Stigmergy is one method of self-organization in a system. It is defined as a method whereby one member of the system changes their local environment, and then another member visits the environment, identifies the changes, and then changes their local environment in response. Eventually, the entire environment is changed by these series of changes in the local environment.

Coined by French biologist Pierre-Paul Grasse in the 1950s, stigmergy is an extension of swarm theory and finds its roots in the customs of eusocial colony insects such as ants and termites.

The primary example of stigmergy is the ant march:

  1. A portion of food is dropped.
  2. An ant finds the food. Upon returning with the food to the nest, it leaves a pheromone trail from the food to home.
  3. Other ants detect the pheromones and follow the trail.
  4. Some ants don't detect the trail, but find alternate paths to the food. If this trail is faster, they will create more pheromone trails in the same amount of time; thus their trail will be stronger, and ants will be more attracted to this shorter, faster trail.
  5. Eventually, the shortest trail is discovered, and all future ants rely on this trail to the food. Thus, though the ants do not act communally, they respond to the collective results of their individual brethren. Pure stigmergy.

Other examples of this include ant corpse-piling (they prefer larger piles to smaller piles, and thus all small corpse piles eventually are combined into one large corpse pile through stigmergy) and termite nest construction practices (where individual termites attach pheromones to bits of mud to encourage similar placement of arches, chambers, and tunnels.)

The Internet is a major source of human stigmergy through the spread of memes, blog trackbacks, and an ever-increasing information/news cycle. Everything2 has elements of stigmergy within it, such as voting (through which better writeups are identified and receive more votes) and the ability for writers to use a single node title to write multiple varying pieces (feeding off each other's individual efforts to create a collectively holistic work at a given node.)