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Plants vs. Insects: An Amazon Epic for the Ages

John Roach
National Geographic News
March 7, 2005
 
Insects are enemy number one to plants the world over: They munch
leaves, suck sap, bore stems, and devour roots. To fight back, plants
have evolved an army's worth of defenses that confuse, repel, deter, and
sicken their attackers.

"As soon as the first insect took a bite out of a plant, what I like to refer to as the arms race began between plants and insects," said Tom Turpin, an entomologist at Purdue University in West Lafayette, Indiana.

Humans have long documented this arms race. They've also taken to consuming many of the plants' "weapons," such as caffeine, nicotine, and cocaine—"all defense chemicals," Turpin said.

According to scientists, the insect-plant arms race in the tropical rain forests of the Peruvian Amazon Basin remains largely undocumented. Scientists hardly know the names of the players or the details of the battlefields.

Paul Fine is an assistant professor of ecology and evolutionary biology at the University of Michigan in Ann Arbor. He's on the case of the Amazon. His studies show that the defenses plants employ against their insect attackers are largely governed by the condition of the soils the plants call home.

Amazon soils, in general, are nutrient poor. "It rains almost every day, and it has been raining for millions and millions of years. Water has just washed everything away," Fine said.

Some soils, however, are more fertile than others. These differences in fertility have helped to spawn mind-boggling plant—and plant defense—diversity.

White and Red

Fine's work to document the arms race in the Peruvian Amazon began by looking at two distinct battlefields: the nutrient-poor white-sand soil and relatively fertile red-clay soil.

White-sand soil is almost pure quartz material derived from sediments that are more than 570 million years old. It has no nutrients and a low cation-exchange complex, which means that any nutrients that flow through the system don't stick to the sand and are washed away.

The lack of nutrients in the white-sand soil means that plants cannot afford to be attacked by insects, because lost leaf tissue is difficult to replace. The result is "strong selective pressure to evolve plant defenses," Fine said.

A more common and more fertile soil is red clay, which derives from rocks that eroded as the Andes mountains formed from around 144 to 65 million years ago.

The clays cover from 50 to 75 percent of the land area in the western Amazon within about 310 miles (500 kilometers) of the Andes and are generally found near rivers and streams.

The clays are young enough to still contain some mineral content and a high cation -exchange complex, "meaning that nitrates, phosphates, et cetera, stick to the clay particles and don't immediately leave the system whenever it rains," Fine said.

With a relative abundance of nutrients, the plants that grow in the red-clay soils can afford some insect predation and devote more of their energy to growth, which helps in plant-to-plant competition, Fine said.

Soil Survival

Fine's preliminary data suggests insects are equally distributed in the white-sand and red-clay soils. However, the plants that grow in the two soil types are unique. White-sand dwellers are not found in the red clay and vice versa.

The difference between the plants comes down to how they allocate their resources—either defense against insects or growth against other plants. White-sand dwellers invest more in defenses, Fine said.

Common plant defenses include greater leaf toughness, as well as resins and latexes that make the plants difficult for insects to eat. Another strategy plants employ is to produce leaves of low nutritional quality so insects have to invest a lot of time and effort in eating.

"This has been shown to increase the risk that the insect will be eaten by something else [because it forces the insect to remain in one place for a long time] and is kind of an indirect plant defense," Fine said.

According to Turpin, the Purdue University entomologist, some of the most impressive and potentially useful defenses are the toxic chemicals plants have developed to ward off insect predation.

"In the rain forest we know very little about the plants, and some may have novel chemicals they use to defend against insects that we may or may not know about, and maybe this chemical is useful," he said.

In a future story, we'll learn how insects help to maintain and accentuate the plant diversity of the Amazon rain forest.


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