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Ecosystem engineering by invasive exotic beavers reduces in-stream diversity and enhances ecosystem function in Cape Horn, Chile


Species invasions are of global significance, but predicting their impacts can be difficult. Introduced ecosystem engineers, however, provide an opportunity to test the underlying mechanisms that may be common to all invasive engineers and link relationships between changes in diversity and ecosystem function, thereby providing explanatory power for observed ecological patterns. Here we test specific predictions for an invasive ecosystem engineer by quantifying the impacts of habitat and resource modifications caused by North American beavers (Castor canadensis) on aquatic macroinvertebrate community structure and stream ecosystem function in the Cape Horn Biosphere Reserve, Chile. We compared responses to beavers in three habitat types: (1) forested (unimpacted) stream reaches, (2) beaver ponds, and (3) sites immediately downstream of beaver dams in four streams. We found that beaver engineering in ponds created taxonomically simplified, but more productive, benthic macroinvertebrate assemblages. Specifically, macroinvertebrate richness, diversity and number of functional feeding groups were reduced by half, while abundance, biomass and secondary production increased three- to fivefold in beaver ponds compared to forested sites. Reaches downstream of beaver ponds were very similar to natural forested sections. Beaver invasion effects on both community and ecosystem parameters occurred predominantly via increased retention of fine particulate organic matter, which was associated with reduced macroinvertebrate richness and diversity (via homogenization of benthic microhabitat) and increased macroinvertebrate biomass and production (via greater food availability). Beaver modifications to macroinvertebrate community structure were largely confined to ponds, but increased benthic production in beaver-modified habitats adds to energy retention and flow for the entire stream ecosystem. Furthermore, the effects of beavers on taxa richness (negative) and measures of macroinvertebrate biomass (positive) were inversely related. Thus, while a generally positive relationship between diversity and ecosystem function has been found in a variety of systems, this work shows how they can be decoupled by responding to alterative mechanisms.

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Thanks to the field and lab assistants who participated in this study, especially Margaret Sherriffs and Michelle Moorman. The following experts helped to identify specimens: W. Shepard, California State University, USA (Elmidae), P. Adler, Clemson University, USA (Simuliidae), I. McLellan, Landcare Research Institute, New Zealand (Gripopterygidae) and M. Mercado, Latoratorio Benthos, Chile (General). The manuscript was improved by comments from A. Mehring, C. R. Carroll, A. T. Fisk, C. M. Pringle, J. B. Wallace, the Rosemond Lab and three reviewers. This research was supported by a Fulbright Scholarship, a Boren Fellowship from the National Security Education Program of the U.S. Department of Defense, a Graduate Assistantship from the University of Georgia, a Tinker Foundation Travel Award, a National Science Foundation Doctoral Dissertation Improvement Grant (DEB-0407875) and an Institute of Ecology and Biodiversity Postdoctoral Fellowship to C. B. A. (ICM P05–002). Logistical support was provided by the Omora Ethnobotanical Park—University of Magallanes (, This research is part of the long-term ecological study network coordinated by the Institute of Ecology and Biodiversity ( and conforms to all pertinent national laws.

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Correspondence to Christopher B. Anderson.

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Communicated by Michael Keough.

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Anderson, C.B., Rosemond, A.D. Ecosystem engineering by invasive exotic beavers reduces in-stream diversity and enhances ecosystem function in Cape Horn, Chile. Oecologia 154, 141–153 (2007).

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  • Ecosystem engineer
  • Exotic
  • Invasion
  • Secondary production
  • Subantarctic