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International Journal of Ecology
Volume 2011 (2011), Article ID 868426, 15 pages
Review Article

The Humpbacked Species Richness-Curve: A Contingent Rule for Community Ecology

1Department of Biology, Berry College, Mount Berry, GA 30149, USA
2Western Fisheries Research Center, U.S. Geological Survey, Seattle, WA 98115, USA

Received 1 January 2011; Revised 22 April 2011; Accepted 23 May 2011

Academic Editor: Shibu Jose

Copyright © 2011 John H. Graham and Jeffrey J. Duda. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.


Functional relationships involving species richness may be unimodal, monotonically increasing, monotonically decreasing, bimodal, multimodal, U-shaped, or with no discernable pattern. The unimodal relationships are the most interesting because they suggest dynamic, nonequilibrium community processes. For that reason, they are also contentious. In this paper, we provide a wide-ranging review of the literature on unimodal (humpbacked) species richness-relationships. Though not as widespread as previously thought, unimodal patterns of species richness are often associated with disturbance, predation and herbivory, productivity, spatial heterogeneity, environmental gradients, time, and latitude. These unimodal patterns are contingent on organism and environment; we examine unimodal species richness-curves involving plants, invertebrates, vertebrates, plankton, and microbes in marine, lacustrine, and terrestrial habitats. A goal of future research is to understand the contingent patterns and the complex, interacting processes that generate them.