AP Biology: Keystone Species and Community Structure Practice Questions
Practice keystone species and community structure with original AP Biology multiple-choice questions. These items connect species interactions to diversity, food-web changes, experimental controls, and evidence-based ecological conclusions.
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1. Which description best identifies a keystone species in a community?
Explanation. A keystone species has an unusually large influence on community composition or ecological processes relative to its population size or biomass. It does not have to be the most abundant species.
2. A predatory fish feeds mainly on a fast-growing herbivorous fish. When the predator is removed from replicated reef enclosures, herbivores increase and algae cover most available surfaces. What is the most direct explanation?
Explanation. Removing the predator releases herbivores from predation. More herbivores can change resource use and community interactions, producing an indirect effect through the food web.
3. In a rocky habitat, a consumer normally prevents one mussel species from occupying most of the surface. After the consumer is removed, mussels dominate and the number of other attached species declines. Which result best supports calling the consumer a keystone species?
Explanation. A strong community-wide change after removal, such as dominance by one competitor and lower diversity, is evidence that the consumer had a disproportionate role in maintaining community structure.
4. Which experimental design would provide the strongest evidence that a species maintains local plant diversity by suppressing a dominant competitor?
Explanation. Replication, an unchanged control, and measurements through time allow researchers to isolate the effect of removing the species from normal variation among plots.
5. A top predator is removed from a forest. Deer browsing increases, tree seedlings become scarce, and several understory plant species decline. This sequence is best described as what?
Explanation. A trophic cascade occurs when a change at one trophic level produces indirect effects at lower trophic levels. Here, predator loss changes herbivore pressure and then plant abundance.
6. Why is a species' low abundance alone insufficient evidence that it is a keystone species?
Explanation. Many rare species have limited measurable effects, while a keystone species is defined by a disproportionately large effect on the community. Evidence must show the effect, often with removal or comparison studies.
7. Two pollinator species visit the same flowering plant. Removing one pollinator causes little change in fruit production because the other increases its visits. What does this finding most strongly suggest?
Explanation. When another species can compensate for a lost ecological function, the immediate community effect may be small. This functional redundancy makes keystone status less likely under the tested conditions.
8. An ecologist observes that removing a burrowing mammal reduces soil aeration and eliminates many small pools used by amphibians. Which role most directly explains the mammal's broad effect?
Explanation. Ecosystem engineers alter physical habitat conditions. By changing soil structure and water retention, the mammal can affect many species even without directly consuming them.
9. A removal experiment finds that species richness declines in treatment plots but not in paired control plots. Which additional observation would most strengthen the conclusion that the removed predator prevented competitive dominance?
Explanation. An increase of a likely dominant competitor specifically in removal plots provides a mechanism linking predator loss to reduced richness: the predator had prevented that species from excluding others.
10. Which outcome would be least consistent with the hypothesis that a sea predator is a keystone species?
Explanation. If well-replicated removal causes no detectable change in the relevant community measures, the study does not support a disproportionate ecological effect in that context.
11. A disease selectively reduces a dominant canopy tree, allowing light-demanding shrubs and herbs to increase. Which community-level prediction is most reasonable during the initial change?
Explanation. Loss of a dominant canopy species can alter a key resource, light. Species that were previously shaded may increase, changing community composition even if no species is formally classified as keystone.
12. Researchers want to distinguish a keystone predator effect from an effect caused by an unrelated drought. Which approach best addresses this alternative explanation?
Explanation. When treatment and control plots experience the same drought, differences between them are more plausibly attributed to predator removal rather than the drought alone.
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FAQ
What makes a species keystone?
A keystone species has a disproportionately large effect on community structure or ecological processes relative to its abundance. Strong evidence usually comes from carefully controlled removal, addition, or comparison studies.
Are all top predators keystone species?
No. A predator can be ecologically important without meeting the keystone definition. Its status depends on the size and mechanism of its effect in a particular community.
How are trophic cascades related to keystone species?
Some keystone predators trigger trophic cascades: changing predator abundance alters herbivores or prey, which then affects organisms at lower trophic levels. Not every keystone effect is a trophic cascade.