The dry forests of western Madagascar are unusual: they have among the greatest diversity of trees in the world but the lowest diversity of animals that disperse the seeds. New research shows that without the brown lemur, these forests fail to regenerate completely on their own.
"This is a unique result. In other forests, the guild of seed dispersers is redundant. If one group drops out--or is shot out--there are others that can fill in," says Jorg Ganzhorn of the German Primate Center at Hamburg University in Goettingen, Germany, who presents this research with his colleagues in the August issue of Conservation Biology.
Since people arrived in Madagascar some 2,000 years ago, an estimated 97% of the island's dry deciduous western forests have been destroyed and those that remain are extremely fragmented. Restoring these forests is one of the highest conservation priorities worldwide, according to a 1995 United Nation's Development Program workshop.
Ganzhorn and his colleagues studied regeneration in both contiguous and fragmented primary forest; the fragments ranged from about seven to 1,500 acres. The researchers found 177 tree species at study sites and determined that 20 (about 10%) of these were dispersed largely or entirely by lemurs.
Fecal analysis showed that these 20 tree species are dispersed primarily by the brown lemur. Of the eight lemur species living in these forests, the brown lemur is the only one that ingests and excretes intact seeds larger than half an inch. Seven of the 20 tree species have seeds this size or bigger.
Ganzhorn and his colleagues compared regeneration in forest fragments with and without brown lemurs and found that the latter had significantly less regeneration of the 20 lemur-dispersed trees. In contrast, there was no difference in regeneration for the 157 tree species that do not depend solely on lemurs for dispersal.
This study shows that "it is insufficient to leave a forest alone to regenerate if lemurs have been eliminated from it," says Ganzhorn. "We should reintroduce lemurs, increase connectivity and especially stop fragmentation of whatever is left."
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