Abstract
The relative effects of sea urchin grazing and physical disturbance on community structure in a Macrocystis pyrifera (L.) C.A. Agardh forest were examined experimentally. Sea urchins, Strongylocentrotus franciscanus (Agassiz), were removed from shallow (8.5 m) and deep water (11.5 m) sites and subsequent changes in the algal and invertebrate populations followed relative to unmanipulated areas for over 12 months. Unusually high water motion also occurred during the experiment, allowing a comparison of grazing and storm effects. The results indicate that algal species composition, abundance, and distribution are strongly influenced by storm-related water motion by (1) initially reducing algal cover, especially the kelp canopy, (2) providing space for recruitment via abrasion and removal of portions of the substratum, and (3) causing apparent clumping of sea urchins in protected areas. Sea urchins dispersed after the storms, and there was a six-fold increase in algal cover, especially of the brown alga Desmarestia ligulata var. ligulata (Lightf.) Lamour. However, canopy-forming brown algal cover was generally higher and remained higher in urchin removal sites, while understory red algal cover was generally higher and remained higher in control sites. Other studies have suggested the sea otter—sea urchin—algae interaction as most important in structuring nearshore kelp communities in the North Pacific. Although changes in community structure related to sea otter predation on sea urchins may vary, our results suggest they will be less important than seasonal and year-to-year changes associated with storms in this study area.
Published Version
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