FRAGMENT FOREST FACES BLEAK FUTURE
(www.newscientist.co.uk — by Fred Pearce, 8 November 2003)
A study of forest fragments in the Brazilian Amazon show the importance of
preserving large natural areas. But it is also a major blow to conservationists trying to mend
the damage to habitats that are increasingly broken into small pieces by human invasions.
Ecologists knew that large chunks of forest can conserve many more species than
smaller chunks. But the new study quantifies the relationship and adds a new dimension:
time. It reveals how fast species disappear in forest fragments of a given size.
Researchers from the US and Brazil’s National Institute for Amazonian Research in
Manaus spent 13 years catching birds in patches of undisturbed Amazon rainforest. The
fragments, which varied in size from 1 hectare to 100 hectares, were cut off from each other
by cattle pasture. During the study, the number of bird species declined in all the fragments.
But the ‘half-life’ for biodiversity, the number of years it took for half the species to
disappear, was dramatically shorter in small forest patches than in larger patches (see
Graph).
The size of forest fragments is more critical for some species than others. It was
concluded in the article (Proceedings of the National Academy of Science) that certain
species seemed much more likely to disappear than others although some species do
relatively well in patchy landscapes. Even in a fast-growing rainforest, it takes 20 years for
the semblance of a new forest to emerge and a hundred years for full regrowth. So the paper
concludes that to be successful, rainforest conservation generally requires fragments larger
than about 10 square kilometres. The findings suggest there is little point in trying to link
small patches because most of the species in them will have disappeared before the new
forest in the gaps has had a chance to grow.
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