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Effects of Local Deforestation on the Diversity and Structure of Southern California Giant Kelp Forest Food Webs


It has been hypothesized that the high diversity of giant kelp forests is due primarily to the provision of energy and habitat by the giant kelp (Macrocystis pyrifera). In this article, I use a 19-year-long kelp forest-monitoring data set from the Channel Islands National Park (a) to identify associations between subtidal species and forested or deforested habitats, (b) to generate an idealized food web for Southern California giant kelp forests in order to identify the primary conduits of energy flow through the system, and (c) to determine changes in the diversity and complexity of this food web due to localized giant kelp deforestation. A total of 275 common species were observed in the park between 1982 and 2000, of which 36% occurred significantly more often in kelp-forested areas than in deforested areas (that is, sea urchin barrens); 25 species were found exclusively in forested areas. Most of these associations were clearly identified as trophic and/or structural associations with giant kelp itself. The producer level of the food web was diverse, although giant kelp apparently represents the greatest single source of fixed carbon through either direct grazing or the production of phytodetritus. Primary, secondary and tertiary consumer levels were also represented by numerous species, and generalist consumers were common. With deforestation, the source of primary production shifts from primarily kelps to ephemeral microalgae, macroalgae, and phytoplankton. These results support the reliance of giant kelp forest food-web structure and diversity on the presence of the forest itself.

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I thank D. Kushner of the CINP kelp forest-monitoring program for providing the CINP species lists, and M. Behrens and K. Lafferty for generously providing me with their unpublished identification of CINP forested and deforested sites. Thanks also to M. Behrens, M. Carr, P. Dayton, M. Edwards, J. Estes, L. Ferry-Graham, M. Foster, R. Grosberg, B. Kinlan, J. Pearse, P. Raimondi, K. Riser, E. Sala, J. Stachowicz, M. Tegner, and numerous others for their discussions of the natural history of giant kelp forest organisms. L. Ferry-Graham, M. Foster, B. Santelices and an anonymous reviewer provided critical reviews of this article. A University of California Faculty Fellowship supported this research.

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Correspondence to Michael H. Graham.

Appendix 1

Appendix 1

Table 1 Common taxa observed in more than 12 of 119 (10%) Channel Island National Park (CINP) monitoring surveys (1982–2000)

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Graham, M. Effects of Local Deforestation on the Diversity and Structure of Southern California Giant Kelp Forest Food Webs . Ecosystems 7, 341–357 (2004).

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  • giant kelp
  • Macrocystis pyrifera
  • kelp forests
  • deforestation
  • sea urchin barrens
  • habitat association
  • food web
  • foundation species