The Smithsonian theory is based on a basic ecological premise called the neutral theory, Condit said, but adds to it the simple yet crucial observation that trees do not generally spread their seeds very far a factor which tends to enhance beta-diversity.

The Science report provides one of the most precise tests of the neutral theory yet published.

The team concludes that the neutral theory cannot account for beta-diversity in tropical forests, and they discount the importance of random events in establishing what grows there. Instead, Panamas high beta-diversity must be due to the abrupt variation in rainfall across the Central American isthmus, from the ever-wet Caribbean shoreline to the dry Pacific slope.

Forests across western Amazonia, however, were more uniform in species composition than the theory allowed, a surprising result.

Explanations for this uniformity will require deeper understanding of how different tropical trees are from one another, said co-author and Smithsonian scientist Egbert G. Leigh, Jr., who devised the mathematical formula that led to the undermining of the neutral theory.

More tedious field work, it seems, is in store, Leigh concluded.

'"/>

Contact: Richard Condit
condit@ctfs.si.edu
202-786-2094 8045
Smithsonian Tropical Research Institute
24-Jan-2002