Can high tree species richness be explained by Hubbell’s null model?

Abstract

We examine several features of Hubbell’s nonequilibrium, or “null”, model of tree dynamics, which holds that species‐rich tropical tree communities are maintained on a local scale by a balance of extinction and immigration, and on a global scale by a balance of extinction and speciation. All species are held to be ecologically equivalent, such that species having equal initial abundances have equal probabilities of extinction or fixation. We show here that the null model is not robust to relaxation of the assumption of ecological equivalence. Recently, Zhang & Lin (1997; J. Theor. Biol. 188: 361–367) showed that persistence times decrease when unequal colonization rates are allowed, but their results still permit very long persistence times in stands of hundreds of thousands of stems or more. We extend their work by allowing tree mortality rates to differ across species, as is seen in all natural tree communities. As a result, persistence times drop dramatically, and forest composition becomes highly deterministic, such that long‐lived species drop out of the community much more slowly than short‐lived species. We also note that the use of tree deaths (instead of years) as a measure of time inflates estimates of persistence times. In summary, calculated persistence times of tropical tree species, even those in very large stands, no longer reach time scales plausible for speciation.