We examine several features of Hubbell's nonequilibrium, or "null", model o
f tree dynamics, which holds that species-rich tropical tree communities ar
e maintained on a local scale by a balance of extinction and immigration, a
nd on a global scale by a balance of extinction and speciation. All species
are held to be ecologically equivalent, such that species having equal ini
tial 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: 3
61-367) showed that persistence times decrease when unequal colonization ra
tes 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 al
l natural tree communities. As a result, persistence times drop dramaticall
y, and forest composition becomes highly deterministic, such that long-live
d species drop out of the community much more slowly than short-lived speci
es. We also note that the use of tree deaths (instead of years) as a measur
e of time inflates estimates of persistence times. In summary, calculated p
ersistence times of tropical tree species, even those in very large stands,
no longer reach time scales plausible for speciation.