Keystone species at Higher Level: The same ideas as SL, with different examples: elephants, Yellowstone's wolves and Scotland's beavers. At HL, expect to weigh the gains and risks of bringing keystone species back.
Practise this as you read
- Trace each link of a chain reaction through the food web.
- Balance gains and risks, each with a named species.
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Small numbers, big effect: A keystone species has a disproportionate effect on community structure. If it is removed, the whole ecosystem can collapse.
The points to remember
- A keystone species has a disproportionate effect on its community.
- It shapes community structure: which species live there, and how many.
- Remove it and the food web can collapse: some species boom, others vanish.
- Keystones can be predators, ecosystem engineers or mutualists such as pollinators.
Remember it as: Pull out the keystone and the arch falls.
Real example: African elephants push over and strip trees and shrubs, which keeps the savanna open. Without them, bush spreads and the grazing animals of the open grassland lose their habitat.
Say why it is a keystone: Link the species to the whole community: 'without it, the food web collapses', not just 'it eats other animals'.
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When a keystone predator disappears, the effect passes down the food web like falling dominoes: a trophic cascade.
The chain reaction, step by step
- A keystone predator controls herbivores, so no one species dominates.
- Remove it: prey numbers rise and they overgraze the producers.
- Fewer producers: less primary productivity, so less energy for higher levels.
- Habitat and niches are lost, so species die out locally: biodiversity falls.
- A small population that is left can lose genetic diversity.
Remember it as: Predator down, grazers up, plants down, species out.
Real example: the last wolves in Yellowstone were killed by 1926, and elk overgrazed the willow and aspen by the rivers. After 31 wolves were brought back in 1995 and 1996, willow recovered in places and beaver colonies on the northern range rose from 1 to 9 by 2009. Scientists still debate how big the effect is.
Use only the links in the figure: Follow the arrows you are given. A change the food web does not show, such as 'sea lions increase because there is less competition', does not count.
Some keystones work by building, not eating. An ecosystem engineer creates new places for other species to live.
The points to remember
- An ecosystem engineer changes the physical habitat and makes homes for others.
- Beaver dams slow the water: ponds, marsh and wet meadow form, so habitat diversity rises.
- Still water suits pond plants, frogs and dragonflies: more species.
- Flooded trees die and give birds nest holes.
- But the pond has less oxygen, so stream fish such as trout decline.
- When a dam breaks, a beaver meadow grows on the nutrient-rich silt.
Reading pond and stream data: Quote a number for each place: 'oxygen is 4 mg per litre in the pond but 11 in the stream'. A change with no numbers is not enough.
Real example: in 2009 beavers were released at Knapdale, Scotland: the first legal return of a wild mammal to the UK. Their ponds let willow and wetland plants recover, and beavers have been protected in Scotland since 2019.
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This is why keystones matter for sustainability: they help an ecosystem stay stable and bounce back. That ability is its resilience.
The points to remember
- Keystones keep the food web complex: many species, many links.
- More biodiversity means more ways to recover after a disturbance: higher resilience.
- Healthy producers mean bigger storages of biomass and more nutrient cycling.
- So the ecosystem can keep itself going for a long time: keystones support sustainability.
Remember it as: Keystone in place: more species, bigger stores, faster recovery.
Real example: in Yellowstone, more beaver ponds along recovering willow streams hold water through dry summers, so the wetland community survives droughts better.
Say the word 'resilience': Link each point to resilience in so many words. An answer that explains the food web but never says why that makes the ecosystem resilient cannot get full marks.
Many conservation projects bring keystone species back, a part of rewilding. The return can raise biodiversity, or lower it.
The points to remember
- An apex predator is at the top of a food chain, with no natural predators.
- Gain: predators cut herbivore numbers and change where they feed, so plants recover.
- Gain: engineers create habitat, and keystones stop one species dominating.
- Risk: a returning species may eat rare native species.
- Risk: in a small, fenced or island habitat it may overpopulate or upset predator-prey balance.
Remember it as: Bringing one back can add species or take them away.
Can raise biodiversity
- Wolves: elk browse less, willow and songbirds return.
- Beavers: new ponds and wetlands.
- Sea stars: mussels cannot crowd out other species.
Can lower biodiversity
- Sea otters eat abalone.
- Dingoes in a fenced reserve ate the rare bettongs they were meant to help.
Real example: in Yellowstone, wolves returned in 1995; they cut elk numbers and made elk avoid open river valleys, so willow could regrow there.
Name a species for every point: Tie each gain or risk to a named species and place. An answer with no named example cannot get full marks.
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Keystone and flagship species are easy to mix up. They describe different things: what a species does, and how people see it.
The points to remember
- Keystone: chosen for its ecological role, found by scientific study.
- Flagship: a charismatic, popular species used to win support and raise funds.
- Protecting a flagship protects its whole habitat and the species in it.
- A keystone may be unpopular: seen as a pest or a threat.
- One species can be both: the jaguar is a keystone top predator and a flagship.
Keystone species
- Its role in the ecosystem
- Found by scientific study
- May be feared or disliked
Flagship species
- Its appeal to people
- Chosen for publicity and funds
- Protects the habitat it lives in
Two traps: 'It controls its prey' alone does not show a keystone: say it is integral to the food web or the ecosystem's structure. And a point made twice in reverse ('flagships are popular, keystones are not') counts once.
Real example: the Bengal tiger is the flagship of the Sundarbans mangrove forest in India and Bangladesh; the grey wolf is a keystone that ranchers have long killed as a pest.
How this comes up: Paper 2, Section B (b): explain, with named examples, how bringing back apex predators and keystone species affects biodiversity [7].
Rewilding projects around the world are bringing back missing species.
Using named examples for both, explain how the reintroduction of apex predators and keystone species can impact biodiversity.
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