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Fragmented habitats no ultimate refuges for forest-dwelling tropical birds


Deep-woods bird species that manage to hang on in remaining patches of a deforested area of Brazil gain no real advantage in avoiding extinction, Duke University ecologists have found. The researchers studied the coastal region harboring the greatest number of threatened birds in the Americas.

"We found that species that also tolerate secondary habitats are not deforestation’s survivors," said Grant Harris, the first author of a paper on the subject published in the December issue of the research journal "Conservation Biology." "If you lose your habitat, everybody is equally threatened," added Harris’ co-author, Stuart Pimm. "There’s no special class of species that seems to adapt well to the habitats we create for them."

Harris recently completed his doctorate under Pimm at Duke’s Nicholas School of the Environment and Earth Sciences and is now employed by the U.S. Forest Service in Alaska. Pimm is the Nicholas School’s Doris Duke Professor of Conservation Ecology who does extensive work in tropical areas.

Their research was funded by the American Philosophical Society.

"Deforestation is rarely total or completely permanent," the two authors wrote in their "Conservation Biology" paper. "Small patches of original habitat remain and, in time, secondary forests, gardens or plantations replace some cleared areas." Given that reality, the two authors designed a study to explore how well some species survive in deforested habitat and whether some surviving species can persist in such landscapes.

Their study focused on Brazil’s Atlantic Forest, which the authors estimate now has been reduced to 119,540 square kilometers, or about 10 percent of its original extent. "There are more species threatened with extinction in this coastal strip of rain forest than anywhere else in the Americas," Pimm said in an interview. "Starting maybe 75 years ago people began moving inland from the coastal cities that have been settled for 500 years. As a consequence, very little of the coastal forest in the lowlands now remains. Much of what remains is in impenetrable mountains."

Deforestation along the approximately 800-mile-long coastal strip is also drying out the highlands further west, Pimm added. "As the lowlands area cleared, they become hot," he said. "And the uplands then begin to suffer because they’re not getting the flow of moist air off the land."

While deforestation affects other kinds of animals -- ranging from mammals to insects -- the scientists focused on birds because "we know birds very well," Pimm said. "The Atlantic Coast Forest has 200 species of birds found only there, which is more than are found in all the forests of eastern North America." Harris labeled the Atlantic Forest "the region with the most threatened birds in the Americas."

The Duke researchers estimated the forest’s original extent by consulting data from remote sensing technologies. They also followed what Pimm called "some very simple rules of thumb: If there are four feet of rain a year and it’s warm and the rain falls in every month you’re going to have lush tropical foliage."

They estimated how many varieties of birds lived in the region before deforestation by consulting available records and superimposing maps of the known ranges of tropical species. Dissatisfied with available data on the extent of the current forest, they created their own map using dry season satellite imagery that accounted for cloud interference. They then digitally superimposed available bird range maps on their own current forest map using the Nicholas School’s advanced Geographical Informational System (GIS) facilities.

What emerged was their own estimate of remaining distributions of two categories of bird species. Those categories were "forest obligate" birds that cannot exist outside the deep woods and purported "survivor" birds that could make do in more marginal "secondary habitats." "There is a subset of forest endemic birds that are also seen in other secondary habitats like gardens, plantations and golf courses," Harris said. "This has led some researchers to claim that if entire regions were completely deforested, this subset of forest endemic birds will not go extinct because they also tolerate secondary habitats."

But their Conservation Biology paper found otherwise. "We found no survivors," they wrote. "Habitat loss threatens forest-obligate birds and those using secondary habitats equally." Harris called the paper "the first analysis incorporating the amount and spatial arrangement of species’ remaining ranges in a regional, scientific context."

Members of Pimm’s research group, Harris included, have worked extensively for years in Brazil’s Atlantic Forest because of the extinction threats.

That group’s research website includes Pimm’s own personal account of some of the difficulties scientists face locating forest obligate bird species in the rugged mountain refuges west of the original lowland forest -- in this case now occupied by bustling Rio.

"Wherever you are in Rio de Janeiro you’ll find a parrot, because it is just a common bird there," Pimm said. It hasn’t lost as much of its habitat as most species have simply because it has a big range. "On the other hand, if a species has lost 95 percent of its range, irrespective of whether is uses human habitats or not, it is still considered to be on the brink of extinction."

Monte Basgall | EurekAlert!
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