Functionally Dominant Herbivores as Keystone Species

Jeff W. Higdon

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Published: August 12, 2002

Both Davic (2000) and Piraino and Fanelli (1999) revisit Paine’s (1966, 1969) original concept of a “keystone” species, and warn against using “key” and “keystone” species interchangeably. These authors define a keystone species, sensu Paine, as one that, through selective predation, has the “potential to maintain high species diversity within the functional groups of the prey community” (Davic 2000). The authors propose that a “keystone” species is a predator, and, following this, major herbivores are not “keystone species,” but rather “key species.”

This definition, although it preserves Paine’s original concept, is too limited in extent. Any species with an important ecosystem function can be a keystone species, and this would include herbivores as well as carnivores. Davic (2000), quoting Schulze and Mooney (1993), states, “keystone species may be regarded as functional groups with no redundant representation.” Herbivores can fit this relatively narrow definition of a keystone species. As an example, in boreal Canada, snowshoe hare (Lepus americanus) influence vegetation patterns through browsing and are an important prey source for a variety of avian and mammalian predators. Studies have clearly shown that the populations of numerous predators, most notably Canada lynx (Lynx canadensis), are closely tied with snowshoe hare abundance.

The presence of hare assists in the maintenance of high species diversity in the predator community. Its loss would result in significant changes in forest composition and a substantial decline in carnivore populations. This decline would be much more pronounced than that created by the loss of other prey species. Snowshoe hare therefore have a “disproportionate effect” (Piraino and Fanelli 1999) on the associated vertebrate community. A keystone predator can be a keystone species, but a keystone species does not necessarily have to be a predator.

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Davic, R. D. 2000. Ecological dominants vs. keystone species: A call for reason. Conservation Ecology 4(1):r2. [online] URL: http://www.consecol.org/Journal/vol4/iss1/resp2

Paine, R. T. 1966. Food web complexity and species diversity. American Naturalist 100:65-75.

Paine, R. T. 1969. A note on trophic complexity and species diversity. American Naturalist 103:91-93

Piraino, S., and G. Fanelli. 1999. Keystone species: what are we talking about? Conservation Ecology 3(1): r4. [online] URL: http://www.consecol.org/vol3/iss1/resp4

Schulze, E.-D., and H. A. Mooney. 1993. Ecosystem function of biodiversity: a summary. Pages 497-510 in E.-D. Shulze and H. A. Mooney, editors. Biodiversity and ecosystem function. Springer-Verlag, Berlin, Germany.