Tropical Tree Richness and Resource-Based Niches

In assessing the role of treefall gaps in maintaining species richness in a tropical forest, S. P. Hubbell et al. (1) provide a test of the intermediate disturbance hypothesis, which predicts greater species richness in gaps than in mature forest. Hubbell et al. find more tree species per quadrat in gaps than in mature forest, but reject the richness difference as “spurious” after showing that the distributions of species per stem do not differ between gap and nongap quadrats. We find two problems with this key conclusion. First, for diverse tropical forests, the intermediate disturbance hypothesis predicts not simply that species richness in any one gap will be greater than in the same area of mature forest matrix, but that gaps collectively will be richer than the matrix, because gaps provide more diverse conditions and resources. The appropriate comparison is between the per-stem species accumulation curve for gaps and the corresponding curve for the nongap matrix (Fig. 1) (2). The species-area plot in figure 3B in the report (1) shows average richness per area, not cumulative richness per stem. Second, richness may actually be higher in individual gaps than in the forest matrix, for matched numbers of stems, despite the absence of any difference in species per stem. This paradox arises from the fact that expected number of species per stem decreases monotonically with number of stems in any species accumulation curve (Fig. 1). Thus, species per stem is an inappropriate statistic for comparing species richness among samples that differ in density (3). The same pitfall undermines their conclusion that pioneer species richness is independent of gap size. Because Hubbell et al. do not provide species accumulation curves, one cannot evaluate the statement that richness is identical in gaps and nongaps. The conclusions reached by Hubbell et al. are further weakened because the contribution of seedlings to gap richness is not taken into account, despite the nearly fivefold higher density of seedlings in gaps. The statement that mortality in gaps is “a random-thinning process” does not account for differential mortality between seedling and sapling stages and between saplings and adults. For these reasons, we question the inference that species richness in tropical forests is decoupled from gap disturbance regimes.