Populations Can Be Less Predictable Than The Weather

The model developed by Higgins and his colleagues suggests that big swings in population size may be caused by random external influences that are amplified by internal responses. The new model stands because it provides an extraordinary match with 42 years of data gathered from eight fishing regions along the West Coast.

The researchers matched Dungeness crab catches with a model that accounted for internal crab population dynamicsnatural survival rates, egg-laying rates, and cannibalism, all of which have been identified by field researchers as effective internal forces that help crabs rebuild shrunken populations or reduce overpopulation.

The scientists added to the model random external forces, also called "noise," representing variations in weather, water temperature, ocean circulation, or other factors. They found that this approach works mathematically, but they believe more research is needed to identify those external influences more specifically.

"The one thing we can predict with this model is that crab populations will fluctuate drastically," says Higgins, now a postdoctoral researcher at the University of Helsinki. "People have felt populations in nature are in equilibrium. Our model says no, small environmental disturbances are amplified by populations, preventing equilibrium. The natural state of populations is constant change."