Remove the Wolves and You Change the Shape of the Rivers
🌿 Remove the Wolves and You Change the Shape of the Rivers
The deer, which had previously grazed anywhere they pleased, suddenly began to avoid open valleys and riverbanks — places where wolves could see them. The vegetation along riverbanks, left ungrazed for the first time in decades, exploded upward. Willows and aspens grew thick. The roots of those trees held the riverbanks together. Rivers that had been wide and shallow began to narrow and deepen. Fish populations recovered. Beaver colonies returned — beavers build dams from willows that hadn't been there before. The dams created ponds that became habitat for otters, muskrats, and waterfowl.
The wolves had not just killed deer. They had changed the behaviour of deer — and that behavioural change had reshaped the rivers. Ecologists call this a trophic cascade: the ripple effect when you change one part of a food web. It is the most dramatic demonstration of what an ecosystem actually is: not a collection of independent species, but a system where everything connects to everything else.
This topic is your introduction to that system. Once you understand it, the tropical rainforest, the hot desert, and every case study that follows will make sense — because they are all ecosystems operating by the same rules.
Geography glossary
- What is an ecosystem?
- A system made up of living and non-living parts that interact with each other.
- What does biotic mean?
- Living parts of an ecosystem.
An ecosystem is a community of living organisms interacting with each other and with their non-living environment. That single sentence contains three ideas that examiners test constantly, so let's unpack each one.
Earn the mark scheme marks
🧠 Memory Aids for Ecosystems
BLEND — The Five Components of an Ecosystem
- B — Biotic (living: plants, animals, fungi, bacteria, decomposers)
- L — Links (food webs, energy flow, feeding relationships)
- E — Energy (flows in one direction; 10% passes between trophic levels)
- N — Nutrient cycling (three stores: Biomass, Litter, Soil)
- D — Distribution (biomes: controlled by latitude, precipitation, ocean currents, altitude)
The 10% Rule — Visualised
Picture a leaky bucket. You pour in 10,000 kJ of sunlight. The bucket has a hole — 90% leaks out at each pour. After the first pour into the grasshopper, only 1,000 kJ remains. Pour again into the frog: 100 kJ. Pour into the snake: 10 kJ. Pour into the hawk: 1 kJ. After four links, you have only one ten-thousandth of the original energy. That is why food chains end — not because predators run out of space, but because they run out of energy.
BLS — The Nutrient Cycle Stores
Remember the three stores as BLS: Biomass → Litter → Soil. Nutrients flow: living things die (B→L), decomposers break them down (L→S), plants absorb nutrients through roots (S→B). The cycle restarts. In a TRF: giant B, tiny L, tiny S. In a desert: tiny B, tiny L, medium S.
LAODA — Why Biomes Are Where They Are
- Latitude (distance from equator → controls temperature)
- Atmospheric pressure belts (rising air at equator = rain; descending air at 25–30° = desert)
- Ocean currents (Gulf Stream warms NW Europe; cold currents create fog deserts)
- Distance from sea / continentality (coasts = moderate; interiors = extreme)
- Altitude (temperature falls 6.5°C per 1,000 m)
Now try it yourself
Quiz · Question 1 of 17
What is an ecosystem?
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This topic in real past papers
Every real exam question we've found on ecosystems overview, with a full worked answer.