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All 12 Flashcards — Keystone species
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Question
What is a keystone species?
Answer
A species with a **disproportionately large effect** on its community relative to its **abundance** — remove it and the community structure changes dramatically.
Question
Where does the term 'keystone' come from?
Answer
The **keystone of an arch** — the small top stone that holds the arch up; remove it and the **whole arch collapses**.
Question
Is a keystone species the same as the most abundant (dominant) species?
Answer
**No** — a keystone species is often present in **small numbers**; its importance comes from **what it does**, not how common it is.
Question
What is a keystone predator?
Answer
A predator that controls the **strongest competitor**, keeping its numbers down so **many other species can coexist** — raising biodiversity.
Question
What is an ecosystem engineer?
Answer
A keystone species that physically **changes the habitat** (e.g. a beaver building a dam), creating conditions many other species depend on.
Question
Why is the beaver a keystone species?
Answer
Its **dams create wetland habitats** that fish, amphibians, insects and birds depend on, so its effect is **far larger than its numbers**.
Question
What is a trophic cascade?
Answer
A chain of **knock-on effects** that spreads through a food web when one species (often a top predator) is added or removed.
Question
What happens to a community when a keystone predator is removed?
Answer
Its prey is no longer controlled, so that prey **takes over and out-competes** other species — **biodiversity falls**.
Question
How does a keystone predator affect biodiversity?
Answer
It **raises** biodiversity, by stopping the strongest competitor from taking over so many species can coexist.
Question
In a sea-star removal experiment, what happens to prey diversity?
Answer
It **falls** — without the predator the mussels dominate the rock and crowd other species out.
Question
Give one keystone predator example and one ecosystem-engineer example.
Answer
Keystone predator: a predatory **sea star** eating mussels. Ecosystem engineer: the **beaver** building dams.
Question
Why does losing a keystone species reduce biodiversity?
Answer
Its stabilising **control is removed**, so one species takes over and **crowds out** the rest, leaving fewer different species.
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Topic 3.8 hub
Populations and communities
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