Quick answer: Green plants capture only about 1% of the sunlight falling on their leaves. Then, at each step of a food chain, only about 10% of the energy is passed on to the next level (the 10 percent law), so food chains usually have only 3 or 4 trophic levels. Harmful, non-biodegradable chemicals such as DDT do the opposite: their concentration increases at each level, which is called biological magnification. The ozone layer high in the atmosphere absorbs harmful ultraviolet (UV) radiation; it has been damaged by CFCs, and in 1987 countries agreed through UNEP to limit CFC production.
Energy flow and the 10 percent law
| Step | Energy available (example) |
|---|---|
| Producers (plants) | 10,000 J |
| Herbivores | 1,000 J |
| Carnivores | 100 J |
| Top carnivores | 10 J |
At each level, about 90% of the energy is used for the organism's own life processes or lost as heat. Only about 10% is stored in its body and available to the next level. This is often called the 10 percent law.
Energy flow is unidirectional: energy captured by plants does not go back to the Sun, and energy passed to herbivores does not return to plants.
Why are food chains short? Because so little energy is left after each step, there is not enough energy to support organisms beyond the third or fourth trophic level.
Worked example
In the chain grass → grasshopper → frog → snake → hawk, the grass has 20,000 J. How much energy reaches the hawk?
Step 1: Multiply by 10% at each step: 20,000 → 2,000 → 200 → 20 → 2.
Answer: About 2 J reaches the hawk.
Biological magnification
Biological magnification is the increase in concentration of harmful chemicals at each higher trophic level of a food chain.
Step 1: Pesticides such as DDT are sprayed on crops and wash into soil and water.
Step 2: Plants and small water organisms take them in.
Step 3: These chemicals are not biodegradable: they are not broken down and they build up in the body, especially in fat.
Step 4: Each consumer eats many organisms from the level below, so the chemical becomes more concentrated at every step.
Step 5: Humans are at the top of many food chains, so the highest concentrations can build up in our bodies.
A well-known study of a coastal estuary found these DDT levels:
| Level | DDT (parts per million) |
|---|---|
| Water | 0.000003 |
| Zooplankton | 0.04 |
| Small fish | 0.5 |
| Large fish | 2 |
| Fish-eating birds | 25 |
That is a rise of about 8 million times from the water to the birds.
The ozone layer
Ozone () is a molecule of three oxygen atoms. At ground level it is a poisonous gas, but in the higher levels of the atmosphere it forms a layer that absorbs UV radiation from the Sun. UV radiation can cause skin cancer and damage to the eyes.
Ozone forms when UV radiation splits oxygen molecules, and the free oxygen atoms join other oxygen molecules:
Damage by CFCs
The amount of ozone began to fall sharply in the 1980s. This was linked to chlorofluorocarbons (CFCs), man-made chemicals then used in refrigerators, air conditioners and fire extinguishers.
In 1987, the United Nations Environment Programme (UNEP) reached an agreement to freeze CFC production at 1986 levels. This agreement is known as the Montreal Protocol, and it has since been strengthened.
Summary table
| Idea | Key fact |
|---|---|
| Energy captured by plants | About 1% of sunlight on leaves |
| 10 percent law | About 10% passes to the next level |
| Food chain length | Usually 3 or 4 trophic levels |
| Biological magnification | Harmful chemicals become more concentrated up the chain |
| Ozone layer | Absorbs harmful UV radiation |
| Main cause of ozone loss | CFCs |
| 1987 agreement | UNEP: freeze CFC production at 1986 levels |
Exam tips
- In energy questions, multiply by 0.1 at each step and show each value.
- Define biological magnification in one line, then explain why it happens (chemicals are not biodegradable).
- Write the two equations for ozone formation with UV over the first arrow.
Common mistakes
- Using 1% at every step. The 1% is only from sunlight to plants; between trophic levels it is about 10%.
- Saying the concentration of chemicals decreases up the chain. It increases.
- Calling ozone harmless everywhere. It is poisonous at ground level but protective high in the atmosphere.
- Saying energy flow is cyclic. It is one-way.
Frequently Asked Questions
What is the 10 percent law?
Only about 10% of the energy at one trophic level is passed on to the next level; the rest is used up or lost as heat.
Why do food chains have only 3 or 4 steps?
So much energy is lost at each step that very little is left to support organisms beyond the third or fourth level.
What is biological magnification?
It is the increase in the concentration of harmful, non-biodegradable chemicals at each higher level of a food chain.
Why do humans have high levels of pesticides in their bodies?
Humans are at the top of many food chains, so chemicals that build up at each level reach their highest concentration in us.
How is the ozone layer formed and why is it important?
UV light splits O₂ into oxygen atoms, which join O₂ to form O₃. The ozone layer absorbs harmful UV radiation from the Sun.