The ants, Azteca instabilis, have a natural history like that of many other ants. A queen establishes a colony in a tree, and once the colony reaches a certain size it splits and a satellite nest is established in a neighboring tree. Over time, you'd expect the ants to spread to every tree on the farm, but that's not the case.
"The ants only occupy about three percent of the trees," Vandermeer said. "But once you find them, you find them in clumps."
How to explain the clumpiness?
"Normally when you have an animal or plant that's distributed in patches like that, you tend to think that there's some kind of underlying habitat variable that's responsible," Vandermeer said. But on the coffee farm, the habitat is about as uniform as a habitat can be, as trees are deliberately planted in a grid pattern. So the non-uniform distribution of ant colonies must be due to something other than habitat---something inherent in the biology of the ants.
Combining computer modeling with field observations, the researchers came up with a scenario that explains the spatial patterns as a case of criticality.
As the ant colonies spread from tree to tree, local clusters develop, but the clusters don't expand indefinitely, all because of another insect with a sinister name: the decapitating fly. The parasitic fly lays its egg on the thorax of an ant; the egg hatches and the fly larva migrates into the ant's head capsule where it feasts on the contents. Then the ant's head falls off and the new adult fly emerges. Unfortunately for the ants, the bigger their clusters, the easier it is for the flies to find their colonies.
"So it's the fly that maintains the ants' spatial distribution," Perfecto said. Looking at the frequencies of various sizes of clumps, the researchers found the telltale power law relationship, the hall
|Contact: Nancy Ross-Flanigan|
University of Michigan