In the past explanations for high species diversity have been sought
at the species level. Theory shows that coexistence requires
substantial differences between species, but species-level data
rarely provide evidence for such differences. Using data from forests
in the southeastern United States, I show that variation evident at
the individual level provides for coexistence of large numbers of
competitors. Variation within populations in many dimensions allows
species to differ in their distributions of responses to the
environment, despite the fact that the populations to which they
belong do not differ on average. Results showing species differences
in higher dimensions require individual level analysis, and they are
consistent with theory predicting that coexistence depends on
competition being stronger within than between species.
