Seventy percent of the world's forest are located within just one kilometer of the forest edge. That's a result of centuries of forest fragmentation, done in the name of agricultural clearing, urban development, and other human interference. As bad as that sounds, the consequences for biodiversity are perhaps even most distressing. A review of fragmentation experiments on five continents over three and a half decades shows how forest fragmentation can reduce biodiversity by up to 75 percent.
Forest fragmentation has been a point of concern for decades, largely because of fears that lost habitat and increased accessibility to predators and parasites will drive some species to extinction. Still, some have called into question the value of habitat fragmentation, on the grounds that it doesn't capture important ecological interdependencies, or that the influence of fragmentation per se can be subsumed under the effects of habitat loss.
That controversy makes the experiments of an international team of 24 researchers all the more interesting. Led by Nick Haddad, a professor of biology at North Carolina State University, researchers conducted two separate analyses to illustrate the significance of forest fragmentation on the environment. First, they used a recent, high-resolution global survey of forest cover to estimate just how fragmented the world's forests are. They found that with the exception of the Amazon and Congo river basins, where it's still possible to find deep forests, 70 percent of the planet's trees lay within a kilometer of the forest edge.
"Fragmentation experiments—some of the largest and longest-running experiments in ecology—provide clear evidence of strong and typically degrading impacts of habitat fragmentation on biodiversity and ecological processes."
That leaves open the matter of whether fragmentation is actually a problem in and of itself. To answer that question, Haddad and colleagues looked at data from seven long-term fragmentation experiments on five continents. Those experiments, some of which have been running for decades, were designed to mimic various aspects of human activity, such as reduced habitat area, increased isolation between forest fragments, and increased ratio of forest edge to forest area. These experiments showed that reduced forest fragment area cut the number of birds, mammals, insects, and plants.
Those effects, however, were often complex, with some individual species actually thriving under increased fragmentation—notably, predators were more successful when the edge-to-area ratio was higher, with negative consequences for birds in particular. What's more, the repercussions often didn't become apparent for years. In the case of an experiment in Kansas, it took 12 years for fragmentation's effects on plant life to show up. Those results, Haddad and colleagues argue, are consistent with recent theories of habitat fragmentation.
"Fragmentation experiments—some of the largest and longest-running experiments in ecology—provide clear evidence of strong and typically degrading impacts of habitat fragmentation on biodiversity and ecological processes," the team writes today in Science Advances. That includes some surprising effects, such as surges for some species and long periods of time before fragmentation's effects become detectable. "In light of these conclusions and ongoing debates, we suggest that fragmentation’s consistency, pervasiveness, and long-term degrading effect on biodiversity and ecosystem function have not been fully appreciated," they write.
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