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By Helen Fosgate
hfosgate@smokey.forestry.uga.edu
A new study by UGA wildlife researchers suggests that the shift from small, shadegrown coffee farms to vast sun-grown coffee fields in Panama may help explain the loss of tropical biodiversity. Researchers say shady coffee farms provide a haven for many forest species, including army ants and ant-following birds, while sun-grown coffee farms support little but coffee.
The study, conducted in western Panama and funded by the University of Georgia, the University of Memphis and the Smithsonian Migratory Bird Center, is the first to show that army ants are a keystone tropical species, even in a landscape that has been modified by humans.
It was published earlier this year in the journal Conservation Biology and is scheduled for publication next month in Ecological Applications.
In the study, authors Dina Roberts and Robert Cooper, of UGA’s Warnell School of Forest Resources, and Lisa Petit, of the Smithsonian Migratory Bird Center in Washington, D.C., show how army ants act as critical links between birds and leaf-litter insects in shade-coffee farms as well as tropical forests. Ant-following birds feed not on the ants, but on the insects flushed from the forest floor by the ants’ swarming advances.
“Shade-grown coffee varieties are tolerant of the shade of high-canopy tropical trees,” says Cooper, a wildlife ecologist in the Warnell School. “These small, traditional farms, especially when adjacent to forests, support a wide variety of species.”
Like forests, shade-coffee farms accumulate layers of leaf litter, woody debris and fallen trunks where army ants can nest and hunt. Researchers worry that as these low-input, traditional farms give way to large, intensive, sun-grown coffee plantations, the ants--and ant-following birds--will decline even further. The ant-following birds include year-round residents like the gray-headed tanager and ruddy woodcreeper, as well as dozens of North American migrant species. Roberts recorded 126 different bird species at ant swarms.
As a graduate student working with Cooper, Roberts lived and worked in Panama for two years. She followed two species of army ants--Eciton burchelli and Labidus praedator--in various habitats, noting their numbers, activities and distribution. She tracked the swarms in various environments, including forests, shade-coffee farms both close to and distant from forests, and in sun-coffee. She says the swarms emerged early each morning and quickly spread out in organized hunting parties, often 25 meters wide.
“It’s incredible,” she says. “Every living thing in their path scrambles to get out of the way. You see tarantulas running and crickets jumping into the air to escape.”
Roberts found plenty of army ant swarms in forests and in shade-coffee farms, even those far from the forests. But she didn’t find a single ant swarm in the hot, exposed soils beneath the sun-coffee shrubs, which are constantly cultivated and sprayed with herbicides and pesticides.
“Historically, coffee farmers cleared portions of the understory in continuous forests and planted coffee shrubs beneath,” she says. “By retaining a diversity of shade trees, the ecosystem could continue to operate, controlling erosion, providing shade, leaf litter, fruit and nesting sites for wildlife.”
The researchers say shade-coffee farms provide a comparable environment, or at least one that is tolerable to the ants. Roberts goes so far as to suggest that, in light of their important role, the ants deserve protected status granted other threatened species.
“Right now, these shade-coffee farms are a safety net for the army ants and the birds that depend on them,” says Roberts. “Unfortunately, they’re quickly disappearing.”
But Roberts is also encouraged by signs that public consciousness is rising.
“Consumers can now buy shade-grown coffee in many grocery stores,” she says. “It costs a little more, but rising sales give the small, traditional farmers in Panama an economic incentive to retain their shade-tree coffee farms.”
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