When it comes to maintaining and accentuating the mind-boggling plant diversity of the Amazon rain forests, insects are a friend, not a foe, according to a new study.
“The point is that insect herbivores magnify the differences between the habitats,” said Paul Fine, an assistant professor of ecology and evolutionary biology at the University of Michigan in Ann Arbor.
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The nutrient-starved white-sand soils are dominated by slow-growing plants that are heavily armed against insect predators. Plants that grow in the more fertile red-clay soils have fewer insect defenses but grow faster than insects can eat them.
“Most people assumed that clay specialists wouldn’t be able to grow in white sand. They thought they would suffer from aluminum toxicity or just not be able to grow in such a nutrient-limited environment,” Fine said.
An alternative idea was put forth in 1974 by Daniel Janzen, a biologist at the University of Pennsylvania in Philadelphia. He hypothesized that plant-eating insects are the main force preventing the red-clay specialists from growing in the white-sand soils, rather than the lack of nutrients.
Fine and his colleagues Phyllis Coley, a biologist at the University of Utah in Salt Lake City, and Italo Mesones of the National University of the Peruvian Amazon in Iquitos, conducted the first test of this theory.
Transplant Experiment
The researchers transplanted trees from each type of soil to the other. Some of the trees were covered with nets to prevent insect attacks. Others were left unprotected. The researchers then monitored the plants’ growth for 21 months between 2001 and 2003.
The trees that normally grow in the red-clay soils thrived in the white-sand soils, growing twice as tall as the white-sand species, but only when protected from insect attacks. Left unprotected, the red-clay specialists were devoured by insects.
Meanwhile, the white-sand plants that were transplanted into the red-clay soils grew more slowly and were consistently outpaced by the faster-growing red-clay specialists. Nets offered no advantage, since the white-sand specialists already had ample insect defenses.
“Certainly, without insects, there would be much less of the selective pressure for defenses in white-sand forests, and the slower growing white-sand specialists would no longer have a competitive advantage in white-sand forests,” Fine said.
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.
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.”
Conservation Implications
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.