Herbivores as Keystone Predators

Robert D. Davic

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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.