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Tuesday, 6 April, 1999, 17:31 GMT 18:31 UK
Mathematics of the crowd
Fish
Schools of fish "dance" together
Like teenage boys hanging out on a street corner or fans cheering at a football game, animals behave differently in a large group compared with when they are by themselves.

The mechanics and patterns of nature's aggregations - schooling fish, flocking birds or swarming insects - provide valuable understanding for how such groups behave in, and survive, trying conditions, says a University of Washington zoologist.

Assistant professor Julia Parrish believes the knowledge we gain about this type of behaviour will provide us with a means of judging the impact on these groups of human activity - such as global warming.

What looks like a complex dance - an entire group suddenly changing direction or exploding and reforming - is actually a series of interactions between members of the group reacting to outside influences, she says.

Beautiful but mathematical

"There's a beautiful, aesthetic, very artistic side of it, but there's also a very mathematical and a very evolutionary aspect of animal aggregation," says Parrish, who writes about the complexity and patterns of animal aggregations in the current issue of Science magazine.

How individuals react to outside influences can determine their own survival, as well as the survival of other group members. A herring that turns right when the school turns left faces certain death, Parrish says. But a herring that always co-operates with the group and never competes might die of starvation.

Finding the threshold between co-operation and conflict eventually could provide scientists with the proverbial "canary in the coal mine" that allows humans to grasp the effects that their actions today will have on the world a century from now.

For example, a slight increase in water temperature because of global warming or a change in the ocean's chemical balance because of coastal pollution could alter the point at which schooling breaks down.

Computer models

Given the added stress of overfishing - humans consume 40 million to 50 million metric tons of schooling organisms each year - fish might end up in groups too small or too unfamiliar to survive.

Documenting how animal groups behave allows computer models to predict what will happen under various conditions in the future. A school of fish, for instance, can sense the approach of a predator and take evasive action.

The group might scatter to avoid being consumed, though stragglers or individuals at the outer edges of the group might be devoured. But once the danger has passed, the group reforms. With a computer model, scientists can change the intensity of predation to see at what level the school is slow to reform or does not get back together at all.

Likewise, the models can assume conditions that do not yet exist - higher water temperature, for instance, or lower fish populations, possibly because of overfishing. The scientists study the models to see how fish react to those conditions.

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