Biological magnification (also known as biomagnification) is the process by which toxins such as pesticides build up in each successive link in the food chain.
For instance, a given population of beetles may have very low levels of a fat-soluble pesticide, but the pesticide will build to much greater levels in the fat of a bird that eats those beetles, and the pesticide will reach greater levels still in a human or panther that eats the beetle-eating birds.
This writeup is based on work I did for the science dictionary at http://biotech.icmb.utexas.edu/