By Andrew Hipp, PhD
Plant Systematist and Herbarium Curator
March 2010

A single square meter of upper Midwest prairie may contain 10 to 15 species [1]. A square foot of land in one of England’s chalk grasslands may contain more than 30 species, many of them no taller than the top of your boot [2]. This is equivalent to a plot of ground approximately the size of a record album cover, with each species covering an average of less than 5 square inches.

How does this kind of diversity persist? As Jonathan Silvertown asks in his book Demons in Eden [2], why doesn’t a single species, a super-fit super-competitor, take over the entire prairie? One answer to this question has long been that coexisting species—species that grow together within a plant community—must differ in how they utilize resources, and physiological trade-offs keep any single plant from dominating in every community. There have been excellent studies demonstrating examples of such trade-offs. In one study conducted in Amazonian forests [3], habitat specialization to soil types in tree species was found to involve an interaction between herbivore defense and growth rate: trees that can grow most rapidly are the best competitors on richer soils, but they were devoured by insects on poorer soils. The trees that fare best on the poorer soils are better defended against insects, but at the cost of their ability to grow rapidly.

However, numerous studies of plant distributions within communities fail to find significant differences among species in how they utilize available resources. If ecological differences between species explain diversity within communities, why can’t we find strong differences? A recent article by James Clark of Duke University [4] demonstrates that even when differences are not evident among species, the differences among individuals within species may allow those species to coexist.

Clark and colleagues studied 33 tree species in 11 forests of the southeastern United States over the course of six to 18 years (depending on the site), studying variation in growth responses for more than 22,000 trees. This is a fantastic dataset, as it is both broad in species and within-species sampling and relatively long in years. Clark studied how each individual tree responded to environmental fluctuations from year to year. He found, first, that different coexisting species tend to respond in the same way to environmental variations. This would suggest that trees do not partition their environment in a way that would promote coexistence. But then he compared the growth rate of every pair of individual trees growing within 20 meters of each other and the reproductive effort (measured in seeds per year) for every pair of individual trees growing within 60 meters of each other. He found something very striking: while among-species differences in response to environmental variation are not significant, the correlation in growth rate and reproductive effort is much higher between individuals of a single species than between individuals of different species. Two individuals of different species will be likelier to live within 20 meters of each other than two individuals of the same species. In Clark’s own example, “On average, both species benefit in wet years, but it is not the same individuals that are benefitting and not to the same degree.” It is variation within species, as much as variation among species, that leads to diversity of plant communities.

In a previous article , I discussed how plant biodiversity entails diversity both within and among species. Clark’s article is an excellent example of how diversity within species leads to diversity among species.

NOTES:
1.            Leach, M.K. and T.J. Givnish, Gradients in the composition, structure, and diversity of remnant oak savannas in southern Wisconsin. Ecological Monographs, 1999. 69(3): p. 353-374.

2.            Silvertown, J., Demons in Eden: The paradox of plant diversity. 2005, Chicago and London: The University of Chicago Press.

3.            Fine, P.V.A., I. Mesones, and P.D. Coley, Herbivores Promote Habitat Specialization by Trees in Amazonian Forests. Science, 2004. 305(5684): p. 663-665.

4.            Clark, J.S., Individuals and the Variation Needed for High Species Diversity in Forest Trees. Science, 2010. 327(5969): p. 1129-1132.

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