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1. Complex Adaptive Systems

Key Concepts

By the end of this chapter you will:

  • Know the definitions of complex adaptive systems, emergence and stigmergy
  • Know examples of these concepts
  • Have seen examples of simulation of emergent phenomena


Figure 1: Ants creating a bridge. [Source]

A little ant is walking on the sandy soil of Arizona looking for food. The food is meant for feeding the brood that is being taken care of by other ants in the colony. Different ants have different tasks in the colony, and the ant we are following is charged with leaving the nest every day in search of food. The amazing aspect of ant colonies is that such a complex organizational structure exists that is not controlled by the queen or a small group of bureaucratic ants. There is also no plan or to-do list that the ants are following. No, the complexity of the ant colony emerges out of the local interactions among the ants. The ant we observe is following a trail of pheromones, indicating that other ants of the colony have found food nearby and dropped the pheromones on their way back to the nest. The use of pheromones is therefore a way in which ants communicate to others “follow my trail and you may find food.” The pheromone trail will evaporate at a certain speed, and will therefore only be of limited use. If other ants do not follow this trail, bring back food and add their own pheromones to the trail, the trail will disappear. But when the trail is enhanced by successful use by others, a highway of ants may emerge. On such a highway, we see one lane of ants empty handed following the pheromone signal, and the other lane of ants bringing food back to the nest.

There is an enormous diversity of ant-species, all of which have some variation in their social organization. Some ant-species produce ants with different physical characteristics that distinguish their role in the colony: workers, foragers, soldiers, etc. In other ant-species, all of the colony’s ants are physically similar and can switch roles when needed. For example, when foraging ants are killed by an ant-eater, the colony will experience a reduction of food being delivered, which then signals other ants to exchange their caring-for-the-brood role for a foraging role.

Ants change the environment, which subsequently changes the behavior of other ants. This indirect influence of agents via the change of the environment is called stigmergy. Another example of stigmergy is the digital trails we develop when we interact with websites. By buying books, renting dvds or listening to music files we leave information “pheromones.” We get recommendations of other books, dvds or music that people “like you” also bought, rented or listen to. These stigmergic interactions can lead to a reinforcement of choices. Popular movies featured on YouTube tend to get even more viewers. If we want to understand how certain books, movies or songs become so popular, we need to look into the various ways choices are influenced and reinforced by others.