Biological magnification (or biomagnification) is the buildup of certain substances, such as DDT, in the bodies of organisms at higher trophic levels of food webs. DDT was once used extensively as an insecticide. DDT builds up in the fatty tissues of organisms. Organisms at lower trophic levels accumulate small amounts. Organisms at the next higher level eat many of these lower-level organisms and hence accumulate larger amounts. At the highest trophic levels the increased concentrations in tissues may become toxic.