Empirical investigations into the drivers of tropical forest community assembly have focused on the effects of competition (niches) and dispersal (neutrality) independently, without looking for patterns expected to arise from their interaction. Recent theory shows that competitive communities open to immigration may self-organize into clusters of species with similar ecological strategies, in contrast with traditional expectations of greater-than-chance trait differences. In combining competition and dispersal, cluster-based predictions effectively unite the coexistence mechanisms of niche theory and neutral theory, two prevailing paradigms of community assembly. Here we look for trait-based clustering among trees in Barro Colorado Island (BCI), Panama, using a method that incorporates species abundances, relies on no free parameters, and identifies the successful niche strategies.
Results/Conclusions
We show that BCI trees cluster by three traits related to strategies for light: maximum height, wood density, and leaf mass per area. They also cluster by seed mass, which may mediate strategies along a tolerance-fecundity tradeoff axis. These results are consistent with predictions of niche differentiation and immigration from a regional pool. Clustering by maximum height is particularly strong, underscoring the importance of light competition in structuring tropical tree communities. Our results point to four optimal strategies---i.e., niches---of light acquisition, aligning well with the classical grouping of Neotropical woody plants into shrubs, understory treelets, midstory trees, and canopy trees.