Biomagnification

Biomagnification, also called bioamplification or biological magnification, is the increase in the concentration of a substance in the tissues of organisms at each successively higher level of a food chain. Pesticides and heavy metals entering aquatic environments are the classic case: as each level of the food chain eats contaminated prey from the level below, the toxin becomes more concentrated in its tissues, and the effect is strongest for chemicals that are fat-soluble and resist excretion. Mercury illustrates the pattern starkly: seawater itself carries only trace amounts, yet herring carry around 0.01 parts per million while sharks, several levels higher in the food chain, carry more than 1 part per million despite similarly small direct exposure. The pesticide DDT, among the least soluble chemicals known, accumulated in the fatty tissue of predatory birds such as bald eagles and peregrine falcons through the 1950s, thinning their eggshells and driving population declines that eventually led to its ban.