Fungi and arthropods that kill seeds and seedlings might be the primary drivers behind the great biodiversity of trees within tropical forests.
Scientists have long struggled to figure out why a handful of tree species do not take over and push other species to die off. They found that the "enemies" of trees, which reside in the soil near individual trees, are necessary to allow different species of trees to thrive within tropical forests.
These findings were published in the journal Proceedings of the National Academy of Sciences.
Why 'Enemies' Of Trees Are Necessary
Taal Levi, an ecologist at the Oregon State University and the lead author of the study, explained that in forests in North America, trees that fight for space often have distinct characters that give them an edge over their competitors. For example, Douglas-firs grow best after a fire while Hemlocks can thrive in the shade.
In contrast, all trees have similar advantages in tropical forests.
"There is an abundance of species, but few individuals of each species. The chances of blinking out should be high," explained Levi. "But there has to be a mechanism that keeps one species from becoming common, becoming dominant."
How Fungi, Arthropods Maintain Biodiversity In Tropical Forests
The researcher added that tropical forests sometimes have as many as 1,000 different species of trees in the same general area. He said that these natural enemies of trees are restricting the recruitment of juvenile trees, an idea that has been posited half a century ago by two scientists.
The Janzen-Connell hypothesis claim that host-specific natural enemies such as herbivores and pathogens maintain the tree species biodiversity in tropical forests. It, however, does not explain how a thousand species of trees can be maintained together.
Levi and his colleague found that the relationship between trees and natural enemies is also important to the biodiversity within tropical forests. If fungi, arthropods, or other natural enemies create small zones around trees where seed or a seedling cannot grow, diversity can be maintained indefinitely.
The researchers also found that seed dispersers, including birds and mammals that bring seeds from different tree species to far places, also play roles in maintaining tree biodiversity. When a seed gets carried to a new region, it gets a better chance to grow because the fungi and arthropods target different species.
Levi said that it is equally important to protect the biodiversity of birds, mammals, and other forest dwellers. If these creatures disappear, the recruitment of new tree species will also decline.