South America's Atlantic Forest was once home to an "exuberant ... megadiverse" swath of animal and plant life, according to ecologist Juliano Bogoni. But in a paper published in the journal PLoS One on September 25th, Bogoni and his colleagues report that the forest's collision with humans over the past 500 years has dramatically cut through its mammal populations.
"We documented thousands of local extinctions," Bogoni, the paper's lead author and currently a post-doctoral scholar at the University of São Paulo in Brazil, writes in an email.
Once stretching from the coast up to mountains topping out above 9,800 feet in southeastern Brazil, the tropical Atlantic Forest has dwindled from 425,000 square miles to just 55,000 square miles as people have cleared away the trees for timber and agriculture and to make room for human settlements. That loss and splintering of habitat, combined with relentless hunting, has driven down the numbers of many mammal species, especially large and medium-sized ones, and biologists have taken note in dozens of studies in parts of the Atlantic Forest.
What's more, we humans have concentrated our farming and hunting in the low-lying areas, which once held greater densities of mammal species than the higher-elevation areas of the Atlantic Forest.
"The historical pressure on those landscapes has been huge," says co-author Carlos Peres, a conservation biologist at the University of East Anglia, where Bogoni earned his doctorate.
Until now, however, no one had looked at the entire Atlantic Forest biome to assess the scale of the hemorrhage of mammal species. So Bogoni set out to pull together the data from those localized studies, examining nearly 500 different sets, or "assemblages," of species throughout the Atlantic Forest. At each site, he and his colleagues compared the sets of mammal species currently present, taken from research from the past 30 years, with those found around the time of Europeans' arrival in the region about 500 years ago.
The team found that, while no single mammal went completely extinct from the entire forest, an average of more than 70 percent of the species disappeared from each of these assemblages.
The researchers also looked beyond species diversity, incorporating analyses of how human activity affected species grouped by their functions in the ecosystem. Larger mammals were more likely to be extirpated, probably because they're favored by hunters and are slower to reproduce than smaller animals, Peres said.
The effects of the loss of these important species could ripple through the ecosystem, Bogoni said, throwing off the forest's ability to regenerate. Larger herbivores play crucial roles as seed dispersers. And the loss of large predators, such as jaguars, could mean that plant eaters, such as capybaras, aren't kept in check.
"A Margay"—a small wild cat—"is [not] able to prey [on] a Capybara," Bogoni says. "Thus, Capybara can increase their population numbers (uncontrollably until they succumb for other reasons)."
In the Amazon, curbing deforestation to save the remaining primary forest is a major priority. But the goals are different for the Atlantic Forest, Bogoni says.
"We're not talking about slowing down deforestation because pretty much there isn't anything else to cut down," Peres says. "All of the attractive forests have already been hammered."
Peres calls for ramped-up restoration efforts throughout the Atlantic Forest. Restoration can be costly, requires the will of political leaders, and can have mixed results. Still, it's worth the effort, Peres says. "This is the right time to start clawing back those bits of nature. If you create the habitat, the animals will come."
This story originally appeared at the website of global conservation news service Mongabay.com. Get updates on their stories delivered to your inbox, or follow @Mongabay on Facebook, Instagram, or Twitter.