Biomagnification

Organisms are exposed to a myriad of chemicals in their environment. Some of these chemicals occur in trace concentrations in the environment, and yet they may be selectively accumulated by organisms to much larger concentrations that can cause toxicity. This tendency is referred to as biomagnification, or bioaccumulation.

Some of the biomagnified chemicals are elements such as selenium, mercury, or nickel, or organic compounds of these such as methylmercury. Diverse others are in the class of chemicals known as chlorinated hydrocarbons (or organochlorines). These are extremely insoluble in water, but are freely soluble in organic solvents, including animal fats and plant oils (these are collectively known as lipids). Many of the chlorinated hydrocarbons are also very persistent in the environment, because they are not easily broken down to simpler chemicals through the metabolism of microorganisms, or by ultraviolet radiation or other inorganic processes. Common examples of bioaccumulating chlorinated hydrocarbons are the insecticides DDT and dieldrin, and a class of industrial chemicals known as PCBs.

Food-web accumulation is a special case of biomagnification, in which certain chemicals occur in their largest ecological concentration in predators at the top of the food web. An ecological food web is a complex of species that are linked through their trophic interactions, that is, their feeding relationships. In terms of energy flow, food webs are supported by inputs of solar energy, which is fixed by green plants through photosynthesis. Some of this fixed energy is used by the plants in their own respiration, and the rest, as plant biomass, is available to be passed along to animals, which are incapable of metabolizing any other type of energy. Within the food web, animals that eat plants are known as herbivores. These are eaten by first-level carnivores, which in turn may be eaten by higher-level carnivores. Top predators occur at the summit of the food web. In general, food webs have a pyramidal structure, with plant productivity being much greater than that of herbivores, and these being more productive than their predators. Top predators are usually quite uncommon. Within food webs, biomagnifying chemicals such as DDT, dieldrin, and PCBs have their largest concentrations, and cause the greatest damage, in top predators.