Some species matter far more than their numbers suggest. You saw in Stability, Disturbance and Change that losing one species can shift a whole ecosystem, and in Energy Flow that predators are few. Put those together and you get the keystone species.
Key idea
A keystone species has a much larger effect on its ecosystem than its numbers or mass would suggest. Removing it sets off a trophic cascade: effects that pass down the food chain, level by level, each level rising or falling in turn.
Why it works
A cascade alternates. If a top predator is removed, its prey increases; with more prey eating them, the level below that decreases; and the level below that increases. Each step flips the sign. So removing one predator can end up wiping out the plants two levels down.
- The ochre sea star: in the 1960s ecologist Robert Paine removed the sea star Pisaster from patches of rocky shore in Washington. Mussels it had been eating took over the rock and crowded out many other species. In 1969 he coined the term "keystone species" for it.
- Sea otters eat sea urchins, which graze kelp. Where otters disappeared, urchins boomed and stripped the kelp forests; where otters returned, kelp came back.
- Wolves were brought back to Yellowstone National Park in 1995. Elk changed in number and behavior. How much of the later change in streamside plants comes from the wolves, and how much from other causes, is still studied and debated by ecologists.
Worked example: Following a cascade
In a lake, bass eat minnows, minnows eat water fleas, and water fleas eat algae. Fishing removes most of the bass. What happens to the algae?
- Fewer bass: minnows increase.
- More minnows: water fleas decrease.
- Fewer water fleas: algae increase.
Answer: The algae increase, and the water may turn green.
Worked example: Three levels
Wolves eat deer, and deer eat young trees. If the wolves are removed, what happens to the young trees?
- Fewer wolves: more deer.
- More deer: more young trees eaten.
Answer: Fewer young trees.
Common mistake
It is natural to think the most numerous species must be the most important. Keystone species are often uncommon predators. The sea star was far less common than the mussels it ate, yet the whole shore community depended on it. Importance is measured by effect, not by count.