4.10.3Additional lessons
Keystone Species — IB Biology SL Revision Notes & Topical Questions
5 min read 4 parts Lesson 137 of 142
Part 1 of 4What Makes a Species Keystone
The word keystone comes from the central stone of an arch, which holds the other stones in place.
Definition
- A keystone species has an effect on its ecosystem that is much larger than its abundance or biomass would suggest.
Removal
- Taking it out can cause loss of other species, major changes in structure and even ecosystem collapse.
Protection
- Conserving a keystone species helps keep the whole community stable and protects the species that depend on it.
Not always dominant
- Keystone species may be small in number; what matters is the role they play, not how common they are.
Evidence
- Keystone roles are identified by removal or exclusion experiments, or by observing what happens when a species is lost.
Part 2 of 4Examples and Their Roles
Each example shows a different way a single species can control habitat structure or food supply, whether it is a predator or an engineer of the habitat.
Examples and Their Roles
Sea otter
- Otters eat sea urchins, which keeps urchins from overgrazing kelp, so the kelp forest remains as habitat and food for other species.
Beaver
- Beavers build dams that slow water flow and create wetlands, which provide habitats for a wide variety of organisms.
African elephant
- Elephants eat shrubs and small trees, keeping grassland open so that grazers such as zebra continue to have food.
Result of loss
- In each case the loss of the species alters habitat structure and the food supply of many others.
| Species | Effect | If removed |
|---|---|---|
| Sea otter | Controls urchin numbers | Urchins overgraze kelp |
| Beaver | Creates wetland habitat | Fewer wetland species |
| Elephant | Maintains grassland | Woody plants spread |
Part 3 of 4Sequence of a Trophic Cascade
Removing a keystone predator sets off a trophic cascade, with effects travelling down through several levels.
1
A predator that is a keystone species is removed by hunting or disease.
2
The population of its prey increases because predation pressure falls.
3
The prey species overuses its food resource, such as kelp or grass.
4
Loss of that vegetation removes habitat and food for many other species.
5
Biodiversity falls and the ecosystem may become less stable.
6
Reintroducing the species can reverse the cascade, though full recovery may take many years.
Part 4 of 4Exam Checklist & Tips
4.10.3 – Key Points for the Exam
- Keystone species A species whose effect on its ecosystem is disproportionately large compared with its abundance.
- Trophic cascade A chain of effects that runs through several trophic levels after a change at one level.
- Overgrazing Eating vegetation faster than it can regrow.
- Habitat The environment in which a species normally lives.
- Apex predator A predator at the top of a food chain with no natural predators of its own.
Common Mistakes
- Assuming the most abundant species is always the keystone species.
- Saying keystone species only means predators; beavers and elephants are not top predators.
- Forgetting to explain the effect on other species when giving an example.
- Treating all species as equally important to ecosystem stability.
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