Biomagnification

All ecosystems are built from food chains, in which producers and consumers occupy successive trophic levels connected by the flow of energy through eating and being eaten. Normally, food and biomolecules that organisms take in are used, stored, and eventually broken down (metabolized) as part of everyday physiology, and since all naturally occurring substances are degradable, this breakdown is a routine and harmless part of any food chain. The problem arises specifically with non-degradable substances: when these enter a food chain, they cannot be metabolized, broken down, or excreted by the organisms that take them in, so instead of disappearing they get passed up to the next trophic level nearly intact. Because each predator eats many prey organisms over its lifetime, the pollutant's concentration increases at every step up the chain -- this stepwise increase in concentration up successive trophic levels is called biomagnification, and it results in increasing toxicity that can eventually become lethal at the top of the chain even though the original environmental concentration was low. This phenomenon is well established for mercury and for the pesticide DDT. In a classic aquatic food chain -- water, to producer (phytoplankton), to zooplankton, to small fish, to large fish, to fish-eating birds -- the concentration of DDT is magnified at each successiv …