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Ecological energy: bigger is better.

Ecological energy: Bigger is better

A large bird like the wild turkey takes in more food than, say, a sparrow. Moreover, despite their typically smaller numbers, large birds as a group may use a disproportionately large share of the resources available within an ecological community, this doesn't compensate for their lower food needs per individual.

The finding that larger animals seem to dominate an ecosystem may help answer some evolutionary questions and appears to contradict earlier studies concluding that species of small body size use at least as large a proportion of the resources within ecosystems as their larger relatives. "Our evidence suggests that this is not the case," says James H. Brown of the University of Arizona in Tucson. "On the average, the energy flow through the larger species outweighs that through the smaller species." He and Brian A. Maurer of Brigham Young University in Provo, Utah, report on their work in the Nov. 20 NATURE.

Brown and Maurer analyzed data covering the population density and individual body mass of related species within an ecosystem. They looked at birds in a variety of habitats across North America, rodents in a desert environment, marine fish in tidal pools and perennial plants in five different desert habitats. In all of these groups, the researchers say, species made up of large individuals account for most of the energy flow and resource use within local ecosystems.

What isn't clear is whether the results apply over the whole range of animals or plants within large areas. "I'm quite willing," says John Damuth of the Smithsonian Institution in Washington, D.C., "to accept the idea that within [groups of related species] in local communities, there might very well be an advantage to large size." Nevertheless, he says, according to his analyses, that advantage may disappear on larger scales across broader groups of animals.

Brown and Maurer say their results may help explain evolutionary patterns in which small organisms eventually give rise to gigantic forms that often become Extinct. "There are certain roles in communities that can be filled efficiently only by these large species," says Brown, but population densities and sizes also go down. The evolutionary process pits the advantages of being an individual of large size (greater likelihood of survival, more mobility, etc.) against the greater probability of species-wide extinction because of smaller populations and slower growth.
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Title Annotation:large animals found to dominate ecosystems
Author:Peterson, Ivars
Publication:Science News
Date:Nov 29, 1986
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