigating grazing impact on the benthic macrophyte community and community recovery. The changes induced by grazing are compared with changes in a similar plot located nearby which was not grazed. A total of 154 quantitative 0.25 m2 quadrats were collected in mid-summer, from the two sites, over a 4-year period. The results presented indicate thatS. droebachiensis populations in the Strait of Georgia undergo periodic, environmentally controlled, outbreaks and that these outbreaks are responsible for localized perturbation of the benthic macrophyte community. Short-term grazing, where the urchins do not remain in the area, result in a 60 percent reduction in most community parameters measured. The community requires 2 or 3 years to regain or exceed pre-grazing biomass levels while other parameters had not recovered after 3 years. An estimated 4–6 year period is required to achieve “ecological climax” (Fishelson, 1977) and “species enrichment” follows. The largest macrophyte in the Strait of Georgia, the kelpNereocystis luetkeana, is a successional species whose abundance is controlled, in part, by the successional status of the community.