Energy Flow: Why Food Chains Are Short
About a tenth of the energy survives each step up a food chain. That single number explains the shape of every ecosystem.
Builds on: 1.1 Photosynthesis: Making Food from Light1.2 Respiration: Spending the Sugar
Energy enters once and leaves as heat
Nutrients cycle — the carbon in you has been through countless organisms and will go round again. Energy does not. It arrives as sunlight, is captured by photosynthesis, passes up the food chain, and leaves as heat at every step. An ecosystem needs a continuous supply because it is constantly leaking.
The levels are called trophic levels:
- Producers — plants and algae, capturing light. Everything above them is spending their work.
- Primary consumers — herbivores.
- Secondary consumers — carnivores that eat herbivores.
- Tertiary consumers — carnivores that eat carnivores.
- Decomposers — bacteria and fungi, feeding on every level and returning the nutrients.
The ten percent rule
Only about 10% of the energy at one level ends up in the next. The other 90% is lost, and the reasons are all things you have already studied:
- Respiration. Most of what an organism eats is burnt for ATP, and that energy leaves as heat. It was never available to whatever eats it.
- Undigested waste. Cellulose, bone, fur — eaten but not absorbed.
- Parts never eaten. Roots, skeletons, anything that dies unconsumed (though decomposers get it).
This is why food chains are rarely longer than four or five links: there is simply not enough energy left to support another level. It is also why top predators are necessarily rare and need enormous territories, and why losing them destabilises an ecosystem — nothing else is holding that position.
The uncomfortable arithmetic
The same rule explains a fact about food production. Feeding grain to cattle and then eating the cattle throws away roughly 90% of the energy in the grain. Eating the grain directly does not. That is why a given area of land can support far more people on a plant-based diet than on a meat-heavy one — a conclusion that falls straight out of trophic efficiency, whatever anyone’s preferences.
Biomagnification follows the same pyramid. Persistent poisons — DDT, mercury — are not excreted, so they concentrate as they move up. A predator eating a thousand contaminated fish accumulates the poison of all thousand. This is why Silent Spring found eagles, not insects, with collapsing eggshells, and why advice on mercury in fish singles out large predatory species.
In the open ocean, a pyramid of biomass can be inverted: the mass of phytoplankton at any instant is less than the mass of the zooplankton eating them. That is not a violation — the phytoplankton reproduce so fast that the small standing stock supplies far more energy over time than a single snapshot suggests. Energy pyramids are never inverted; biomass pyramids sometimes are.
⚗️ Lab — The Energy Pyramid
Four trophic levels, one dial for how much energy survives each step.
- At 10%, look at what reaches level 4 — a thousandth of what the plants captured.
- Raise the efficiency to 25% and see how much more a top predator could be supported.
- Drop it to 2%. A fourth level becomes effectively impossible.
Work it by hand — 0 / 2
No multiple choice here: compute the value and type it. Suffixes like 2.5k or 20m are understood; answers within ±2% count.