Rainforest Trees Are Like Islands

In the 1970s, biologists realized something interesting about islands: There was a correlation between their size and the number of species they harbored, apparently a kind of evolutionary natural law. Soon, the idea was extended to other kinds of geography—the number of mammal species on mountaintops, similar to islands in their isolation, can also be predicted by their area. The relationship between species number and area has become one of the abiding fascinations of modern ecology. Now, drawing on six years of fieldwork in rainforest trees, perching in their crowns and watching the comings and goings of ants, researchers can report that the leafy giants also follow this rule.

There are more than 400 ant species roaming the rain forest, with about 120 of them living or spending their time in trees, says Steve Yanoviak, an ecologist at University of Louisville and the Smithsonian Tropical Research Institute. Yanoviak has spent the last 25 years ascending to the treetops with rock-climbing equipment to study ants as they roam around in those floating worlds, connected only with woody vines called lianas. “As I was climbing trees, looking around in the canopy,” he says, recalling the beginning of this project, “it became pretty clear to me that ants actually were using lianas to get from place to place.” Did trees connected by lianas—part of a network of high-wire highways—have more species diversity than those that stood alone? And did those that stood alone, unconnected, have patterns in species numbers like islands and mountains?

He and his graduate student Benjamin Adams have been working to answer those questions in the Panamanian forest of the Barro Colorado National Monument. They have recently published a new study, for which the researchers and assistants climbed up to the crowns of more than 200 trees and put out a dollop of tuna or meat mixed with honey. “That's basically a dream food for an ant,” Yanoviak says, and it draws them in over the course of the one-hour observation period. The researchers kept track of which ants showed up to feed or just wander through.

Over the six years of the study, they found that larger standalone trees do tend to have larger numbers of ant species than smaller ones, much like islands or mountain tops. That’s interesting, Yanoviak says, because the trees are not all that far from each other, even for ants. “We were really impressed to see that mathematically, the size of the tree determines how many ant species live in it,” he says, “as long as it doesn’t have lianas.”

Because when the trees are connected by lianas, that pattern disappears. Trees with vines, regardless of size, have more ant species than unconnected trees, on average an increase of 25 percent. “It doesn’t matter if you have one or 100 lianas, you will have more species,” Yanoviak says.

Essentially, once the trees are connected with vines, they don’t function as individual trees anymore, from the perspective of ant biodiversity. That suggests that these vines are important thoroughfares and promoters of biodiversity in the forest, turning it from an archipelago of tree islands to something larger and more connected.

The researchers expect to release another chapter in the tale of the ants and vines in the near future. In these same research plots, the study’s third author, Stefan Schnitzer, has been running a parallel experiment concerning the effect of lianas on the trees themselves. Part of the experiment involves removing the lianas from half the plots to see how the trees respond. Adams and Yanoviak have been watching to see what the ants do. Even with their super-highways gone, so far, the ants seem to be surprisingly resilient, and loyal to their now-isolated home trees.

“You could tell they didn’t like it,” he says. “They shut down their activities and stopped foraging. But they didn't disappear or pack up and leave.”