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Science Magazine (EUA)

The Amazon in 4D

Publicado em 19 julho 2013

Por Lizzie Wade

NEW YORK CITY — In a chilly specimen storage room on the sixth floor of the American Museum of Natural History (AMNH), Joel Cracraft opens a cabinet and slides out two trays filled with delicate, palm-sized songbirds, inhabitants of the Amazon rainforest. One tray holds a jumble of red-headed manakins—eyeless, boneless, and stuffed with cotton. The other holds their close relative, golden-headed manakins. The two species seem to differ only in the color of the males' crown feathers. But they are never found in the same place. Red-headed manakins live south of the Amazon River; golden-headed manakins live north of it.

How that pattern arose is a mystery, says Cracraft, the museum's curator of ornithology. Scientists don't know what drove the birds to evolve different colored crowns. They don't know when the two species began to diverge, or the precise age of the river that separates them—much less, when it became wide enough to deter birds from flying across. They don't know just how these birds, and the plants and animals living alongside them, may have adapted to the Amazonian climate, or exactly how that climate may have changed over time. In short, scientists don't know how the Amazon rainforest and its stunning biodiversity came to be.

"It's one of the great questions of all time," says Thomas Lovejoy, biodiversity chair of the Heinz Center in Washington, D.C., and an ecologist who has worked in the Amazon for more than 45 years. Now, dozens of scientists from Brazil and the United States have teamed up across disciplines to try to find an answer. Called Dimensions of Amazonian Biodiversity, their project aims to create a "four-dimensional evolutionary atlas" of the Amazon. By combining data from a variety of fields such as geology, evolutionary biology, DNA analysis, and climate modeling, the team plans to track the region's biodiversity and environment through space and time to understand how they interacted and evolved.

A volatile place

For many years, biologists took the Amazon's abundance of plant and animal species to mean that its rainforest was an old, stable ecosystem—a kind of ecological "museum" that has "had more time to accumulate more species," says botanist Lúcia Lohmann of the University of São Paulo in Brazil, who is, along with Cracraft, a principal investigator for Dimensions. But geological evidence points to a much more volatile history for the region, according to Lohmann and her collaborators. From at least 17 million to 11 million years ago, in the middle of an epoch known as the Miocene, the Amazon Basin was covered in an extensive system of wetlands that scientists refer to as Lake Pebas. At some point—exactly when is still disputed—the lake's water began flowing east, eventually breaking through into the Atlantic and forming the beginnings of what is now the Amazon River.

As the wetlands drained and gave way to forest, the biodiversity that we see in today's Amazon began to take shape, Lohmann says. New evidence garnered from DNA sequencing and other evolutionary data point to an image of the region not as an environmental museum but as what Lohmann calls a biological "cradle" where large numbers of species have diversified fairly recently. As a result, says Alan Graham, a paleobotanist at the Missouri Botanical Garden in St. Louis, scientists now "need a whole new suite of explanatory factors" to account for the region's biodiversity. Dimensions, launched in September 2012 with a promise of $2 million over 5 years from the U.S. government and an equivalent offer from the state of São Paulo in Brazil, aims to find them.

To untangle the Amazon's complicated history, scientists from many disciplines will need to work together, says Carina Hoorn, a geologist at the University of Amsterdam. Hoorn is involved in CLIM-AMAZON, a European-Brazilian collaboration begun in 2011 that is gleaning data on the region's climate and environmental history from sediments throughout the Amazon River Basin. It's not enough to focus on a single critical field, such as geology or evolutionary biology, she says. "You really have to see it in an integrated, interdisciplinary way."

Cracraft's manakins provide an example of how such a strategy might work. If you were studying Amazonian birds, he says, you might plot the location where each of AMNH's manakins was collected and see a sharp north-south separation between the golden-headed and red-headed species. But only if you overlaid the data on a map of the region's waterways would it be clear that the barrier separating them is the Amazon River, the formation of which may have isolated two groups that evolved into separate species. Knowing precisely how long ago the two manakins split off from their common ancestor might give a good clue as to when the river became the major waterway that it is today. "This is one of the purposes of the grant, to try to tease apart that layered history of Amazonia and [its] biota in a way that can explain it all," Cracraft says. "Or at least most of it."

