In a temperate forest, ecologists study two species: a large herbivore comprising 30% of total vertebrate biomass whose removal causes little change in biodiversity, and a rare fig tree (less than 1% of canopy cover) whose removal causes several frugivore species to go locally extinct. Which conclusion is best supported?
AThe large herbivore is a keystone species because it contributes the greatest biomass to the community
BThe fig tree is likely a keystone species because its removal causes disproportionate community disruption relative to its abundance
CNeither qualifies as a keystone species because keystones must be top predators
DThe large herbivore is a keystone because keystone status correlates with population size
Keystone status is defined by disproportionate per-capita impact on community structure, not by abundance or biomass. The fig tree's removal causes cascading extinctions far out of proportion to its rarity — this is the defining feature of a keystone. The large herbivore, despite its abundance, turns out to be functionally redundant. This scenario illustrates a keystone mutualist: a species whose removal triggers secondary extinctions because it provides a critical resource (fruit during lean seasons) that many other species depend on. Paine's original sea star work and subsequent research have repeatedly shown that the most biomass-heavy species are often not keystones.
Question 2 Multiple Choice
After wolves are reintroduced to a valley where elk had been overgrazing streamside vegetation, willows and aspens recover, riverbanks stabilize, and beaver and songbird populations increase. This sequence is best described as:
ABottom-up control, because plant recovery drove increases in animal diversity
BCompetitive exclusion, because wolves displaced elk from their ecological niche
CA trophic cascade, in which the top predator indirectly benefited primary producers by suppressing herbivore pressure
DEcological succession, because the community progressed through predictable developmental stages
A trophic cascade is the indirect effect of a top predator on lower trophic levels — in this case, wolves suppress elk numbers and behavior, reducing overgrazing, which allows vegetation to recover, which supports beavers, stabilizes stream habitat, and increases songbird diversity. This is top-down control: the structure of the community is regulated from the apex predator downward. Bottom-up control would mean nutrients or primary productivity driving the system from below. The Yellowstone wolf reintroduction is the canonical terrestrial trophic cascade example.
Question 3 True / False
Keystone status is not an intrinsic property of a species — a species may be a keystone in one community and play a minor role in another, depending on community context.
TTrue
FFalse
Answer: True
This is one of the key refinements to the keystone concept since Paine's original work. Whether a species is a keystone depends on the community in which it is embedded — specifically, whether its removal would trigger competitive exclusion, secondary extinctions, or dramatic restructuring. The same predator species might suppress the dominant competitor in one community (creating space for many other species) but simply be one predator among many in a different community with different competitive dynamics. Conservation decisions based on keystone status must therefore consider community context, not just species identity.
Question 4 True / False
The strongest evidence that a species is a keystone comes from observational studies showing it is the most abundant or conspicuous predator in the community.
TTrue
FFalse
Answer: False
The definitive evidence for keystone status comes from removal experiments, not abundance surveys. Robert Paine's work established this standard: by physically removing Pisaster sea stars from experimental plots and observing that mussels monopolized the substrate and diversity collapsed, he demonstrated a causal relationship between the predator's presence and community diversity. Sea stars were relatively rare — their keystone status would never have been predicted from abundance alone. Observational surveys can generate hypotheses, but only experimental removal (or opportunistic natural experiments like reintroduction/extirpation) can establish the disproportionate-impact criterion.
Question 5 Short Answer
What makes a species a 'keystone' species, and why can a relatively rare or low-biomass species qualify for this status?
Think about your answer, then reveal below.
Model answer: A keystone species has a disproportionately large impact on community structure relative to its abundance or biomass — tested by what happens when it is removed. A rare species can qualify because keystone status is about per-capita impact: by preferentially consuming the dominant competitor or providing a critical resource, the species prevents competitive exclusion or sustains dependent species far beyond what its biomass share would predict. The key test is a removal experiment.
Paine's Pisaster sea star is the paradigm case: sea stars were relatively uncommon on the rocky intertidal shore, but their selective predation on mussels (the dominant competitor for space) prevented mussels from monopolizing the substrate. Remove the sea star, and mussels crowd out barnacles, algae, limpets, and other species — diversity plummets. The mechanism is per-capita selectivity, not sheer numbers. This insight reframed conservation thinking: protecting an ecosystem may depend less on preserving the most abundant species and more on identifying and protecting the rare species that anchor community structure.