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.
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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.
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