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."

related report
Bird song changes sound alarm over habitat fragmentation
London, UK (SPX) Nov 30 - Changes in bird song could be used as an early warning system to detect man-made ecological disturbances, new research published in the British Ecological Society's Journal of Applied Ecology has found. Although much previous research has focused on bird song and vocal mimicry, this is the first study to analyse the role played by habitat loss and fragmentation on song-matching.

Ecologists recorded and analysed the songs of more than 200 Dupont's larks, Chersophilus duponti, in Spain and Morocco and found that in fragmented habitats, song-sharing among neighbours was enhanced whereas song-sharing among non-neighbours declined. Having ruled out other explanations, such as the stage of the breeding season and competition intensity, the researchers say this change in song-sharing is due to lack of interaction between individuals isolated by habitat barriers.

According to the study's authors, Dr Paola Laiolo and Dr Jose Tella of the Estaci�n Biologica de Do�ana in Seville, Spain: "We found that habitat loss in the steppe matrix markedly affected song-type sharing mechanisms in Dupont's lark. The occurrence of anthropogenic habitat barriers seems to hinder cultural transmission of song types over distances, resulting in an intensification of the differences between non-neighbours and increasing mimicry between neighbours. This suggests that males from fragmented habitats perceived as rivals only the close neighbours with which they engaged in counter-singing."

"Communication systems of habitat-sensitive species might be used as a behavioural indicator of anthropogenic environmental deterioration. Because of their rapidly evolving cultural nature, bird vocalisations might become an early warning system detecting the effects of fragmentation over relatively short times and before other indicators - such as genetic markers - show any change," they say.

Sharing song types - when a male replies to a rival's song with the same song sequence - with neighbours is common in birds, and is thought to act as a threat signal between males, which would explain why birds have evolved such complex song repertoires.

The Dupont's lark is a rare and specialised steppe passerine, and its song unit is made up of up to 11 discrete sequences, each sequence being composed of up to 13 notes. The most common sequence is known as the 'whee-ur-wheeee'. To be considered as shared songs in this study, two sequences had to match at least 75% of their component notes and the matching portions had to be similar in note shape, timing and frequency.

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