On the causes of gradients in tropical tree diversity

Abstract

1. The number of woody species in tropical forests tends to increase with precipitation, forest stature, soil fertility, rate of canopy turnover and time since catastrophic disturbance, and decrease with seasonality, latitude, altitude, and diameter at breast height (d.b.h.). 2. A model is presented to account for these trends. Novel hypotheses include how increased rainfall and substrate fertility, and decreased seasonality, might (i) increase attacks by natural enemies, and thus the overall level of density-dependent plant mortality; (ii) increase shade tolerance, canopy turnover, and stem density of the species-rich understorey; and (iii) increase reliance on relatively sedentary forest-interior birds for seed dispersal, fostering high rates of speciation in understorey genera. 3. High rainfall and low seasonality in the tropics favour two key groups of natural plant enemies - insects and fungi - that are directly responsible for promoting high rates of density-dependent plant mortality. Lower rainfall, greater seasonality, soil infertility, or unfavourable rooting conditions favour greater allocation to anti-herbivore defences, and thus lead to lower rates of such mortality and thence to lower tree diversity. The increased number of individuals on rainier sites is a minor contributor to increased tree diversity, accounting for only about 17% of the 8.3-fold increase with rainfall in the lowland Neotropics. 4. Predictions of the model are consistent with many ecological patterns of variation in tropical tree diversity within regions, and may help explain the decrease in tree diversity with elevation and the accompanying decrease in horizontal patchiness (within-habitat β diversity). 5. Random drift over evolutionary time in the relative effectiveness of density-dependent control of individual tree species by specialized natural enemies may better account for the observed distribution of tropical tree abundance than a random walk of species abundance through ecological time.