A keystone species has an effect on ecosystem structure and sustainability that is disproportionately large relative to its abundance. Removing it can trigger cascading changes across a food web, reducing biodiversity and potentially shifting the ecosystem toward a new, less stable equilibrium.
Level and syllabus: SL and HL, Subtopic 2.1: Individuals, populations, communities and ecosystems.
How Removal Can Destabilize an Ecosystem
Keystone species maintain important interactions among populations. They may regulate prey, modify habitats, disperse seeds or support mutualistic relationships.
- The keystone species is removed or its population declines.
- Populations previously controlled by it increase or decrease sharply.
- These changes affect other trophic levels, producing a trophic cascade.
- Habitat structure, resource availability and species interactions change.
- Biodiversity falls, weakening ecosystem resilience.
- Positive feedback may amplify the disturbance, potentially pushing the system across a tipping point into a new equilibrium.
A named example is the sea otter in North Pacific kelp forests. Sea otters prey on sea urchins. Where otters decline, urchin populations can increase and overgraze kelp, converting biodiverse kelp forests into lower-diversity “urchin barrens.” This reduces habitat, food-web complexity and ecosystem services such as fisheries support and carbon storage.
The common misconception is that every predator is automatically a keystone species or that removing one always causes immediate collapse. A species is keystone only when its ecological effect is disproportionately large, and the outcome depends on food-web complexity, alternative species and the size of ecosystem storages. Removal raises the risk of a regime shift; it does not guarantee total collapse.
Management also involves societal perspectives. Protecting sea otters may support ecocentric conservation and tourism but can conflict with shellfish harvesters concerned about income and access to resources.
Exam Technique
For an explain question, give a linked causal chain: removal, population change, trophic cascade, biodiversity loss and reduced resilience. Include a named example and avoid merely defining the term; examiners reward the mechanism connecting the species to ecosystem stability.