Biological magnification is the resulting situation when the organisms at the bottom of the food chain exceed the chemical concentration in their body compared to their surrounding soil or water. As a result, other organisms who prey on these chemical concentrated organisms get contaminated, and so forth. The concentration of the chemical spreads throughout the food web, and the end result is a food web poisoning from chemicals, such as DDT.
http://waynesword.palomar.edu/lmexer10.htm
"A simplified illustration of biological magnification. The red dots represent units of DDT. Starting with a low concentration in the water, the DDT concentration (density of red dots) increases within the bodies of each successive trophic level, ultimately reaching a lethal concentration in the Grebes."
http://gillchair.lamar.edu/Environmental/25.htm
The picture above shows how DDT can spread throughout a food chain, causing all of the animals to become contaminated.
