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Copyright © 2002 by the author(s). Published here under license by The Resilience Alliance.

The following is the established format for referencing this article:
Davic, R. D. 2002. Herbivores as keystone predators. Conservation Ecology 6(2): r8. [online] URL: http://www.consecol.org/vol6/iss2/resp8/

Response to Jeff W. Higdon 2002. "Functionally dominant herbivores as keystone species"

Herbivores as Keystone Predators

Robert D. Davic

Ohio Environmental Protection Agency

I offer the following response to Higdon (2002). Although loss of snowshoe hare in boreal Canada most likely would have a large effect on vertebrate predators and ecosystem processes, this fact alone does not a “keystone species” make. Paine (1969) not only coined the term “keystone species,” but also presented the characteristics required for a species to be given keystone status: (1) it provides top-down effects (such as predation) on lower trophic levels, and (2) it prevents the monopolization of a critical resource (such as competition for space) in lower trophic levels. The synergy of this dualistic top-down (e.g., predation) and bottom-up (e.g., competition) interaction must (3) stabilize community diversity. As presented by Paine (1969), this narrowly defined keystone process is relatively rare and functionally non-redundant in ecosystems—thus, its great ecological significance.

It is my view that Higdon (2002) incorrectly proposes that “any species with an important ecosystem function can be keystone species.” Obviously, many species in ecosystems (such as the hare example given by Higdon, earthworms, termites, beaver) are of great importance to ecosystem function but, unless they attain the dualistic criteria of Paine (1969) as summarized above, such species are best coined “key species” or “ecologically dominant species” (Davic 2000), or, in the case of the beaver, “ecosystem engineers” (Jones et al. 1994). Higdon (2002) presents a strong argument that the boreal snowshoe hare is a key species for top carnivores. However, whether or not the hare is also a keystone species must await the results of its experimental removal to determine if such a removal regulates competitive interactions, and species diversity, within the plant functional group that it eats.

Realizing that plants can be “prey species” of hare corrects the false assumption by Higdon (2002) that herbivores cannot be keystone predators. For those uneasy with the suggestion that plants can function as prey species for herbivores, I offer the following from Wilson and Bossert (1971, p. 127):

....ecologists define predation in its broadest possible sense: the eating of live organisms, regardless of the identity of the organism. Predation includes the consumption of plants by animals, called herbivores....

For example, Paine (1992) experimentally documented that a few strong keystone interactions within a herbivore guild can regulate algae prey species diversity within an intertidal habitat.

I agree with Higdon (2002) that not all keystone species must be predators. The keystone species concept of Paine (1969) could be expanded to primary producers, fungi, and bacteria yet maintain a bi-directional (top-down, bottom-up) food web focus. If we consider elements in soil, especially cations that compete for attachment sites on clay and humus particles, to be “chemical species,” then plants, fungi, and bacteria may be keystones if they act to regulate the functional diversity of essential elements in ecosystems.

In summary, herbivores are predators of plant species and snowshoe hare may be a keystone species if it regulates competitive interactions within the functional group of its prey, but not because it is prey for top carnivores.

Published: September 6, 2002


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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://.consecol.org/Journal/vol4/iss1/resp2.

Higdon, J. W. 2002. Functionally dominant herbivores as keystone species. Conservation Ecology 6(2): r4. [Online] URL: http://www.consecol.org/Journal/vol6/iss2/resp4

Jones, C. G., J. H. Lawton, and M. Shachak. 1994. Organisms as ecosystem engineers. Oikos 69:373–386.

Paine, R. T. 1969. A note on trophic complexity and species diversity. American Naturalist 100:91–93.

Paine, R. T. 1992. Food-web analysis through field measurement of per capita interaction strength. Nature 355:73–75.

Wilson, E. O., and W. H. Bossert. 1971. A primer of population biology. Sinauer Associates, Inc., Sunderland, Massachusetts, USA.

Address of Correspondent:
Robert D. Davic
2110 East Aurora Road
Twinsburg, Ohio 44087 USA
Phone: (330)963-1132
Fax: (330)487-0769

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