Simple rules predict the outcome of predator-prey...( can be filled with water and stocked...)

can be filled with water and stocked with predator and prey species, allowing researchers to control the supply of an essential nutrient -- in this case, nitrogen. At high nitrogen supply, the algae can be expected to reproduce quickly; farther up in the food chain, the rotifer population also should thrive. At low nitrogen supply, however, both algae and rotifers might struggle to survive.

In a series of chemostat trials, the researchers varied the rate at which fresh water or nitrogen entered to the system. Then they sat back to watch and found these results:

• When nitrogen concentrations or supply rates were relatively low, the system remained at equilibrium with just enough algae for the rotifers to eat and enough rotifers to prevent overpopulation of the algae.

• But lower nitrogen supply rates were detrimental for predator populations for another reason: rotifer individuals usually remained longer in the chemostat vessels before they were washed out, and this shifted the population toward older, less reproductive predators.

• And when large amounts of nitrogen entered the system, numbers of both the predators and prey oscillated wildly, resulting in the extinction of both.

The predators and prey in the laboratory microcosm had responded just the way the model predicted, given the cards that "life," in the form of pumps and chemostat valves, had dealt them.

"Similarly simple models have been successful in a variety of circumstances, including measles 'preying on' people in large cities like London, on commercially important crab populations in the Pacific and on forest insect pests, despite the fact that the patterns of population variation look quite complicated," said Stephen P. Ellner, the Cornell professor of ecology and evolutionary biology who was a professor of biomathematics at North Carolina State when the study began.

"More complicated systems can be broken down to examine and model individually," a
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Contact: Roger Segelken
hrs2@cornell.edu
607-255-9736
Cornell University News Service
5-Dec-2000

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