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The "intermediate disturbance hypothesis," which postulates maximum diversity at intermediate regimes of disturbance, has never been clearly proved to apply to species-rich tropical forest tree communities and to local-scale canopy disturbances that modify light environments. This hypothesis was tested on a sample of 17,000 trees in a Guianan forest, 10 years after a silvicultural experiment that added to natural treefall gaps a wide range of disturbance intensities. Species richness, standardized to eliminate density effects, peaked at intermediate disturbance levels, particularly when disturbance intensity was estimated through the percentage of stems of strongly light-dependent species.
The proposition that "the highest diversity is maintained at intermediate scales of disturbance" (1), known as the "intermediate disturbance hypothesis," remains largely untested for the highly diverse rain forest tree communities, and for the most prevalent patch-size disturbances (2) in such communities, canopy light gaps (3). This model is particularly difficult to study in this case because (i) trees are longlived, thus, data sets large enough to cover temporal and spatial variations of tree species richness and their relationships with disturbances are very few (46), and (ii) the mechanisms through which light gaps influence tree regeneration are still not fully elucidated (3). Until now, most studies have focused on the pioneer guild and early gap phases, and on the pioneer/ nonpioneer and gap/nongap contrasts, because both conceptual objects are rather easily circumscribed in the field: shade-intolerant pioneer species germinate exclusively in open places, and newly opened gaps can be delimited through canopy height measurement (7). These approaches have demonstrated the existence of interspecific differences in light requirements among tropical forest trees, but have given little evidence of gap partitioning by pioneers (3), thus failing to convince all ecologists that the intermediate disturbance hypothesis could explain the maintenance of high tree species diversity in tropical rain forests.
In a study of the variation in species richness of tree saplings during early gap-phase regeneration in a 50-ha plot at Barro Colorado Island (BCI), Panama, Hubbell et al. concluded that this hypothesis should be rejected in favor of another model, the recruitment limitation hypothesis (8). According to this hypothesis, the effects of dispersal- and recruitment-limitation on tree species diversity outweigh those of disturbance. Although the opening of gaps in mature forest does enlarge...