New Research Bolsters Theory That Forest Edges Are Hotbeds of Speciation, Kicking Off Global Investigation Of Evolutionary Processes In Tropical Rainforests
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SAN FRANCISCO, Nov. 22, 1999 - Challenging long-held views that geographic isolation is the singular driver of species diversity in tropical rainforests, a team of researchers from Boston University, San Francisco State University and the University of Queensland report in the Proceedings of the National Academy of Science (PNAS) that natural selection in forest peripheries, or "ecotones," may play an equally important role in the evolution of new species.
The research compliments SFSU team member Thomas B. Smith's groundbreaking work with Robert Wayne (UCLA), et al, published in Science two years ago, which revealed that West African ecotones are hotbeds of evolution, functioning as engines of biodiversity in the region's tropical rainforests.
The research presented in PNAS extends the ecotone theory to Australia.
Based on studies of Carlia rubrigularis, a common skink, or lizard, prevalent throughout Australia's wet tropical rainforests and dry open forests, lead author Chris Schneider of Boston University and co-authors Smith, Brenda J. Larison (SFSU) and Craig Moritz (U. of Queensland) found that despite evidence of genetic exchange, skink populations living within the ecological gradient, or ecotone, between the two forests exhibited significant differences in their physical appearance compared to their rainforest counterparts.
In striking contrast, rainforest skink populations that have been geographically isolated by a mountain barrier for millions of years were uniformly similar, despite ancient genetic divergence.
The team sampled adult skinks from eight paired sights in rainforest and open forest in the Wet Tropics World Heritage area of North Queensland, Australia.
"The differences in the shape, size and sexual maturity of skinks between the rainforest and adjacent open forest populations, but not between historically isolated populations, suggests that natural selection rather than isolation is promoting these physical differences," says Schneider. "This stands in stark contrast to the prevailing view that geographic isolation alone is the key to population divergence and speciation."
"The skink work strengthens our ideas that this is not just something happening in a few bird species in Cameroon. It's happening in another rainforest, with different taxa," says Smith.
Because preserving the processes that maintain diversity in rainforests is fundamental to effective conservation, the researchers are launching a comprehensive investigation of the mechanisms responsible for generating biodiversity in the world's tropical rainforests. With a $2.6 million grant from the National Science Foundation (NSF), Smith will lead an international team of scientists, students and policy makers on a three-continent study to test alternative hypothesis of speciation, with the goal of defining better conservation policy. Collaborating institutions include NASA, the World Resources Institute, Boston University, UCLA and the University of Queensland.
"The general belief is that if we preserve rainforests, we're also preserving the processes that create biodiversity. But considering the role of ecotones, that may not be the case," says Smith, an evolutionary biologist and director of SFSU's Center for Tropical Research (CTR), which received the NSF grant.
In a 1997 Science article, veteran evolutionary biologist John Endler hailed the ecotone work as a "major first step" in supporting the hypothesis that natural selection not only shapes the physical appearance of all living organisms, but also may be important to forming new species.
Schneider, who is a principal investigator for CTR, says: "When we think about the processes that generate biodiversity in rainforests, we need to move beyond the traditional view of geographic isolation and focus on the ecological opportunities that are provided by habitat gradients and newly formed habitats."
In their study, the researchers found that open-forest lizards were smaller, had shorter limbs and a bigger head, and became sexually mature earlier than their rainforest counterparts.
To test for the selective forces influencing the rapid changes in the skinks' appearance and reproductive maturity, the researchers looked at predation. Because theoretical studies suggest that natural selection caused by predation favors the evolution of smaller bodies and earlier reproduction, they suspected that lizard-eating birds hunting in open forests were the agents of natural selection.
The researchers placed 480 plastic lizard decoys, painted to match the striped, reddish skink, throughout the dense rainforest and the open dry forest. By looking for the telltale bite marks created by bird bills, the researchers identified how many models were attacked. According to Schneider, it was no contest between the two sites. Twenty-one models were targeted in the open, transitional forest, versus only four in the closed rainforest habitat.
"We have identified a potential selective mechanism that would explain the difference in size between rainforest and open forest habitats," says Schneider. "The changes in morphology across habitats, in spite of high levels of gene flow, suggest rapid adaptive evolution in response to natural selection."
The evolutionary mechanisms that have fostered the rich tapestry of plant and animal species found in tropical rainforests, which harbor roughly half the Earth's species, have been hotly debated for decades. In their new study, the researchers will test competing speciation hypotheses across various landscapes and diverse taxa-including birds, mammals, reptiles and amphibians-in Africa, Australia and South America. Their goal is to identify the key factors important to speciation in tropical rainforests.
Using remote satellite imaging technology employed by NASA's Jet Propulsion Laboratory, remote sensing expert Dr. Sason Saatchi and post-doctoral researcher Dr. Catherine Graham (SFSU) will identify the rate of habitat loss using historical data, and paint a clearer picture of the geographic elements of rainforest systems that are associated with species diversity.
Additionally, to insure that the research results are used, the team is collaborating with the World Resources Institute in Washington, D.C., an international environmental policy center.
"Ultimately, what we're trying to do with this grant is put science and policy on the same track," says Smith. "At present, conservation programs tend to emphasize preserving areas of high species richness, with little attention to the evolutionary processes that generate biodiversity. Our research model is designed to provide the data necessary to define more effective conservation policy."
An auxiliary research unit of SFSU's College of Science and Engineering, the Center for Tropical Research conducts basic and applied biological research in order to better understand essential biotic processes that produce and maintain tropical biodiversity worldwide.
EDITORS: Images of field research, sample species and ecotones are available in slide form and as downloadable digital files at the following website: http://www.sfsu.edu/~pubaff/ecotone.htm.
Materials provided by San Francisco State University. Note: Content may be edited for style and length.
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