Amazonian Treefrogs Exhibit ‘Megadiversity’
Deep in the heart of the Amazon rainforest, in the sweltering heat and humidity, about 2.5 million insect and tens of thousands of plant and animal species thrive in one of the most biologically diverse places on the planet.
Scientists have long believed that the hot, wet climate of the Amazon region is a major reason for the variation, but a recent study of tree frogs has found that while the climate may be a reason so many species are able to thrive in an area, time may be an even more important factor for why so many of them exist in the first place, at least for these amphibians.
Other hypotheses had included taking into account how these species use resources such as food and space. Size, competition and predation were also leading theories.
John J. Wiens, the study’s lead author, studies phylogenetics — the relation between groups of species through millions of years of evolution. Tree frogs in the Hylidae family number over 800 species worldwide, and they can be found on every major continent except for Antarctica. Wiens is also a professor in the Department of Ecology and Evolution at Stony Brook University.
His study, published in early May of 2011 in Ecology Letters, showed that tracts of the South American rainforest dwarf similar ecosystems in Asia and Africa in terms of tree frog biodiversity, despite those regions’ similar tropical climates.
The Amazon Basin is home to 142 known species of tree frog, and in certain smaller habitats the number of species can reach 36. The most biodiverse tracts are in the Amazon and Atlantic rain forests in Brazil and Ecuador.
In order to test his hypothesis, whether time is a major factor in diversity, Wiens built a phylogenetic tree (a chart depicting the branches of evolution as different species evolved from a common ancestor) of 362 tree frog species. The placement of each on the tree was based on its DNA sequence.
Kevin de Queiroz, a vertebrate zoologist and curator at the Smithsonian Institute, declined to comment on the study because he does not study amphibians, but said that calculating the exact time of origin of a species is impossible . The methods researchers use now involve calibrating the phylogenetic tree against points in the fossil record, which they can use to determine the approximate time each branch of the tree broke off and began to diversify.
Wiens estimated that many of the most species-rich areas in the Amazon Basin have populations of tree frogs that began to diversify about 60 million years ago. In another study that is now being peer-reviewed, Wiens said he has found evidence of similar trends in salamanders where the evidence for time-based diversification is even stronger. “So it seems like time can be a really important factor,” he said.
In the Amazon, tree frogs can run the gamut from fingernail to palm-sized, and the study showed that body size did not factor into rates of diversification. Of the 36 local species, seven were in the same thumbnail-sized range, all co-existing peacefully, so he ruled out body size as a factor in tree frog diversity.
Wiens collected information from 12 sites representing the same number of regions, including historical climate records, precipitation and the sizes of the frogs inhabiting them.
There was some correlation between climate and species diversification in an area, but the most significant correlation that Wiens said he had found was based on time. The longer a group had inhabited an area, the higher the likelihood of a more species-rich region.
The Dardanelos Dam and Estacion Vera Cruz sites in Brazil, La Selva region in Costa Rica, and Santa Cecilia area in Ecuador are some of the most species-rich areas, and also have some of the earliest colonization ages of tree frogs, the study showed.
Diversity & Conservation
But the diversity in the South and Central American rainforests is at risk. The Amazon and Atlantic rainforests, the latter of which is also located in Brazil, are not just repositories for millions of plant, insect, bird, mammal, reptile and amphibian species— they also function as carbon sinks.
With ongoing deforestation and the less well-defined consequences of global warming posing a significant risk to the Amazon Basin, conservationists are not just concerned with losing species because of the shrinking habitat—global warming may change the rainforests in unpredictable and potentially unwelcome ways.
Because of the density of its trees and year-round greenery, these rainforests are constantly pulling carbon dioxide out of the atmosphere, converting it to oxygen and helping manage the harm caused by greenhouse gas emissions. For this reason, the Amazon rainforest is known as the “Lungs of the World.”
Deforestation is the most significant threat facing the Amazon rainforest in South America. Cattle ranchers are responsible for up to two-thirds of the annual loss of the Brazilian rainforest, cutting down trees in order to make room for more pastureland. Squatters, taking advantage of a policy by the Brazilian government allowing the use of an unclaimed piece of public land, cut down and burn trees to clear space for crops like bananas, palms, rice and corn.
Rhett Butler, founder of MongaBay.com, a major resource cited by the Select Committee on Energy Independence and Global Warming for conservation information, wrote in an e-mail that although Brazil has some of the most environmentally-conscious laws in the world, it is lax on enforcement.
But Butler also said that he is optimistic about Brazil. “Civil society is strong and strategic and there is real interest among Brazilians in the environment,” he wrote. “These interests increasingly affect how Brazilian business conducts itself in the Amazon.”
In the last half a decade, the area being deforested each year in Brazil has dropped from about 5,500 square miles in 2006 to just fewer than 2,500 in 2010, partly because of enforcement picking up and also because Brazilians are focusing more on conservation of the environment.
Areas of the Amazon outside of Brazil, such as the section of it located in Peru, are more at risk, Butler said.
Researchers for the Select Committee on Energy Independence and Global Warming, a bipartisan committee formed by members of Congress to investigate the consequences of global warming, are less positive. They say on their website that by the middle of the century, the Amazon forest will have shrunk enough to become a source of carbon dioxide in the atmosphere rather than one of the few places in the world that pulls in so much of this greenhouse gas.
Wiens said that he has also visited deforested areas, and while several species of treefrogs will still be present and thriving, an area that began with over 30 species may decrease to only 10 or 15 after it’s been clear-cut. Most tree frogs live in the leaves and branches of the rainforest trees, and lay their eggs in the ground.
“It’s not as if everything goes extinct immediately,” he said, “but it is reduced dramatically.”
He takes a more cautious view of the consequences of global warming. Experts fear rising temperatures could dry out the rainforests. The frogs could migrate north or south or to areas with different elevations to accommodate the change, but Wiens observed the Amazon Basin is all lowland; the frogs would have no place to go, he said.
“Some things are likely to go extinct, and some things are going to be okay, but we don’t know how exactly the climate is going to change,” he said. “The Amazon could stop being this incredible place.”
This article was first published in the Stony Brook Independent.