A healthy ecosystem is not frozen. Succession showed that communities change over years. And community interactions keep populations rising and falling. So what does "stable" mean?
Key idea
An ecosystem is stable when its populations and processes stay within a range over time, even as they go up and down. Small disturbances, like a storm, are followed by recovery. A large or lasting change, like a new predator, the loss of a key species or a changed climate, can shift the ecosystem to a new state.
Why it works
Negative feedback holds populations in a range: more prey feed more predators, who then eat the prey down, so both swing but stay bounded. A disturbance pushes populations off course, and the same feedbacks pull them back, as long as the push is small and the key species survive. Push hard enough, or remove a species that the feedbacks depend on, and different feedbacks take over.
Worked example: Reading a record
A pond's fish population was counted every year: 410, 450, 380, 430, 150 (after a drought), 300, 420, 440. Is the pond stable?
- In normal years the count stays between about 380 and 450.
- The drought pushed it down, and within two years it was back in the range.
Answer: Yes: it fluctuates within a range and recovered from the disturbance.
Worked example: A shift
In some kelp forests, after sea otters were hunted out, sea urchins multiplied and ate the kelp down to bare rock. The barren state lasted for years. Was this a recovery or a shift?
- The ecosystem did not return to its old range.
- A different set of populations took over and held steady.
Answer: A shift to a new state.
Ecosystems with more species, and with several species doing the same job, often recover better, because if one species is lost, another can fill in. This is one reason scientists study biodiversity.
Common mistake
A balanced ecosystem is sometimes imagined as one where nothing changes. Populations always fluctuate. Stability means they stay within a range and come back after small upsets, not that they hold one fixed value.