CORVALLIS, Ore. - The vast forests of the Amazon store enormous amounts of carbon that help moderate the Earth's temperature, but a new study shows that this carbon-storing capacity is being threatened by over-hunting.
Wide-scale reduction of fruit-eating large mammals - especially primates and tapirs - is changing the way seeds are dispersed in the Amazon and changing the composition of forests, the researchers say.
Results of the study are being published this week in Proceedings of the National Academy of Sciences.
"Large mammals including spider monkeys and wooly monkeys are fruit-eaters that historically have made up most of the frugivore (or fruit-eating) biomass in these forests," said Taal Levi, an Oregon State University ecologist and author on the study. "There are many tree species with large seeds that rely on these primates to spread seeds through the forest.
"These large-seeded fruit trees are also slow-growing and populate the forest with dense wood that sequesters a great deal more carbon than in typically stored in trees dispersed by wind or smaller frugivores," Levi added.
As technology has advanced and firearms have spread through tropical forests, hunting success has improved and these primates have been extirpated from vast areas, Levi pointed out.
"When large primates and tapirs, which are the largest frugivores in the neo-tropics, are lost, forests are eventually populated by plants whose seeds are more likely dispersed by wind, rodents or birds," Levi said. "It is not the same aggregation of plants and it is affecting the Amazon's carbon-storing ability."
In fact, the researchers say, over-hunting occurs over much larger areas than the total area of the Amazon forest affected by deforestation. A relatively small loss in the amount of carbon stored in trees can lead to enormous declines in the amount of carbon stored in these vast forests.
The analysis of 166 wildlife surveys across the Amazon basin documents the loss of large primates. Levi's computer model projects that this will result in more than three out of four plots losing forest biomass, with a (conservatively) estimated average loss of 2.5 to 3 percent.
Tapirs are another key seed disperser that is sensitive to over-hunting. When tapirs are lost in addition to large primates, nearly nine out of 10 plots will lose forest biomass with the loss (conservatively) projected to average about 5.8 percent.
"The loss of 2.5 to 5.8 percent of biomass may not sound like a lot," Levi said, "but in an area as vast as the Amazon, the impact could be huge - a projected 313 billion kilograms of carbon not being absorbed."
Levi said the economic value of such a loss on the world carbon markets could range between $5.91 trillion and $13.65 trillion.
The researchers studied data from 2,345 one-hectare forest plots scattered across the Brazilian Amazon containing nearly 130,000 large trees. Simulations showed that 77 to 88 percent of these plots will lose above-ground forest biomass when the forests are over-hunted and trees that require large primates or tapirs to regenerate are replaced by other trees on the same plots.
Carlos Peres, a research ecologist with the University of East Anglia and lead author on the study, said the research uncovers an important - and perhaps under-appreciated - link between wildlife and climate change.
"Amazonian forest wildlife has been declining through a combination of habitat destruction, habitat degradation and overhunting since the 1950s," Peres said, "but until now there was a poor understanding of the status of wildlife populations in hunted forests that otherwise remain intact and free of human disturbance.
"We show that dense-wooded, large-seeded Amazonian tree species are replaced by light-wooded trees that produce smaller seeds, which continue to be dispersed in over-hunted forests by more resilient smaller mammal and bird species," he added.
Levi said trying to manage the forests by manually dispersing seeds would be impractical because of the vastness of the Amazonian forests. There also is evidence that seeds that go through the digestive tract of large mammals are more likely to germinate having been cleansed of flesh that attracts fungal pathogens and other natural enemies.
"Seeds that fall from trees contain a lot of pulp," Levi said, "and in tropical climates become excellent petri dishes for fungus to colonize."
The researchers say the key to protecting optimal forest composition is to recognize the importance of hunting and better manage it.
"These findings highlight an urgent need to manage the sustainability of game hunting in both protected and unprotected tropical forests, and place full biodiversity integrity, including populations of large frugivorous vertebrates, firmly in the agenda of reducing emissions from deforestation and forest degradation (REDD+) programs," the authors noted in the article.
Other authors on the PNAS article are from the National Institute of Amazonian Research and Fiocruz Amazonia.
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Taal Levi, 541-737-4067, firstname.lastname@example.org