THE PETER LAKE FOOD WEB Originally, the food web was dominated by minnows eating zooplankton such as water fleas, which survive by consuming tiny water-borne plants. The few largemouth bass in the lake fed on the minnows. But tweak the top of the chain and an ecosystem shift ensues: the increasingly numerous bass devastated the minnow population, leading to large swings in phytoplankton amounts until the food web settled into its new state. PRECISION GRAPHICS (NOT SHOWN TO SCALE)
Forecasting the future is a tricky art, but a field test of an important ecological theory now shows that drastic environmental shifts can be predicted, potentially paving the way for preventive intervention. Trophic cascades occur when a layer in the food web is added or removed and an entire ecological system shifts catastrophically as a result. For example, if a top predator becomes extinct—say, by overfishing or hunting—pressure is relieved on its prey. This can have destructive effects on other organisms lower down the food chain as they suddenly experience a higher rate of predation. Steve Carpenter at the University of Wisconsin, Madison, and colleagues have now shown that such regime shifts, in this case caused by the deliberate addition of a top predator to ...
