Biomagnification

The concentration of poisonous chemical pollutants increasing from one trophic level to the next along a food chain is called biomagnification.

Figure to be added

Figure 15.30 — biomagnification: a pollutant at 0.000 002 mg/l in the water becomes 0.005 mg/l in plankton, 8–11 mg/l in small animals and fish, 37 mg/l in larger fish, 110 mg/l in a sea bird and 160 mg/l in a seal.

For extra knowledge

Dichlorodiphenyltrichloroethane (DDT), polychlorinated biphenyls (PCB) and heavy metals such as mercury and copper build up in living bodies in this way.

Features of substances that biomagnify:

They last a long time without breaking down.
They can pass from body to body.
They dissolve in fat.
They are biochemically active.

Although they enter the lower trophic levels of food chains in trace amounts, their concentration rises as they pass to higher levels.

Why the top of the chain suffers most

Recall the energy pyramid: a predator must eat a great deal of prey to get enough energy, because 90% is lost at each step. Each prey animal carries a little of the fat-soluble poison, which the predator cannot break down or excrete, so it keeps all of it and adds it to its own fat. A fish eats thousands of plankton, a bird eats hundreds of fish, and the poison is concentrated at every step — in figure 15.30, eighty million times from the water to the seal. This is how DDT thinned the eggshells of hawks and eagles. People are also at the top of many food chains.

Poison concentrates up the chain

Long-lasting, fat-soluble poisons such as DDT, PCBs and mercury become more concentrated at each higher trophic level.