Because a food web links many species, a change to one population travels along the arrows. Ecologists predict these ripples, and then check them against data.
Key idea
When one population grows or shrinks, the species that eat it and the species it eats change too, and then the species linked to those change. Effects can reach plants two or three steps away.
Why it works
Follow the energy. If foxes disappear from a meadow, fewer rabbits are eaten, so the rabbit population grows. More rabbits eat more grass, so the grass shrinks. Later, with too little grass, the rabbits run short of food and their numbers fall again. Every step is just "more eaters means less food" or "fewer eaters means more food."
Worked example: Lose the frogs
In a pond, herons eat frogs and fish. Frogs eat insects. What could happen if a disease wiped out the frogs?
- Frogs eat insects, so with no frogs, more insects survive: insects ▲.
- Herons lose one of their foods, so they eat more fish: fish ▼, or herons may leave or decline.
- Insects that eat plants or algae may then reduce those producers.
Answer: Insects increase; herons eat more fish (fish decrease) or herons decline; the producers the insects eat may decrease.
A real case: gray wolves were missing from Yellowstone National Park for decades, and 31 wolves were brought back in 1995 and 1996. Elk changed where and how they fed. Ecologists still debate how much this helped willows and aspens recover, because drought, other predators and people also affect the elk and the plants.
Common mistake
Students think removing a predator affects only its prey. Trace at least two steps: predator → prey → the prey's food. Also look sideways: other predators of that prey, and other foods of the predator, change too.