Databases Reference
In-Depth Information
A simple example of the hill building behavior of termites provides a
strong analogy to the mechanisms of Termite. This example illustrates the
four principles of self organization.
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Consider a flat surface upon which
termites and pebbles are distributed. The termites would like to build a
hill from the pebbles, i.e., all of the pebbles should be collected into one
place. Termites act independently of all other termites, and move only on
the basis of an observed local pheromone gradient. Pheromone is a chemical
excreted by the insect which evaporates and disperses over time. A termite
is bound by these rules: 1) A termite moves randomly, but is biased towards
the locally observed pheromone gradient. If no pheromone exists, a termite
moves uniformly randomly in any direction. 2) Each termite may carry
only one pebble at a time. 3) If a termite is not carrying a pebble and it
encounters one, the termite will pick it up. 4) If a termite is carrying a
pebble and it encounters one, the termite will put the pebble down. The
pebble will be infused with a certain amount of pheromone. With these
rules, a group of termites can collect dispersed pebbles into one place.
The following paragraphs explain how the principles of swarm intelligence
interplay in the hill building example.
Positive Feedback:
Positive feedback often represents general guide-
lines for a particular behavior. In this example, a termites attraction
towards the pheromone gradient biases it to adding to large piles. This
is positive feedback. The larger the pile, the more pheromone it is likely to
have, and thus a termite is more biased to move towards it and potentially
add to the pile. The greater the bias to the hill, more termites are also likely
to arrive faster, further increasing the pheromone content of the hill.
Negative Feedback:
In order for the pheromone to diffuse over the
environment, it evaporates. This evaporation consequently weakens the
pheromone, lessening the resulting gradient. A diminished gradient will
attract fewer termites as they will be less likely to move in its direction.
While this may seem detrimental to the task of collecting all pebbles into
one pile, it is in fact essential. As the task begins, several small piles will
emerge very quickly. Those piles that are able to attract more termites will
grow faster. As pheromone decays on lesser piles, termites will be less likely
to visit them again, thus preventing them from growing. Negative feedback,
in the form of pheromone decay, helps large piles grow by preventing small
piles from continuing to attract termites. In general, negative feedback is
used to remove old or poor solutions from the collective memory of the
system. It is important that the decay rate of pheromone be well tuned to
