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?