The team plans to go through a similar mapping process with all the region's birds, primates, and butterflies, as well as two families of plants. "To start with, we just want to understand general patterns of diversity and see how those relate to environmental features," Lohmann explains. A project like this is "long overdue," Lovejoy says.

Finding hidden patterns

Although museum specimens are at the heart of the Dimensions project, much of the information that they contain is not easily accessible because it's not yet online. Improving digital archives is one of the project's main objectives. The team plans to create a comprehensive, freely available database, including details on when, where, and how specimens were acquired, which anyone will be able to use to analyze the region's biodiversity. "The key will be to make this information comparable so that we can combine stuff and analyze bigger data sets," Cracraft says. If this succeeds, "suddenly it will be possible to look at patterns and questions that have been hard to actually get at before," predicts Lovejoy, who is not involved in the Dimensions project.

In this first year of their grant, Dimensions researchers have been focusing on adding precise geographical coordinates to records of museum and herbarium specimens. Location is the common factor that runs through everything, from plants to birds to primates, and even "the paleo and geological data," says Barbara Thiers, director of the New York Botanical Garden's herbarium, which is contributing to the Dimensions project. "Collectors didn't routinely record [latitude and longitude] information on the specimens" until handheld GPS devices came along, she says. So the Dimensions team is reconstructing coordinates as best they can, trying to match up places mentioned in collectors' original notes with the localities included in geographical directories called gazetteers, or by pulling out paper maps and retracing a collector's route.

Some patterns line up, like the distribution of plants and their pollinators, while others diverge in curious ways. Amazonian birds, for example, tend not to fly across rivers, meaning that scientists often find strikingly different species on either side of a waterway. Many plant species, however, "seem to cross rivers without any problems," Lohmann says. So while the formation of rivers might be a driving force behind bird evolution in the basin, researchers need to come up with other ideas about how populations of plants might have become isolated and evolved into separate species.

The atlas coordinates should reveal not just how species are distributed but also where collectors in the past focused their efforts. Most fieldwork in the Amazon has been done along major rivers or near cities. But collecting in just a few areas, no matter how extensive, does not guarantee a representative sample of diversity—especially because Amazonian species tend to be very particular about where they live, even when barriers separating one area from another aren't yet clear to scientists. "People have basically been looking at nice, cute things at eye-height … things that were easy to get at," says Alexandre Antonelli, an evolutionary biologist at the University of Gothenburg in Sweden.

This can skew conclusions and bias ongoing research, Lohmann agrees. "We often go back to the most collected areas as being the centers of diversity just because our data sets don't allow a very fair comparison throughout the area," she says. If the Dimensions atlas can reveal which places and species are already well studied, it could help scientists target future fieldwork on undercollected areas.

Coordinating such a sprawl ing, interdisciplinary effort is not easy, and often requires getting "out of your comfort zone," Lohmann admits. As the Dimensions project progresses, it's "essential" that research leaders "be very closely attuned to other specialists, and especially those that have alternate viewpoints," agrees Graham, who is not involved in the project. For example, the Dimensions and CLIM-AMAZON researchers disagree on some fundamental questions about the region, particularly the age of the Amazon River. Hoorn says that she welcomes the discussions. "The better people work together, the better the results will be," she says.

Although Dimensions focuses on Amazonia's history, it may help scientists and policymakers protect the increasingly at-risk ecosystem. The atlas, for example, may pinpoint areas where new species are likely to evolve. Preserving those hotspots of biodiversity might give Amazonia's inhabitants a leg up in the race to adapt to Earth's changing climate, says George Gilchrist, the National Science Foundation (NSF) program director in charge of the Dimensions grant. "If we can't keep the full diversity of species there, can we keep the evolutionary potential that generated [it]?" he wonders.

Ultimately, scientists and NSF officials hope that Dimensions will serve as a model for interdisciplinary studies about the evolutionary history of all kinds of ecosystems. Cracraft, for one, seems to have already taken the Dimensions' strategy to heart. Outside the room where the AMNH manakins are stored, he calls to a group of museum employees about to go bird watching in Central Park, "Tell me what you see and where!"