Trees’ natural ‘enemies’ make tropical forests so biodiverse

It isn’t just God who works in mysterious ways. Nature does, too.

To wit: how tropical forests manage to maintain their wondrous floral biodiversity where tree species rarely come to dominate at the expense of others has long been a bit of a puzzle. For instance, a mere half a square-kilometer of virgin forest in Borneo or the Amazon contain as many tree species as 4.2 million kilometers of temperate zone forest in Europe, North American and Asia combined.

The answer for this hyper-diversity of tree species in the tropics, an international team of scientists says, lies in humble fungi and arthropods. By targeting specific species in specific areas, these “enemies” of the trees devour or destroy many seeds and seedlings near host trees, whereby those trees are unable to spread thickly in small areas and muscle out other species in the process.

Also playing a part are a range of animals from mammals to birds, which spread seeds far and wide. By doing so, they help maintain an equilibrium of sorts in forests, ensuring that numerous plant and tree species get a fair fighting chance.

“In many North American forests, trees compete for space and some have a niche that allows them to outcompete others,” says Taal Levi, an ecologist at Oregon State University who was lead author of a study on the matter. “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,” he adds.

“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,” Levi explains. “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.”

Thanks to these natural “enemies,” individual tree species are kept in check, which means that as many as 1,000 different tree may thrive in the same general area within certain tropical forests. If fungi, arthropods and other natural enemies of trees create protective zones around trees where a new tree of the same species cannot take root or grow to maturity, then the very high levels of tree diversity in tropical forests can carry on indefinitely, without human interference such as logging or forest clearing.

“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,” Levi says. “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.”