be able to support. The duo tested their model by trying to predict how many herbivore species would be found in each of 28 different East African wildlife reserves (where the quantity of resources, in the form of plant food, can be predicted from the amount of rainfall) and how many grass species should be found in experimental plots on the Minnesota prairie (where the amount of nitrogen in the soil is a good measure of total resources). The results were impressive: The number of species at each site, and their size differences, matched the model’s predictions well. But Ritchie and Olff’s idea is not all-embracing. It works well only on smallish scales, areas up to a few thousand times larger than the individual home ranges of the organisms under study. For soil bacteria this would be a few cubic centimeters; for a plant it’s a field; for a grazing herbivore it’s the area of a game park. Over greater areas other factors, such as whether organisms can get between distant patches, come into play and so its predictions start to break down—it can’t tell you why there are 700 species of birds in North America and not 7.

Both Tilman’s and Ritchie and Olff’s theories are based on the Hutchinsonian idea that biodiversity is the result of some sort of balance in how species divide up the environment. But other ecologists think that nature is not in balance and that it is the very disturbances that Hutchinson ignored—the droughts, floods, and fires—that allow species to live together. Even if two species share identical niches, one does not instantly eliminate the other. The speed with which this happens depends on what else is going on in the environment. In a stable environment, competition will run quickly. Nothing else is going on. But in the real world, lots of things can send the competitors back to the starting line, shift the balance between them, or so preoccupy them that they never get around to competing at all. A tree cannot crowd out its neighbor if it is struck by lightning or blown over. But the next lightning bolt will strike another species of tree, allowing a third species to survive. On a human timescale, it looks as if the trees in a rain forest have struck a happy balance. In fact, they might be strangling one another, but so slowly and haphazardly that we never see a killer blow. Taking competitors out of the game reduces the strength of competition, perhaps to a level where competition is not strong enough to



The National Academies | 500 Fifth St. N.W. | Washington, D.C. 20001
Copyright © National Academy of Sciences. All rights reserved.
Terms of Use and Privacy Statement