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.
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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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