Hubbell's neutral theory of biodiversity suggests a neutral birth-death-migration process that yields the observed species abundance ratio statistics.  A spatially explicit generalization of this theory provides some predictions for the spatial patterns of biodiversity like species-area relationships. Motivated by recent works on patch size distribution in the semi-arid climatic zone we compare the cluster size statistics of a single species, as predicted by Hubbell's theory, with the empirical distribution observed in the Barro Colorado Island. Our preliminary study shows that the observed patch statistics admits, for the most abundant species, a power-law tail, in contrast with the exponential decay predicted by the neutral dynamics. The observed patterns fit quite nicely a generalized stochastic model in which the birth-death ratio is positive for some species and negative for others.