Diversifying the Rainforest

But a recent University study of the Amazon rainforest finds that in fact, insects are little farmers of biodiversity — they play key roles in developing the region’s plant diversity.

Conducted by ecology Prof. Paul Fine, the study overturned a long-standing belief among researchers that soil type alone is the main reason for plant diversity of an environment.

The conventional wisdom was that “soils were different, and thus plants adapted to that environment, end of story,” Fine said.

Fine’s study aimed to discover why there are so many different kinds of trees in the Amazon rainforest and which factors play a role in developing the many unique habitats of the environment.

Unlike other rainforests, Fine said the Amazon rainforest possesses great cleavages in biodiversity despite the geological similarities in the environment.

“In Malaysia you have (many species) in one area. But it’s the same species all across Malaysia; regional diversity is lower.

In the Amazon you seem to get all these different habitats that have their own specialized tree species,” said Phyllis Coley, co-author of the study and biology professor at the University of Utah.

In other words, a random sampling of a Malaysian rainforest would be representative of the entire rainforest. However, in the Amazon rainforest, one sampling could be completely different from another sampling a mile away.

“A big question (was) why? Why do species replace themselves as they go across habitats? Why isn’t one species good across the whole shebang? Do they need different adaptations in these different habitats — is that why one species doesn’t dominate?” Coley asked.

To answer these questions, Fine and his team of researchers studied the vegetation from two very distinct habitats in the Peruvian Amazon, one characterized by fertile red-clay soil and the other nutrient-starved white-sand soil.

Fine said he was interested in investigating a hypothesis put forth in 1974 by researcher Daniel Janzen from the University of Pennsylvania, who argued herbivore predation is the main reason plants are restricted to one soil type.

To test the theory, Fine and his team identified 20 species of plants that live in either the red-clay soil or the starved white-sand soil.  The plants have specialized defenses against feeding insects that enable them to live in each soil type.  

The white-sand plants grow slowly but are highly armed with poison defenses against herbivore insects.

“It’s so hard for the white-sand plants to get nutrients out of that dreadful beach sand that they have to protect those nutrients as much as they can. You invest a lot of resources in chemical defenses, so that makes you grow slower,” Coley said.

On the other hand, the red-clay specialists have low protection against insects and other predators, but because of a surplus in nutrients from the red-clay soil, the plants’ growth rate is faster than the rate that they are eaten.

“They can tolerate losing more leaves because they don’t have to pay the cost to defend themselves,” he added.

Fine said the researchers moved plants from their native soil type to the other. They left some trees in each soil type unprotected from predatory herbivores and built netting houses around the others as a barrier against the insects.

Over the course of 21 months, the researchers observed that the fast-growing trees, which were protected from insects and were moved to the nutrient-deprived soil, vastly outgrew the white-sands plants. However, the plants that were transplanted from white-sand to red-clay grew at a slow pace despite being in a more fertile soil.

 Without the threat of insects, red-clay plants would dominate both soil types, which would, in turn, decrease the diversity of the Amazon, Fine said.

After completing the study, Fine concluded the fight between herbivores and plants increases the number of habitats in the rainforest by regulating their growth and preventing the domination of one type of plant.

“People have been interested for so long in how there are more species in (the Amazon). I’d say this was a fairly major breakthrough in showing that alone, these two factors would not make a difference, but together, they show a sharp boundary between (differences in habitats),” Coley said.

Identifying differences between habitats and which species live in each one is important to understanding future conservation implications.

The study showed that very specialized plants live in white-sand forests and that both the soil type and herbivores are necessary to maintain this specialization.

“White-sand forest is a very rare habitat in Peru. These habitats are very fragile because once logged they do not regenerate, or it takes thousands of years. Most white-sand are not even protected,” Fine said.

Fine said the study also sheds some light on an explanation to how plants evolve and differentiate by showing that plants may adapt differently to neighboring environments despite lack of a geographic boundary.

“The fact that these two habitat types were right next to each other meant the insects could cross back and forth to one another,” yet the plants still demonstrated different adaptations as a result of interaction between the plants and the insects and soil, Fine said.

“These remarkable forests are in good shape. These aren’t the only two things, but they are two things that look like they have a big effect,” Coley said.