Insects Key to Rain Forest Diversity, Study Shows

March 10, 2005

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.

Fine conducted his research in the Amazon rain forest of Peru near the town of Iquitos. There, two different soil types are home to completely different communities of plants.

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.

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