community and the expected distribution of relative species abundance and species range sizes. The “metacommunity” refers to the evolutionary–biogeographic unit in which most member species spend their entire evolutionary lifetimes, from origination to extinction, a concept apropos to the entire Amazon Basin. Neutral theory generates a biodiversity number θ that uniquely specifies not only how many species are expected to be present at steady state between speciation and extinction in the metacommunity but also the expected abundances of each species. The number θ is a fundamental quantity in neutral theory that is proportional to the product of the average per capita speciation rate in the metacommunity and the size of the metacommunity. Metacommunity size is simply the sum of the population sizes of all species in the metacommunity. An important discovery from neutral theory is that the expected distribution of metacommunity relative species abundance is Fisher’s logseries (Hubbell, 2001; Volkov et al., 2003).
The logseries distribution applies in cases when the metacommunity is continuous, as in continental tropical forests, but not necessarily if an island model is more appropriate for the metacommunity, as in the case of isolated coral reefs scattered across the Pacific Ocean (Volkov et al., 2007). Remarkably, it also turns out that the fundamental biodiversity number θ of neutral theory is identical to Fisher’s α, the celebrated diversity index of Fisher’s logseries, and parameter x of the logseries is the ratio of the average per capita birth rate to per capita death rate in the metacommunity. The reason Fisher’s α is so stable, according to neutral theory, is that it is proportional to the average speciation rate in the metacommunity and to the size of the metacommunity, both very stable numbers.
How do we fit Fisher’s logseries when the total number of tree species in the Amazon and their relative abundances are unknown? Extensive areas of the Amazon have not yet been adequately collected. Moreover, a large amount of material already collected remains to be described and classified for the first time, and many groups need revision to eliminate synonyms for species described multiple times from collections made by different museums at different times from different parts of Amazonia. Despite current problems with species-level identifications, however, generic-level determinations of Amazonian trees are much more reliable. Most undescribed tree species can at least be placed with reasonable confidence into a known genus. This is fortunate because we can test the fit of the logseries and the lognormal to the abundances of Amazonian genera.
Neutral theory asserts that generic- and familial-level clades should also obey the same metacommunity dynamics as species, the only difference being that they should have lower rates of origination and extinction than species do. Over the last two decades, a dataset comprising over