Coaching Centre
Energy flow in ecosystems
Why only about 10% of the energy passes from one trophic level to the next, where the rest goes, energy pyramids, why there are fewer predators than prey, why food chains are short, and sustainable ecosystems.
A wedge-tailed eagle needs a territory of up to 100 square kilometres to find enough food. A grasshopper can live its whole life in one paddock. The reason has nothing to do with how big they are, and everything to do with how much energy gets lost each time something is eaten.
Where does the energy go?
When a grasshopper eats a mouthful of grass, only a small part of the energy in that grass ends up as new grasshopper. Most of it goes elsewhere:
- Waste. Plant material is hard to digest, so a lot passes straight through and leaves as faeces. That energy goes to dung beetles and decomposers instead.
- Respiration. The grasshopper uses energy to move, to keep its cells working and to repair itself. This energy ends up as heat that escapes into the surroundings. It can't be passed on.
- Uneaten parts. Animals rarely eat everything. Roots, bones, fur and shells are often left behind.
Only the energy that's built into new body tissue, the growth, is available to the next animal in the chain. On average that's only about 10%. This is called the 10% rule. It's an average: the real figure ranges from about 5% to 20%, depending on the animals. Warm-blooded birds and mammals pass on less than insects or fish, because they burn so much energy keeping their bodies warm.
Energy pyramids
Because so much is lost at each step, the energy available shrinks quickly as you move up a food chain. Drawing each trophic level as a bar gives a shape called an energy pyramid.
If producers in an area capture 10 000 kJ, the herbivores get about 1000 kJ, the animals that eat them about 100 kJ, and a top predator about 10 kJ, just one-thousandth of what the plants captured.
The plants in a paddock capture 50 000 kJ of energy in a year. How much reaches the third-order consumers?
- With 10% passed on, how much energy reaches the 5th level from 10 000 kJ?
- Change the percentage to 20%, then to 5%. How does that change the top of the pyramid?
- Switch between "Bars to scale" and the classic shape. Why do textbooks usually draw the classic shape?
- Try 6 levels. Why do real food chains rarely have more than 4 or 5?
What this explains
The 10% rule explains a lot about the living world:
- There are far more herbivores than carnivores. A savannah might support thousands of zebras but only a few dozen lions. If it were the other way round, the lions would eat their way through the zebras in weeks.
- Top predators need big territories. There isn't much energy left at the top, so eagles, dingoes and sharks have to cover a huge area to find enough food. This also makes them the first to disappear when habitat is cut up.
- Food chains are short. After four or five steps there's simply not enough energy left to support another level.
- Eating lower down the food chain feeds more people. About 10 times more people can be fed from a field of grain eaten directly than from the same grain fed to cattle first. That's one reason plant-based diets have a smaller environmental footprint.
Sustainable ecosystems
An ecosystem is sustainable if energy keeps flowing in and nutrients keep cycling, so that all of its populations can keep going year after year. Energy flows through an ecosystem in one direction: in as sunlight, out as heat. Nutrients such as carbon and nitrogen go round and round, passed along food chains and returned to the soil by decomposers. An ecosystem becomes unsustainable when something uses up energy or nutrients faster than they can be replaced, for example when a paddock is overgrazed or a fishery is overfished.
Test yourself
Producers in a pond capture 20 000 kJ. Using the 10% rule, how much energy reaches the second-order consumers?
20 000 → 2000 (first-order) → 200 (second-order).
Most of the energy an animal takes in is lost as
Only the energy in new tissue can be passed on.
Why are there usually fewer lions than zebras in an area?
With only about 10% passed on, the zebra level can support far fewer lions.
Which statement about energy and nutrients in ecosystems is correct?
Energy enters as light and leaves as heat; decomposers return nutrients to the soil.
A grasshopper eats grass containing 200 J. About how much of this becomes new grasshopper tissue?
About 10% is built into growth.
Label the levels of the energy pyramid.
Order from most energy to least energy.
On average only about % of the energy at one trophic level is passed on to the next. Most of the rest is used in and escapes as , or leaves the animal in . This is why food chains are usually .
Explain why a field of wheat can feed more people if the wheat is eaten as bread than if it is fed to cattle and the people eat the beef.
Eating bread puts people one step from the producer, but eating beef adds an extra trophic level: wheat → cattle → people. Each time energy is passed up a food chain, only about 10% is passed on, because the cattle use most of the energy in the wheat for respiration (moving and keeping warm), which is lost as heat, and some is lost in their waste. So the same field gives people roughly ten times as much energy as bread as it does as beef.