Experts at Oregon State University have identified a remarkable way that tropical forests manage to maintain their biodiversity without certain species taking over or others dying out. The scientists have found that their are natural “enemies” in the form of fungi and arthropods that kill seedlings in the soil surrounding the host tree, which prevents a tree species from becoming dominant.
Seed dispersal also plays a critical role in promoting biodiversity across tropical forests. Seeds have a chance to become established when mammals or birds carry them to a new region where the fungi and arthropods in the soil target other tree species.
According to the researchers, the restriction of tree recruitment near the adult trees creates a long-term stabilizing effect that favors rare species and hinders common ones.
“In many North American forests, trees compete for space and some have a niche that allows them to outcompete others,” said study lead author Taal Levi. “Douglas-firs are the species that grow best after a fire. Hemlock thrives in the shade and grows well under a canopy. Some species do well at elevation.”
“But in the tropics, all of the tree species appear to have a similar competitive advantage. There is an abundance of species, but few individuals of each species. The chances of blinking out should be high. But there has to be a mechanism that keeps one species from becoming common, becoming dominant. And it is these natural enemies that have a high host-specificity.”
Egbert Leigh of the Smithsonian Tropical Research Institute pointed out that a half square-kilometer of forest in Borneo or in the Amazon contains as many tree species as 4.2 million kilometers of temperate zone forest in Europe, North America, and Asia combined.
Levi said some tropical forests have as many as 1,000 different tree species living in the same general area. The study authors, which are also from the University of Florida and James Cook University in Australia, found that this close relationship between trees and their natural enemies is the key driver of this diversity.
“There is a ‘seed shadow’ around adult trees and some escape the curve and get out, allowing recruitment in other areas until the host-specific enemies get established in the new location,” said Levi. “That’s why it is critically important to maintain the biodiversity of birds and mammals in these forests, or recruitment eventually will decline – especially in over-hunted areas.”
The study is published in the journal Proceedings of the National Academy of Sciences.
Image Credit: Oregon State University