Edge Effects May Extend Much Farther than Suspected
Life on the forest edge is perilous for many species. Edges can attract predators from surrounding areas and can drive many insects, insect-eating birds, and small mammals further into the forest, thus reducing their effective habitat. Recent studies suggest that some edge effects may extend more than 60 times deeper into forests than previously thought, according to William Laurance of the Smithsonian Institution in the April issue of Trends in Ecology and Evolution.
Fragmenting forests greatly increases their total edge, and biologists are concerned that this will threaten diversity. Until recently, however, most research showed that edge effects extend only up to 150 meters into forests.
The first recent study suggesting that edge effects extend further was on canopy tree reproduction in the 90,000 hectares Gunung Palung National Park in Western Borneo. Lisa Curran of the University of Michigan and her coauthors published this study in 1999.
The researchers studied diptocarps, tall hardwood trees that produce massive seed crops just a few consecutive weeks out of every few years, presumably to swamp seed predators. Called masting, these episodes appear to be triggered by severe El Nino droughts.
Whereas there were about 150,000 diptocarp seedlings per hectare after the 1991 masting episode, Curran and her co-authors did not find any new diptocarp seedlings in study plots after the masting episode that followed the 1998 El Nino.
The researchers attributed this lack of seedlings to the fact that the park has been flooded with seed predators from the surrounding degraded areas. Seed-eaters, from weevil larvae to parakeets to orangutans, can’t find enough food outside the park because the forest there has been fragmented by uncontrolled logging, exotic tree plantations, and human-caused wildfires.
The study plots were at least 10 kilometers from the nearest areas in the park that have been illegally hand-logged and more than 20 kilometers from the park border. Thus, Curran’s study suggests that edge effects can exceed 10 kilometers.
The researchers also found that 47 other diptocarp species “have largely failed to reproduce [in the park] since 1991, apparently because of dramatic ecological changes outside the park,” says Laurance. Fires have increased near the park since the 1998 El Nino drought, and illegal hand-logging has also increased due to Indonesia’s recent economic crash.
The conclusion that edge effects can occur over significant distances is supported by a study in press by Halton Peters, now at the University of Cambridge in England. He found that the highly invasive exotic shrub Climedia hirta, which ranges from Mexico to Argentina, has become established in undisturbed forests in the 2,600-hectare Pasoh Forest Reserve in Peninsular Malaysia. This was a surprise because such invasions are rare in intact forests.
Peters found that a 50-hectare study plot more than two kilometers from the edge had more than 1,000 of these exotic shrubs, primarily in treefall gaps and areas that had been disturbed by wild pigs. While the pigs are native to Malaysia, they have increased dramatically in some agricultural areas. The conversion of regenerating forests near the reserve to oil-palm plantations led to more wild pigs, which disturbed more soil in the reserve, which in turn facilitated the invasion of weeds.
“Land conversion outside Pasoh Forest Reserve appears to be having important effects on the viability of the reserve itself,” says Laurance.
If edge effects can extend as far as 10 kilometers into forests, then reserves may need to be larger than 500,000 hectares to preserve intact ecosystems and ecological processes, says Laurance. He also calls for wide, carefully managed buffer zones to mitigate the adverse effects of edges on forest reserves.
Laurance, William F. 2000. Do edge effects occur over large spatial scales? Trends in Ecology and Evolution 15(4):134-136.
Biological Dynamics of Forest Fragments Project, Smithsonian Institution/INPA. Manaus, Brazil and National Museum of Natural History, Smithsonian Institution, Washington DC