Special section: Habitat fragmentation can amplify

November 30, 2001

Habitat fragmentation is even more devastating than we thought. Fragments are well-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 issue of Conservation Biology.

The findings in the special section include:

--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-, 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.

--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% higher at the edge than in the interior. Moreover, nitrogen in the forest understory is about 45% 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.

--forest fragments can also be more susceptible to fire. A study of fire in the Brazilian Amazon found that more than 90% of burned forest was within a third of an 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.

--Amazon forest fragments are also more susceptible to damage from El Nino-Southern Oscillation droughts. During the 1997 drought, trees near fragment edges were 50% 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, Louisiana.

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 focussing on the fact that fragments are small and isolated. "The current fragmentation paradigm...is dangerously inadequate for conservation purposes," say Laurance and Cochrane.
-end-
For faxes of papers, contact Robin Meadows robin@nasw.org

For more information about the Society for Conservation Biology: http://conbio.net/scb/

Society for Conservation Biology

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