Researchers find that when seed-dispersing animals disappear, woodlands lose their vitality and ability to store carbon.
Sep 30, 2016·
Richard Conniff is the author of House of Lost Worlds: Dinosaurs, Dynasties, and the Story of Life on Earth and other books.
(Photo: Wolfgang Kaehler/LightRocket via Getty Images)
In the aftermath of bushmeat hunting, pet trade harvesting, habitat fragmentation, selective logging, and other human intrusions, forests and national parks can still look surprisingly healthy. They may even feel like beautiful places for a walk in the woods. But these forests face what researchers in one recent study describe as a “silent threat” due to the rapid decimation of wildlife almost everywhere in the tropics. Without wildlife, forests rapidly deteriorate, losing their value for carbon storage and becoming unsustainable.
That’s because tropical forests, particularly in the Americas, Africa, and South Asia, are primarily composed of tree species that depend on animals to disperse their seeds. This is especially true for the tall, dense canopy trees that are best at carbon storage, a critical factor in climate change calculations. For instance, in a Smithsonian Institution research forest on Barro Colorado Island in the Panama Canal, roughly 80 percent of canopy trees produce large, fleshy fruits. A ravenous cacophony of animals zeroes in on these trees as the fruit ripens.
In a healthy forest like Barro Colorado, the customers may include monkeys, big pig-like tapirs, large rodents like the agouti, toucans with their long, colorful bills adapted for snatching fruit, and a long list of hungry birds, fruit bats, and other species. After these happy diners have eaten and wandered off, they excrete the seeds of the fruit they have eaten all around the forest. Most of the resulting seedlings inevitably die. But in patches of sunlight or otherwise suitable ecological niches, some find the conditions they need to flourish, ensuring that the tree species remains a part of the forest.
Without wildlife to disperse the fruit, on the other hand, the result is a rain of fruit and seeds in the immediate vicinity of a tree. Even more seedlings die, and the survivors grow more slowly because of crowding and competition for sunlight and water. That’s bad news for the tree species.
In a new study in Proceedings of the Royal Society B, a team led by Trevor Caughlin, then a forestry student at the University of Florida, looked at the effects of hunting on a wildlife sanctuary in western Thailand. With the help of sophisticated computer modeling, they calculated that the loss of seed-dispersing wildlife multiplied the risk that a tree species would become extinct more than 10-fold over just a century. But the effects of losing wildlife can show up even sooner. Local extinctions of tree species are occurring in Thailand, said Caughlin, as elephants, gibbons, bears, and other seed-dispersing species vanish.
A recent study published in the journal Nature Communications found that declining populations of large seed-dispersing animals can lead to a 60 percent reduction in the abundance of the trees that depend on them. Wind-dispersed species, which tend to be less effective at carbon storage, proliferate in their place. Those researchers calculated that removing just half the wildlife from forests in tropical Africa and America—a modest loss compared with current reality—would reduce aboveground carbon storage there by 2.1 percent. The coauthors note that this would release the carbon equivalent of 14 years of Amazonian deforestation.
All this has come as “a surprise for many of us in forest conservation,” said Caughlin in an interview. “Trees seem really permanent and stable. A single tree can live for hundreds of years and produce hundreds of thousands of seeds, and from that perspective, it shouldn’t seem like the loss of seed-dispersing animals should be a big deal. They can wait it out.”
“But these studies suggest that when you lose the seed-dispersal services that the animals provide, you will lose biodiversity value, and you will lose carbon storage in that forest,” he added. “You would expect it would take a long time. But it doesn’t.” He cited another recent study showing that the loss of toucans and other animal seed dispersers is causing palm trees in the Brazilian Atlantic Forest to adapt, over a matter of decades, by evolving smaller seeds.
People who aren’t worried about climate change may think it’s no big thing if forests lose a little carbon storage. But storing carbon is the basis for an entire system of “green finance,” in which companies and countries try to mitigate their carbon footprint by paying to protect tropical forests. These forests are an attractive target because they store about 40 percent of all the terrestrial carbon on Earth—and the worldwide penchant for cutting them down now releases as much as 17 percent of global carbon emissions each year.
Beyond carbon storage, the people putting up the money also often take credit for other “ecosystem services” these forests ostensibly provide. Studies increasingly suggest, however, that what conservationists call “empty forests,” forests without wildlife, lose their ability to provide many of those same ecosystem services. They are no longer nearly so good at protecting water quality, preventing soil erosion, and nurturing pollinators and biological pest control. Caughlin adds that losing seed-dispersing wildlife also compromises the wild stock for fruits beloved by humans, including mangoes, tamarinds, and passion fruit—sacrificing genetic resources that might prove critical when new diseases or a changing climate threatens domesticated varieties of these fruits.
Here is the bottom line. “Empty forests” are not really forests at all until we act to bring back the wildlife that enables them to function.