4.11.3Additional lessons
Keystone Species — IB Biology HL Revision Notes & Topical Questions
5 min read 5 parts Lesson 94 of 101
Part 1 of 5What Makes a Species Keystone
Keystone species hold a community together in the way a keystone holds an arch.
Definition
- A keystone species has an effect on its community far larger than would be expected from its biomass or abundance.
Mechanism
- Effects usually operate through feeding links, where the keystone controls the numbers of a dominant competitor.
Removal
- Loss of the species can cause large changes in the community, including loss of biodiversity.
Habitat role
- Some keystone species, such as beavers, change the physical habitat as well as the food web.
| Species | Role | Effect of removal |
|---|---|---|
| Sea otter | Eats sea urchins | Urchins overgraze kelp forests |
| Pisaster ochraceus (starfish) | Eats mussels | Mussels crowd out other species |
| Beaver | Builds dams | Wetland habitat disappears |
Part 2 of 5Sea Otter Cascade
1
Sea otters feed on sea urchins, which graze on the large brown algae that form kelp forests.
2
When otters were hunted heavily, urchin numbers rose because their predator was removed.
3
Urchins overgrazed the kelp, leaving bare rocky areas called urchin barrens with few fish.
4
When otters were protected and returned, urchin numbers fell and kelp forests recovered, restoring habitat for many other species.
Part 3 of 5Identifying and Protecting Them
Identifying and Protecting Them
Removal experiments
- Comparing plots with and without the species shows its effect on the number of other species present.
Conservation priority
- Protecting a keystone species can protect many others at the same time, so it is cost-effective.
Trophic cascade
- An effect that passes down several feeding levels, as with otters, urchins and kelp.
Caution
- A species may be keystone in one ecosystem but not in another, so each case must be tested.
Examples in practice
- In the Pacific coast intertidal zone, removing the starfish Pisaster lets mussels spread,
- and the number of other species on the rocks falls.
Part 4 of 5Worked Example
Question
- In a removal experiment a rocky shore plot with a starfish has 15 species and a plot without it has 6.
- Calculate the percentage loss of species richness when the starfish is removed.
- Loss = 15 − 6 = 9 species.
- Percentage loss = 9 ÷ 15 × 100 = 60%.
- The large loss shows the starfish is acting as a keystone predator (values are illustrative).
Answer 60% loss of species richness
Part 5 of 5Exam Checklist & Tips
4.11.3 – Key Points for the Exam
- Keystone species A species with a disproportionately large effect on community structure.
- Trophic cascade A chain of effects passing through several trophic levels.
- Overgrazing Excess feeding on plants that damages the plant community.
- Removal experiment A field test in which a species is taken out to observe the effects.
- Predation Feeding by one organism on another.
Common Mistakes
- Defining keystone species as the most abundant species; abundance is not the key.
- Forgetting to describe the whole chain of effects in a cascade.
- Saying that all top predators are keystone species.
- Giving the effect without linking it to a change in biodiversity.
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