Beta-Diversity in Tropical Forests

Condit et al. (1) and Duivenvoorden et al. (2) discussed beta-diversity of tropical rainforest trees and came to some conclusions that we find problematic. Condit et al. argued that beta-diversity of lowland rainforest trees is higher in Panama than in western Amazonia, based on observation of a steeper distance-decay of floristic similarity in 34 Panamanian tree plots compared with 16 Ecuadorian and 14 Peruvian tree plots. That constitutes a rather bold claim, given the small number of plots in their study; more important, such a result could have been predicted from their sampling strategy even without Hubbell's dispersal-based neutral theory (3), which was the focus of the Condit et al. study. The Panamanian plots spanned an annual precipitation range of 1900 to 3100 mm, included both secondary and old growth forests, and contained several base rock types from limestone and lavas to sandstone. In contrast, the two Amazonian regions were sampled in relatively homogeneous environments. The variation in annual precipitation within each region was negligible (4), only old growth forests were sampled, and the geological formations that are known to increase beta-diversity among lowland western Amazonian tree communities (5–7) were not represented. Consequently, the data merely confirmed that if one samples more heterogeneous terrain, one finds more floristic variability. That agrees both with common wisdom in plant ecology and with numerous earlier studies that have emphasized the role of environmental factors for species composition in tropical rainforests (5–11).