Background/Question/Methods More that half of all species in the network of CTFS plots are so rare that collectively they constitute less than 1 % of all individuals.  Explaining tropical tree rarity is thus a central issue in explaining tropical tree species richness.  In this talk I will outline a new hypothesis, the Enemy Susceptibility Hypothesis (Hubbell in press), to explain commonness and rarity in tropical tree species. New data indicate that spatial population dynamics of tropical trees are dynamically coupled on landscape scales. 

Results/Conclusions The data suggests that populations of rare species are knocked down before they become as abundant as common species, perhaps from slow-killing heart-rot and root-rot fungi.  Although this hypothesis is still speculative, there are both theoretical results and empirical data in its support.  I suggest that this may be a general if not universal explanation for patterns of commonness and rarity in tropical tree communities.