AP Syllabus focus:
‘Keystone species have disproportionately large effects on ecosystem diversity and overall stability.’
Keystone species are a central idea in ecology because they show how a single species can structure entire communities. Understanding their roles helps explain why some ecosystems resist change while others rapidly destabilise.
Keystone species and why they matter
Core concept: disproportionate impact
A keystone species can be relatively low in abundance yet strongly influences ecosystem diversity and overall stability by controlling species interactions, resource availability, and habitat conditions.
Keystone species: a species whose impact on community structure and ecosystem function is disproportionately large relative to its abundance or biomass.
Practice Questions
FAQ
They compare community change to the species’ abundance/biomass using field experiments or models.
Common approaches include:
Removal/exclosure studies with before–after comparisons
Per-capita interaction strength estimates
Network metrics showing how many interactions depend on that species
A dominant species is abundant and can strongly influence biomass and energy flow because it is common.
A keystone species may be uncommon but has an unusually large effect through regulation of interactions or maintenance of key habitat features.
Yes.
Keystone effects depend on:
Which competitors, prey, or mutualists are present
Environmental conditions that change which resource is limiting
Community complexity (alternative pathways can buffer impacts)
Manipulations can be constrained by ethics, scale, and time.
Also:
Effects may take years to appear
Multiple species can partially compensate
Natural variability can mask causal patterns without replication
Not necessarily.
A keystone predator can increase richness by preventing dominance, but a keystone engineer might increase stability without large richness changes by maintaining critical habitat functions that support persistence and recovery after disturbance.
