AP Syllabus focus:
‘Removing keystone species can cause ecosystems to collapse because their effects are disproportionate to their abundance.’
Ecosystems are held together by a small number of highly influential species. When these keystone species are removed, indirect effects can propagate through food webs and habitats, sometimes pushing the entire system toward rapid, difficult-to-reverse collapse.
What a keystone species is (and why abundance can mislead)
A keystone species is defined by its impact, not how common it is. Some keystones are rare, yet they regulate many other populations or maintain essential habitat features.
Keystone species: a species whose removal causes disproportionately large changes in community structure and ecosystem function relative to its abundance.
Practice Questions
FAQ
They may use temporary exclosures, controlled reductions, or simulated removals, then measure community-wide responses.
Evidence is strongest when many taxa and functions shift, and controls rule out abiotic changes.
Yes; keystone effects are context-dependent.
Differences in food-web structure, productivity, or presence of potential functional substitutes can weaken or strengthen keystone influence.
Recovery can be blocked by altered habitat, missing recruits, or new dominant species that prevent re-establishment.
These “alternative states” can persist because feedback loops stabilise the new conditions.
No. Some are engineers or mutualists.
Their keystone status comes from controlling limiting resources or enabling many species, not trophic position.
Time-series showing rising variability, synchronised declines across unrelated species, and loss of structural habitat metrics are useful.
Network-based indicators (interaction changes) can also flag increasing fragility.
