When loggers level swaths of tropical forest and developers build roads and new towns, they leave behind forest fragments made highly vulnerable due to their high ratio of perimeter to total area. Now, a series of papers in the December issue of Conservation Biology presents new evidence that forest fragments are even more susceptible to ecological stresses like drought and fire than previously thought and that these stresses can interact and exacerbate one another.
One team--ecologists William Laurance of the Smithsonian Tropical Research Institute, Balboa, Panama, and Bruce Williamson of Louisiana State University, Baton Rouge--investigated the effects of the drought during the 1997 El Niño. Not unexpectedly, more trees died during the drought than before or after. But swaths of forest within 70 meters of edges suffered the most extreme losses--25% to 33% more casualties--meaning that fragments were especially hard hit.
Drought deaths can lead to further damage in a forest fragment. When trees die, gaps open up in the canopy. The added light raises temperatures and lowers humidity. This builds up woody debris and undergrowth that fuels devastating fires, as well as hurting sensitive wildlife.
Another investigator, who had previously found that patchy forests are more vulnerable to fire than larger ones, was interested in studying more closely fire patterns and frequency in fragmented forests. Mark Cochrane, an ecologist at Michigan State University in East Lansing, used satellite images and field surveys to reconstruct the fire history for a highly fragmented 3750-square-kilometer area of eastern Brazil from 1984 to 1997. He found that forest edges are highly prone to fire damage and that 90% of all burned areas included an edge. Worse, every fire hiked the risk of future blazes.
Three additional papers report on synergistic effects between habitat fragmentation and other environmental impacts:
* A study of more than 5000 Amazon forest fragments by Carlos Peres of the University of East Anglia, U.K., revealed that subsistence hunters take the most animals from the smallest forest parcels, pushing many species toward local extinction.
* Kathleen Weathers and colleagues at the Institute of Ecosystem Studies in Millbrook, New York, determined that atmospheric deposits of sulfate and other ecologically significant pollutants are much higher at fragment edges of deciduous forests in New York state than in their interiors and adjacent lands.
* Finally, research by Richard Hobbs of Murdoch University, Australia, suggested that forest fragments in southwestern Australia are harder hit by livestock grazing and invasions of weedy plants.
The scientists believe that taken together, their work reveals that drought, fire, hunting, and other stressors interact with each other and have a disproportionate impact on forest fragments. Unless scientists look at whole systems and the interacting factors affecting them, says Cochrane, it will be difficult to develop successful models for conservation management. If that holistic approach isn't applied, he says, "we'll simply be left doing forensic studies on dead ecosystems."
"This is a wakeup call," says Francis Putz of the University of Florida in Gainesville, referring to the work by Cochrane and by Laurance and Williamson. "We talk in vague terms about deforestation in the Amazon, but these other processes also are occurring at alarming rates. These papers bring to light the magnitude of ecosystem alteration there."