A team of scientists led by SFSU researcher Thomas B. Smith reports in the journal Science that ecotones, the transition areas between rain forests and savannas, may be integral to generating biodiversity in tropical rain forests.
Conserving these ecotones may be critical to preserving the processes that maintain diversity in rain forests, says Smith and his colleagues, Robert K. Wayne, UCLA; Derek Girman, SFSU Romberg Tiburon Center; and Michael Bruford, the Institute of Zoology, London.
"Ecotones are dynamic environments that have typically been overlooked," says Smith, an evolutionary biologist who specializes in the tropics. "The general belief is that if we preserve rain forests we're also preserving the processes that create biodiversity. But our findings suggest that the engines generating new species and increased biodiversity may lie in the unprotected ecotones at the forest periphery. "
Funded by the National Geographic Society, the National Environment Research Council of the UK, the World Wildlife Fund and the Royal Society, the 6-year-long study of West African rain forests and ecotones suggests a new paradigm for how biodiversity is generated.
Using molecular genetic techniques to examine the DNA of 12 populations of the little greenbul (Andropadus virens), a common West African bird species that inhabits both rain forests and adjacent ecotones, and by measuring their distinct physical characteristics, the team uncovered evidence linking ecotones with speciation.
"The findings are significant because they contrast with past theories of rain forest speciation which attribute the evolution to new species to geographic isolation, to the dynamics within the forest during glacial periods," says 41-year-old Smith. "Although much more work is needed, our results suggest that, instead, ecotones may be vital to the production and maintenance of biodiversity in tropical rain forests by creating new species through the process of natural selection."
In an accompanying Science news article, evolutionary biologist John Endler, who in 1977 was the first to suggest that natural selection may overcome gene flow, says Smith's work is "a major first step" in supporting the long-held hypothesis that natural selection not only shapes the physical appearance of all living organisms, but also may be important to the formation of new species.
From 1990 to 1996, the team compared birds from the forest and ecotone sites. Using mist nets to trap them, they drew blood for genetic analysis and measured five physical characteristics--weight, bill depth, and wing, tarsus and upper mandible length.
Smith found that although the ecotone populations were significantly different physically from their rain forest counterparts, there was considerable gene flow between them. Smith says this means that differences in natural selection may drive populations apart, despite gene flow.
"If new species are formed this way," reports Smith in Science, "they may move from their ecotone cradle to the forest and contribute to the biodiversity. If the work is confirmed by other studies, it may reinforce an idea that many biologists have suggested in the past, that rain forests are sinks for new species, rather than areas where new species are generated."
Rain forests contain 50 percent of the world's species, yet constitute only 7 percent of the earth's land mass. As forests shrink, says Smith, ecotones are some of the first habitats to disappear as a result of burning, wood gathering and grazing.
"If further research supports the role of ecotones as centers for speciation, they will need to be preserved," he says. "If we lose these habitats, we may be losing the processes that generate biodiversity."
Smith's team is continuing their work at 22 sites in West Africa. They plan to expand the study to include plants and mammals.
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