Dr. Fangliang He is part of a research team that studied fundamental questions plaguing scientists since Darwin's time: why are some species so common while others are rare? How do common and rare species interact? And how do hundreds, even thousands, of tree species coexist in a limited space in the tropics?

He, along with Igor Volkov and Jayanth Banavar, from Pennsylvania State University, Stephen Hubbell from the University of Georgia and Amos Maritan from the Universita di Padova in Italy, offer a new theory to explain why tropical rain forests are so species rich and how species are assembled in a community. Their work is published in the current edition of "Nature".

Species must meet certain conditions to live in a community. Understanding the rules that make up community assemblages is one of the most challenging scientific questions facing scientists today. Niche theory, which assumes species differ from one another in various aspects, has been traditionally used to explain community assemblages. However, this theory offers little to predict community assemblage patterns � the way species share a limited space.

He's work attempts to address community assembly rules based on Hubbell's recently developed neutral theory. "The basic idea of the neutral theory is that community membership is determined by five fundamental processes: birth, death, immigration, speciation and random drift. Furthermore, the theory assumes that every individual in the community, regardless of species identity, has the same rates of birth, death, immigration and mutating into a new species," said He, who is a Canada Research Chair from the Department of Renewable Resources.

The research team modified this theory by arguing that the birth rate and mortality rate are not identical across species, but there is a "density-dependent" probability of birth and death. The more abundant species have lower birth rates and higher mortality rates. "The consequence is that when a species becomes rare, its birth rate will increase and death rate will reduce," said He. In other words, species will regulate themselves to make room for each other if they follow the membership rules. "If not, they're out."

The scientists tested their model using data from six tropical rain forests�these tiny areas can accommodate more than 1000 tree species--across the world. "Our theory offers a better understanding of why tropical rain forests are so species rich," said He. "This rare species advantage regulates dynamics and therefore permits the coexistence of many species in a community."

This work was supported by NASA, by the NSF and the NSERC (Canada). The fieldwork has also received long-term support from the John D. and Catherine T. MacArthur Foundation, the Mellon Foundation, Earthwatch, Frank Levinson and the Celera Foundation, and other private foundations and individual donors.

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