Special Section: Habitat Fragmentation Can Amplify Ecological Stress

Habitat fragmentation is even more devastating than we thought. Fragments are known to be inferior to intact habitat because they are more likely to lose species. New research shows that fragments are also more vulnerable to hunting, fire, drought, and other kinds of ecological stress.

“Such negative synergisms could potentially be one of the most important — and least understood — aspects of the modern environmental crisis,” say William Laurance of the Smithsonian Tropical Research Institute in Balboa, Panama, and Mark Cochrane of Michigan State University in East Lansing, who co-edited a five-paper special section called “Synergistic Effects in Fragmented Landscapes” in the December 2001 issue of Conservation Biology.

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The findings in this special section include:

  1. hunting may accelerate extinction in fragments. A study of hunting in Amazon forest fragments found that the smaller the fragment, the greater the overharvesting of animals from peccaries to monkeys to curassows (turkey-like, tree-dwelling birds). The disproportionate impact of hunting on fragments is presumably due partly to the fact that fragments are more accessible to hunters. This work is by Carlos Peres of the University of East Anglia in Norwich, the United Kingdom.
  2. fragments may also be more vulnerable to airborne pollutants. A study of atmospheric deposition in deciduous forest fragments in New York State found that during the growing season, sulfate is about 20 percent higher at the edge than it is in the interior. Moreover, nitrogen in the forest understory is about 45 percent higher at the edge than in the interior of fragments. Considered to limit the growth of many temperate trees, excess nitrogen could increase the growth of nitrogen-loving species along forest edges. This work is by Kathleen Weathers, Mary Cadenasso, and Steward Pickett of the Institute of Ecosystem Studies in Millbrook, New York.
  3. forest fragments can also be more susceptible to fire. A study of fire in the Brazilian Amazon found that more than 90 percent of burned forest was associated with a forest edge. Once burned, many fragments are likely to burn again within a decade or two. The estimated historical fire interval is at least 100 years, and tropical trees cannot withstand more frequent fires because their bark is too thin.
  4. Amazon forest fragments are also more susceptible to damage from El Niño-Southern Oscillation droughts. During the 1997 drought, trees near fragment edges were 50 percent more likely to die than trees in the interior. These fragments are already particularly vulnerable to fire because they have dry edges and often adjoin cattle pastures, which are burned regularly. Moreover, global warming could make Amazon forest fragments even more vulnerable to fire by exacerbating the periodic droughts. This work is by William Laurance of the Smithsonian Tropical Research Institute in Balboa, Panama, and Bruce Williamson of Louisiana State University in Baton Rouge.

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This small but compelling body of research shows that other environmental stresses can amplify the effects of fragmentation. However, most habitat fragmentation studies fail to take other environmental changes into account, simply focusing on the fact that fragments are small and isolated. “The current fragmentation paradigm… is dangerously inadequate for conservation purposes,” say Laurance and Cochrane.

For more Information
Laurance, W.F. and M.A. Cochrane. 2001 Introduction to the special section: Synergistic effects in fragmented landscapes. Conservation Biology 15(6):1488-1489.

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