The tropics are hotbeds of biodiversity. Compared to other regions, the variety of birds, insects, plants and other species in the steamy forests of the equator is off the charts. For example, about 20,000 tree species occur in the rainforests of the Amazon, versus roughly 600 in the temperate forests of the United States and Canada.
“In the Amazon, an area the size of a football field can have 300 different species of trees. You’re lucky in California if you get 10 or 15,” says Paul Fine, a Berkeley professor of integrative biology. “How can all of these species coexist? Why don’t they outcompete each other?” Fine is experimenting with rainforest trees to find the answers. In the process, he aims to identify the factors that help generate biological diversity.
Much of his research takes place in Iquitos, Peru. Over the eons, rivers have carried soils to this portion of the Western Amazon from many corners of South America. Today, soil mosaics of white sand, clay, and flooded forests (habitats that are inundated for several months of the year) are found within a few hectares of one another.
These conditions make Iquitos an ideal place to study how habitat variety affects speciation. To do so, Fine has been transplanting trees from one habitat to another. He is observing, for example, whether slow-growing trees associated with poor nutrient soils will grow faster on richer soils. “We want to see whether plants can turn these strategies on and off, whether they have the genetic makeup to deal with a variety of different environments,” Fine says.
When the transplants were protected from insects, the faster-growing clay species grew more rapidly on both white sand and clay habitats. But when exposed to insects, white sand species did better on white sand, and clay species on clay. This bolsters the idea that insect abundance accentuates tropical diversity. And because plants near the equator must contend with more enemies than they would in high latitude areas, where cold winters tend to decimate insect populations, they may need to develop specific chemical defenses against local pests.
This effect could be most pronounced for plants growing in poor soils, Fine reasons. “If you’re growing in nutrient-rich soil, and an insect eats half your leaves, it’s much easier to recoup the tissue you’ve lost. Whereas in the world’s worst soil, like these white sand areas, losing half your leaves can cause death,” Fine says.
Fine is now examining the chemical defenses trees produce in certain habitats. He is collecting all of the insects living on a single tree species, from multiple populations separated by distances of between 100 and 3,000 miles of rainforest. “We’re guessing that widely separated populations are not likely to have the same kinds of insect assemblages eating them. If we are right, we predict that this is going to drive selection for different kinds of plant defenses. It would be a kind of local adaptation to an ‘enemy environment’,” Fine says. He is also attempting to cross-pollinate the tree populations from different localities and habitats, to compare what insect-repelling chemicals the hybrids produce versus their parents.
Fine suspects, however, that soil type may be an equally strong factor promoting speciation. To evaluate habitat influence, he is identifying every tree in the genus Protium, which is very common in the Amazon, and reconstructing the group’s family lineage with DNA sequencing. Fine then maps the habitat type associated with each species on top of this evolutionary tree.
Fine has published results for 35 species so far. He found that trees growing in a given habitat type did not hail from the same branch of the family tree. The same pattern seems to be holding for the 65 additional species he has analyzed to date. “It seems to be very easy for these plants to evolve new associations with habitat types,” Fine concludes.
Temperate forests, by contrast, have moved often with the waxing and waning of glaciers. That means temperate trees may not have existed long enough in a single place to evolve such soil specializations. “There are many places on earth where tropical rain forests have been continuously present for at least 50 million years. Maybe that makes it more likely that species will arise in these aseasonal areas; they have a long history of having adapted to the conditions of those localities,” Fine says. The result? A planet with a remarkable fondness for tropical creatures.
by Kathleen M. Wong