Tuesday, June 06, 2006

Emergent behavior in desert locusts

312 1402 F3One of the key notions in modern science, one which can yield tremendous insights into the nature of any number of phenomena, is emergence. The idea, as I described it in a post arguing that it's the one thing I'd most want people to understand,
Emergent properties rely on large numbers and often on interactions between simple agents. Out of that large number of interactions, complex and often predictable results emerge. A probability or an average is a very simple emergent property of a system. Biological fitness can be expressed as such a simple probability.… Most of the challenges we face in modern society relate to our ability to manage the complexity of these sorts of processes.
A key concept in trying to understand what makes certain properties emerge from interactions is that it generally seems that some critical density is necessary, below which no complex behavior is observed and above which the group of interacting agents take on those behaviors.

A paper out in Science describes a transition From Disorder to Order in Marching Locusts. The figure reproduced here shows how actual observations of the movement and alignment of locusts matches the predictions of a simple model of emergent behavior. As density rises, the model predicts that the individuals should become ever more homogeneous in their alignment, so that the population of locusts turns into a marching column.

When the locusts molt into flying adults, they can have devastating effects on agriculture for hundreds of acres. Swarms of locusts have been observed in satellite images as the sweep across fields, leaving devastation in their wake.

This research gives a hint about ways to avert such outbreaks. Knowing the critical density at which the locusts start marching will tell pest control specialists when they need to worry about locust populations. While the density stays low, the locusts will not form horrific swarms.

The research is more important than that. There's a broad literature showing how seemingly complex insect behaviors are actually results of very simple rules. What this research shows is how the transition to criticality happens. Density rises, but the behavior of individual locusts doesn't change. What changes is the behavior of the entire group.

The figure shows comparisons between experimental movement of actual locusts with a theoretical model of the behaviors of a system in which each agent orients itself in the average direction of the other agents in its neighborhood. The model makes very few assumptions, and seems to model the behavior of the locusts wonderfully.

Understanding the nature of that shift in behavior of the group without changing the individual behavior can give us insight not just into insect behavior, but into the origins of life, the organization of ecosystems, the development of organisms, and the way consciousness comes into existence.

This suggests that the locusts are not thinking carefully about their behavior. The semblance of thoughtful behavior is a consequence of very simple responses to stimuli. Neurons in the brain respond to equally simple stimuli. At the low density of a locust's brain, such simple interacting agents don't yield complex behaviors, but in the density we find in human brains, or dolphin brains, we get different behavior. The neurons aren't different, but the behavior of the system is. One brain is conscious, the other isn't.

Where Is My Mind?” by Pixies from the album Surfer Rosa (1988, 3:52).