Without a competitive advantage against insect predation, the white-sand specialists could die out, reducing the diversity of the rain forest, Fine added.
The study shows that different soil types alone are not sufficient to determine what plant species grow where, contradicting a longstanding theory and confirming Janzen's hypothesis, according to the researchers.
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Fine added that insects might have a similar effect on accentuating plant diversity in habitats defined by differences in altitude, rainfall, and other factors.
The study was published in the research journal Science in July 2004. Writing in a commentary on the paper, Robert Marquis, of the University of Missouri in St. Louis, said the research "adds to the mounting evidence that herbivory is a major factor in determining the plant composition of tropical forests."
According to Fine, no studies suggest that insects are endangered in the Amazon. But the white-sand soils, which cover small, isolated islands of habitat, are becoming increasingly altered by human activities.
Government programs have settled the rural poor on the white-sand soils and told them to grow crops. The crops fail after a year. And because the soils are so nutrient-poor the forests are unable to grow back.
"There are no uses for these forests beyond what you can get from a natural forest growing there," Fine said. "Once they're cut down, they don't grow back, and what you get are white-sand wastelands."
Intact and sustainably managed, however, the forests can provide valuable woods used in housing construction and handicrafts. Apart from that, at least six species of birds unknown to science have been discovered in white-sand forests in the last decade, Fine said.
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