When toxic chemicals enter a food chain, they often build up in living tissue rather than being broken down. This causes two related problems:
- Bioaccumulation is the build-up of a toxic chemical inside a single organism over time. The organism absorbs the chemical faster than it can excrete it, so the concentration in its body rises.
- Biomagnification is the increase in concentration of a toxic chemical at successive trophic levels of a food chain. By the time you reach top predators, the concentration may be thousands of times higher than in the water or soil.
How biomagnification works
- A small amount of a toxin (e.g. a pesticide, or a heavy metal like mercury) enters the water or soil.
- Producers absorb it. Each producer ends up with a small dose.
- A herbivore eats many producers, accumulating the doses from all of them.
- A carnivore eats many herbivores, accumulating their doses.
- At each trophic level the concentration in body tissues climbs.
A famous example: DDT and birds of prey
In the 1950s and 60s, the pesticide DDT was used heavily to kill insects on farmland. DDT does not break down easily; it persists in soil and water for decades and dissolves in fat. The chain:
- Water had tiny amounts of DDT (a few parts per billion).
- Plankton absorbed it as they grew.
- Small fish ate plankton and accumulated DDT in their fat.
- Big fish ate small fish, accumulating ever more.
- Birds of prey (ospreys, falcons, bald eagles) ate the big fish, ending up with millions of times higher DDT concentration than the water.
The high DDT levels made the birds lay eggs with thin, fragile shells that broke under the parent's weight. Populations of bald eagles, peregrine falcons and ospreys crashed. After DDT was banned in many countries in the 1970s, these populations have recovered.
Other examples
- Mercury in fish: industrial mercury pollution accumulates in fish. Large predator fish (tuna, swordfish) have higher mercury levels than smaller fish, which is why pregnant women are advised to limit their intake.
- Lead: built up in wildlife from leaded petrol and lead shot, contributing to declines in many bird species. Now mostly phased out.
- PCBs (polychlorinated biphenyls): industrial chemicals that biomagnify; banned in most countries but still persist in the environment.