Lizard from Oz a clue to rainforest biodiversity
New research, based on skinks in Australian tropical rainforests, supports the recent theory that forest fringe areas are a rich and important source of species diversity.
Challenging long-held views that geographic isolation is the singular driver of species diversity in tropical rainforests, a team of American and Australian researchers report, in the latest Proceedings of the National Academy of Science, that natural selection in forest peripheries, or 'ecotones,' may play an equally important role in the evolution of new species.
The research compliments groundbreaking work, 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 current research extends the ecotone theory to Australia.
"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 team member, Thomas B. Smith, also a member of the original African team.
The study was based on Carlia rubrigularis, a common skink, or lizard, prevalent throughout Australia's wet tropical rainforests and dry open forests. 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.
Chris Schneider of Boston University and co-authors Smith, Brenda J. Larison (San Francisco State University) and Craig Moritz (University of Queensland) found that skink populations living within the ecotone between the two forests exhibited significant differences in their physical appearance compared to their rainforest counterparts, despite evidence of genetic exchange. 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.
To test for the selective forces influencing rapid changes in the skinks' appearance and reproductive maturity, the researchers looked at predation. They suspected that lizard-eating birds hunting in open forests were the agents of natural selection.
They 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. "The changes in morphology across habitats, in spite of high levels of gene flow, suggest rapid adaptive evolution in response to natural selection".
Smith will now 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.
"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.