Biomagnification

Species at Risk

Biomagnification is a special concern for species that occupy the higher trophic levels and those that have long average life spans. Particularly at risk are terrestrial mammalian carnivores, marine mammals, birds of prey, and predator fish species. Studies have demonstrated that the degree of biomagnification, however, is not merely related to the trophic levels of the consumer, but also to the species' bioenergetic conversion efficiency, which relates activity and energy expenditures to growth and bioaccumulation rates. It is also associated with the species' physiological ability to detoxify the chemical.

Predatory birds are especially vulnerable to biomagnification because their detoxification capability is poor. In fish-eating birds—such as the bald eagle and osprey—the biomagnification of DDT created toxic levels of DDE, a metabolic product of DDT that interferes with calcium absorption. This led to the thinning of eggshells and caused eggs to crack under the weight of the roosting parent, leading to plummeting numbers of these species. In the decades following the prohibition of DDT use in the United States in the 1970s, some of these populations have recovered.

As a high trophic level feeder, humans are not excluded from the process of biomagnification. A tragic example of this occurred in Minamata, Japan, in the 1950s. Mercury was released into a nearby river by a factory and then methylated by bacteria. The methyl mercury was bioconcentrated by organisms and subject to biomagnification, eventually poisoning the humans who consumed contaminated fish. This resulted in a host of health disorders and deaths, creating health problems even for successive human generations.

Mobile and migratory organisms have the capacity to move chemical contaminants in their bodies from one ecosystem to another. Global air and ocean circulatory patterns, in addition to transport by rivers, can move contaminants long distances. Thus, biomagnification of toxins occurs in organisms living in ecosystems far removed from the original sources of contamination.

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