Under the co-ordination of the Centre for Tropical Forest Science (CTFS) at the Smithsonian Tropical Research Institute, Panama, a study was conducted on seven 'tropical forest observatories' in Borneo, India, Malaysia, Panama, Puerto Rico and Thailand.
"Ecologists have debated for decades whether there is ecological value to species diversity," says Christopher Wills, a biologist at the University of California, San Diego, who headed the study. "We found that in forests throughout the...tropics, older trees are more diverse than younger ones. In other words, diversity is actually selected for as each of the forests matures. This means diversity does indeed matter and is an essential property of these complex ecosystems.
"Are the same processes operating in temperate forests?" he questions and, "How much damage can a forest sustain before its diversity begins to decline? Are other complex ecosystems, like coral reefs, also selected for increased diversity? This paper provides insights into a dynamic and evolving natural world and shows that diversity is not just an aesthetic ideal, but is also an important property of natural ecosystems."
The specialised monitoring of forests on two major continents revealed that nature promotes diversity by selecting for less commonplace species as trees mature. Thirty-three ecologists from twelve countries produced results which demonstrate that diversity has important ecological implications for tropical forest survival. The findings were published in last week's issue of the journal Science.
Diverse to begin with, the forest plots studied range from dense and species-rich wet rainforest to dry, open, fire-prone areas. Even so, all the tree populations manifested local diversity increasing decidedly with age.
"Each forest in our study is a highly dynamic community," explains project collaborator, Kyle Harms, a biologist at Louisiana State University. "We found that the diversity of each local area increased regardless of the species that were present. This is because trees that were locally common tended to die more often than those that were locally rare, giving a survival advantage to rare species." The effect was even noticed within individual species. Harms went on to say that, "If a species was common in one part of a plot and rare in another, its death rate was higher where it was common."
According to CTFS director, Stuart Davies. "The great scientific value of these tropical forest observatories is that each of them has undergone a complete census more than once, so that the researchers know what has happened to hundreds of thousands of trees from one census to the next." He elaborated that the observatories had facilitated vanguard work at the forefront of today's world ecology studies.
Scientists are still to understand exactly what is responsible for the increased diversity selection but they are able to point out that none of the three processes they identified as being involved are seen to operate in forests where trees are all of a single species.
The first of three possibilities focussed on by the research is that rare species may be better off because the animals, fungi, bacteria and viruses preying on them are less likely to cause damage when their hosts are rare. The second is that rarer species may be at an advantage in the competition for certain physical resources, because individuals of the same species have more similar resource needs than individuals of different species. Thirdly, rare species are favoured when tree species have direct, positive influences on one another because a high proportion of trees that are different from them usually surround a tree of a rare species. Indeed same species forests are highly susceptible to diseases where individual appear to be in direct competition with others like themselves.
These diversity-enhancing processes are probably absent from grossly denuded forests. When forests are completely felled, soil is rapidly eroded and depleted of nutrients. Furthermore the insect, bacterial and fungal life intrinsic to the sustenance of diversity largely disappears.
The authors of the study however believe that tropical forests which have only been damaged and not entirely cleared, can, by meticulously managed and selective logging for example, regain previous diversity levels providing that the assault has not been too severe or long-term.
"If you damage a forest a little bit, the forest can recover. Even damaged ecosystems can be restored to their former diversity through natural processes if they are allowed to do so." Wills says. He further explained that the new study opens the path to even more detailed investigations into the processes by which forest diversity is maintained. It also raises fresh questions and reveals uncharted avenues of research for forest managers as well as ecologists.
"This study addresses a fundamental question in tropical ecology," said Jess Zimmerman, currently a programme director in the National Science Foundation (NSF)'s Division of Environmental Biology and co-author of the paper. "In a snapshot of time," Zimmerman wonders, "are rare species rare because they are on the brink of local extinction, or because they are on their way to becoming more common?"
"This long-term study shows that there is an advantage to being rare, and that this advantage causes rare species to become more common."
In addition to the Smithsonian Tropical Research Institute, the Indian Institute of Science, the Royal Forest Department of Thailand, the University of Peradeniya of Sri Lanka, the University of Puerto Rico, and the Forest Research Institutes in Peninsular Malaysia and Sarawak managed the tropical forest observatories included in this study which was funded by grants from the National Science Foundation and the Centre for Tropical Forest Science of the Smithsonian Tropical Research Institute.
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