Birds and Bats Act as Rainforest Gardeners

Rainforests are increasingly thinning out due to deforestation and logging, but these tropical forests are getting a boost from some unlikely gardeners. Birds and bats are introducing a surprising number of trees into new forests, a new study published in the journal PLOS ONE shows.

A team of Mexican and American biologists examined the vegetation in a restored tropical forest in southern Mexico, and found that these animals were responsible for 94 percent of the plant species not reintroduced by conservationists.

By regurgitating, defecating or burying seeds, birds, bats and other fruit-eating animals are unknowingly helping most tropical trees reproduce. The authors note that some plants also grow sticky or spiky seeds that latch onto birds' feet and feathers, and there's even evidence that the color of certain fruits evolved to attract birds.

"Seed dispersal by birds and mammals plays a key role in tropical forest dynamics, and will play a critical role in determining which tree species migrate or vanish in response to changes in land use and climate," the researchers wrote.

Over six years, the study team monitored plots of land, previously used as cattle pasture, in a rainforest in southern Mexico. Over time, the number of "recruited" plants - sprouting from bird- or bat-borne seedlings - shot up, contributing at least 19 new species in the restored rainforest.

Though birds and bats took slightly different approaches to how they dispersed such seedlings. Bats, for instance, had more of an affect on pioneer shrubs and trees - hardy plants that don't need nutrient-rich soil to survive - that grew in grassy areas. Once these pioneer plants prepared these areas, the birds' seeds then took root, growing into fruit-bearing trees.

"With half of the tropical rainforest biome cleared at least once in the last 100 years," the authors wrote, "forest conservation and restoration using birds and mammals that transport seeds should become a central theme in ecology of this century."

The Lacandon rainforest region in the eastern Chiapas in Mexico, for instance, was once covered by more than 13,000 square kilometers of tropical rain forest, but over half has been cleared and burned. And now what's remaining is being converted by modern agricultural colonization, logging and cattle-ranching, according to the Smithsonian Institution. Help from effective gardeners like birds and bats may be able to help in part restore such rainforests and others alike.