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 betadiversity 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). As a further argument for low Amazonian beta-diversity, Condit et al. pointed out that plot-to-plot comparisons between Peru and Ecuador (1400 km apart) show 20% shared species, on average, which roughly equals the similarity observed for plots only 50 km apart in Panama. However, their data are not appropriate for such a comparison. Their Panamanian data included all tree species, but comparisons between Peru and Ecuador were based on fully identified species only and thus excluded about 25% of taxa in both regions because they had only morphospecies identifications. Morphospecies tend to have more restricted distributions than identified species (12), so the true similarity between Peruvian and Ecuadorian plots is most likely lower than Condit et al. estimated. Furthermore, stressing the relatively high floristic similarity between Peruvian and Ecuadorian plots gives an unbalanced view of known floristic variation in the region. Very different tree floras have been documented elsewhere in lowland western Amazonia, within much shorter geographical distances than that between the Peruvian and Ecuadorian plots of Condit et al. (5–7, 13). Condit et al. suggested that the observed deviation from the predictions of the neutral theory in the Panama data may be due to environmental heterogeneity, but they did not test this suggestion. Duivenvoorden et al. (2) attempted a formal testing by using multiple regression on distance matrices (14) to partition the variation in floristic similarites of the Panamanian tree plots to fractions explainable by either environmental difference alone, geographical distance alone, or environmental and geographical distance together. Their main result was that most of the floristic variation (59%) remained unexplained. However, their analysis used linear geographical distances, even though logarithmic distances provide a better fit to the distance decay predicted by Hubbell’s neutral theory. Furthermore, they used a floristic similarity measure that takes into account species abundances, although beta-diversity as addressed in Hubbell’s theory is not about species abundances but species turnover, and hence should be modeled using presenceabsence data. When we reanalyzed the data using logarithmic distances and species presence-absence data (15), only 41% of the variation remained unexplained. Duivenvoorden et al. compared their results with an earlier study, where variation partitioning was based on correspondence analysis (16). This comparison is uninformative because the two analysis methods focus on different aspects of the data, do not partition the same variation, and hence do not produce comparable results. The most serious problem with their study, however, is that the possible role of Hubbell’s neutral theory in explaining floristic variation cannot be tested using a variation partitioning approach, because the theory is not falsifiable in this way. It is a probabilistic theory, which can thus produce an unknown amount of variation in any of the four fractions. As a result, none of the fractions can be used as unequivocal proof against it, and new approaches need to be developed to test the extent to which dispersal and speciation affect species distributions.