Seeds of understanding of plant diversity

Forests, grasslands, and other plant communities often contain large numbers of plant species; in the extreme, over a thousand tree species may be present in a single square kilometer of tropical forest (1). Ecologists have long sought to understand what factors govern the array of plant species present in any particular area, and indeed, how so many species can coexist at all given that all plants require essentially the same resources: light, water, and nutrients (2, 3). The classical explanation is that species can coexist if they are sufficiently different in their “niches,” the particular habitats or conditions in which they thrive or in the resources they exploit, and have corresponding weaknesses in other areas, so that no one species is able to out-compete the others (4). In this view, species composition of a community essentially reflects which niches are available. Alternatively, some ecologists have argued that chance events of immigration, local extinction, and speciation play a dominant role in determining which species are present in a community, so that species composition essentially “drifts” (5, 6). Empirical studies have demonstrated the importance of both sets of processes: there are meaningful differences among plant species in competitive ability under varying conditions, and yet historical factors are also involved in determining whether a species that could thrive in an area is actually present.

Traits important to competitive ability are variable and subject to evolutionary change.

A full understanding of plant communities thus requires consideration of both niche- and drift-related factors, as well as of both evolutionary and ecological forces (7). Such an integrated approach is attempted by Uriarte and Reeve (8) in this issue of PNAS, in their theoretical study of the conditions for long-term ecological and evolutionary stability of coexisting plant species. Previous studies often focused exclusively on ecological …