Tropical forests include the most diverse plant communities on earth, with up to 473 tree and liana species in a single hectare. Such forests also display striking patterns in species richness along several ecological gradients: the number of woody species 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. Potential causes for these patterns have been little explored. A model is presented to account for these trends, in terms of the potential effects of rainfall, seasonality, and soil fertility on (i) attacks by natural enemies, defenses against such enemies, and the overall level of density-dependent plant mortality; (ii) shade tolerance and/or canopy turnover and the density of the species-rich understory; and (iii) the incidence of endozoochory in understory plants, which rely on relatively sedentary forest-interior birds for seed dispersal. High rainfall and low seasonality in the tropics should favor two key groups of natural plant enemies ­ insects and fungi ­ directly promoting high rates of density-dependent plant mortality; lower rainfall, greater seasonality, soil infertility, or unfavorable rooting conditions all favor increased allocation to anti-herbivore defenses, promoting lower rates of such mortality and 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. 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.

Key words: anti-herbivore defenses, density-dependent mortality, tree turnover, tropical forests