Predicting species diversity in tropical forests

A fundamental question in ecology is how many species occur within a given area. Despite the complexity and diversity of different ecosystems. there e xists a surprisingly simple, approximate answer: the number of species is p roportional to the size of the area raised to some exponent. The exponent o ften turns out to be roughly 1/4. This power law can be derived from assump tions about the relative abundances of species or from notions of self-simi larity. Here we analyze the largest existing data set of location-mapped sp ecies: over one million, individually identified trees from five tropical f orests on three continents. Although the power law is a reasonable, zeroth- order approximation of our data, we find consistent deviations from it on a ll spatial scales. Furthermore, tropical forests are not self-similar at ar eas less than or equal to 50 hectares. We develop an extended model of the species-area relationship, which enables us to predict large-scale species diversity from small-scale data samples more accurately than any other avai lable method.