Trophic cascade
When removing or adding a predator ripples down a food chain, changing the numbers of its prey and of what the prey eats.
A food web maps who eats whom. A trophic cascade happens when a change at one level ripples through the others. Remove a predator and its prey multiplies; the prey then eats more of whatever it feeds on, which declines. Bring the predator back and the chain reverses.
The ecologist Robert Paine showed this by removing starfish from tidal pools on a rocky shore, which had a big effect on the numbers of many other species. Off the Pacific coasts of North America, sea otters eat sea urchins and sea urchins eat kelp, so whether otters are present or absent regulates whole kelp forests. Species with such outsized effects are called keystone species.
People have set off cascades by accident. In China's Four Pests campaign, launched in 1958, sparrows were declared pests because they ate grain, and nearly 2 billion were reported killed that year. But sparrows also eat insects. With them gone, locusts and other insects surged and destroyed crops, adding to the disaster of the Great Chinese Famine. In 1960 sparrows were taken off the list and replaced by bedbugs.
Introducing a predator can go just as wrong. Predators eat whatever is easiest to catch, which is not always the pest people had in mind; mongooses taken to Hawaii to control rats in the sugar-cane fields fed on native birds and their eggs as well.
Coming up in future stories
- Hawaii's Mongoose Mistake 1883 · Hamakua Coast, Hawai'i
- The Night Captain Cook Ate Pufferfish Liver 7 Sep 1774 · New Caledonia
- When China Declared War on Sparrows 1958 · China