Biological Magnification Description
Biological magnification, or biomagnification, refers to the accumulation of toxins in the higher trophic levels of a food web. The toxins enter the food web at lower trophic levels accumulating in greater concentrations as the pollutant moves up the food web. Through biological magnification, toxins in low concentrations in the environment may accumulate to detrimental levels in top predators.
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First Stage of Biological Magnification
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Biological magnification begins with a pollutant entering the food web, often as part of a producer taking up inorganic nutrients. Producers form the basis of ecosystem food webs, incorporating nutrients and energy from the surrounding environment into their bodies. The indiscriminate uptake of pollutants with nutrients by the producer brings the pollutant into the food web. These pollutants are then stored along with the nutrients within the producer at levels higher than those present in the surrounding environment.
Second Stage of Biological Magnification
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The next stage of biological magnification occurs when this producer is eaten by a primary consumer. Primary consumers provide an important link between the energy and nutrients stored within the cells and tissues of producers and other organisms within the ecosystem. Each primary consumer must ingest a large amount of biomass from the producer to sustain itself. The pollutants in the producers are also consumed and accumulate in the tissues of the primary consumer in even greater concentrations.
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Subsequent Stages of Biological Magnification
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Secondary and tertiary consumers that feed upon the primary consumers accumulate even greater amounts of the pollutant in their tissues. The further an organism's trophic level is from the initial input of the pollutant, the greater the concentration of the pollutant in each meal. Since the pollutants accumulate over the life of the organism, long-lived organisms also tend to accumulate more toxins than short-lived organisms. A pollutant that is at benign concentration levels in the producers can quickly reach detrimental levels as it is increasingly magnified up each level of consumption. The highest concentration levels are in long-lived, top-level predators.
Pollutants
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Pollutants of concern share several traits important for biological magnification. The first is the ease with which they enter the food web. Many are taken in by primary producers when they ingest inorganic nutrients needed for life though they may also enter the food web at higher trophic levels. A second trait these pollutants share is a relatively long life in the environment and in living tissues. If the pollutants are rapidly excreted by organisms or degrade rapidly in the environment, little risk exists of their accumulation at dangerous levels. A third trait of these pollutants is their detrimental effects on living organisms. While not all chemicals that play a role in biological magnification are dangerous, those of interest to public health and the environment are toxic to some degree.
Biological Magnification of Mercury
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Mercury and other heavy metals are an ongoing concern for public health agencies throughout the United States due to biological magnification. Florida's Department of Health, for example, has issued fish consumption advisories for many freshwater and marine fish within the state, including shark, largemouth bass, cobia, yellowfin tuna, snook and bluegill. These advisories are specific to the size and location of the fish and give the frequency with which persons may dine on the species from each locale.
Other Examples of Biological Magnification
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Published in 1962, Rachel Carson's book "Silent Spring" described the effects of DDT, a pesticide, on birds. A later study found the concentration of pesticides in porpoises to be 800,000 times greater than the surrounding water. The ecological consequences of DDT and similar pesticides have led to their ban in the United States and other countries. PCBs used in electrical components, plastics and insulation have likewise been banned in the United States due to their persistence in the environment and accumulation in living organisms. While DDT is not known to have a direct adverse affect on humans, PCBs are associated with potentially severe effects in humans.
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References
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- Photo Credit Bald Eagle image by Penny Williams from Fotolia.com
