Biomagnification is the increase in concentration of a chemical or substance in a population as trophic levels increase. This occurs when chemicals are persistent and cannot be broken down easily in the environment. An example of biomagnification is the increase in concentrations of toxic pesticide, DDT, as it moves up the food chain. DDT enters freshwater through agricultural run-off. It gets taken up by small organisms in the water and bioaccumulates as the small fish eat the plankton, and then as the larger fish eat the smaller fish, and then again as birds eat the larger fish . Concentrations of DDT ultimately become much more highly concentrated in birds than in fish or plankton.