Biological magnification occurs when organisms cannot excrete or break down a toxin quickly enough. As that toxin travels up the food chain, it becomes more and more concentrated, since each predator eats many prey and absorbs the contaminants stored in their bodies. For example, algae store mercury, plankton eat algae, small fish eat plankton and larger fish eat small fish. Although each individual algae only consumes a little mercury, by the time the mercury works its way up the food chain it can reach toxic levels.
Biological magnification has wreaked havoc on ecosystems many times before. One of the most famous cases involved the bald eagle and the pesticide DDT. The DDT used in farming would eventually find its way into the bodies of bald eagles, where it would cause them to lay thin, fragile, eggs. These fragile egg shells would often be damaged or crushed by the eagles, resulting in fewer eagles being born. The bald eagle population crashed until DDT was banned in the United States.
Biological accumulation poses many risks. Pesticides, heavy metals and other accumulants have been linked to neurological problems, cancer, reproductive problems and depressed immune systems. The biggest risk for people is mercury accumulation from fish. Women of childbearing age and young children are especially at risk. You should have high mercury content fish such as swordfish once a week at most and, ideally, less than that. You can safely consume a few six-ounce helpings each week of fish with low mercury content, such as herring and halibut. For detailed information about which fish to eat and which to avoid see Resources below. Pesticides on fruits and vegetables are another risk of biological accumulation. Always wash your fruits and vegetables thoroughly. Throughout your lifetime, you will eat thousands of pounds worth of plant matter, so even a little bit of pesticide contamination on each apple is a bad thing.
Biological magnification is a worldwide issue. Pollution from mining, manufacturing, farming runoff, dumping and other sources of contamination wash into streams and rivers, which in turn wash into oceans. Ocean currents and evaporation carry it to every corner of the earth, so even isolated environments can become polluted. For example, polar bears now have elevated levels of mercury in their bodies. The good news is that reducing the amount of a contaminant in an environment can reduce the damage caused by biomagnification. For example, peregrine falcons and bald eagles have made a dramatic recovery since DDT was banned in the United states.
Biological magnification occurs mainly with two types of substances: persistent organic pollutants and heavy metals. Persistent organic pollutants are generally man-made chemicals such as pesticides and herbicides. Heavy metals are naturally occuring, but are usually released into the environment by human activities. Both kinds of substances are stored in body fat, but they build up for slightly different reasons. Persistent organic pollutants are chemicals to which most organisms have not had a lot of exposure. As a result, organisms don't have enzymes to break them down quickly enough. Metals don't have to be broken down, but most organisms naturally get rid of them slowly. When they are too common in the environment, they can build up faster than the body can get rid of them.