In fact, when protected from insects, the red clay tree species transplanted onto white sand soil grew roughly twice as tall and had twice the leaf area as white sand species. That's because the white sand species grow slowly because they put more energy into defenses against insects either tough leaves or chemical defenses.
"A plant can't be extremely well-defended from insects and grow very fast," says Fine. "It's similar to why you can't have the heaviest, safest car and the fastest car."
Unprotected, the red clay trees transplanted onto white sand soil died at twice the rate of trees that normally live on white sand soil.
Meanwhile, white sand trees transplanted onto red clay soil grew more slowly than the trees that normally live on clay soil. Net enclosures didn't increase their survival because they already had natural defenses against insects.
The findings mean red clay tree species would grow on and dominate both habitats red clay soils and white sand soils were it not for the presence of insects that normally prevent the red clay tree species from living on white sand soil.
"My results suggest that if bugs weren't around, the faster-growing clay soil plants could live in both soils, and the white sand tree species would go extinct," Fine says. "Therefore, bugs promote diversity in the rainforest by making it impossible for red clay trees with low defenses [against insects] to live in poor white sand soils."
Coley says: "Most people think of herbivores as detrimental pests bugs eat plants, which is not good for the plants. Yet without insects, clay tree species would take over all forest types. Bugs beget tree diversity."
Implications for diversity and the origin of species
The study shows different soil types are not adequate by themselves to define habitats or niches what trees grow