Final answer:
The increased concentration of toxins as they move up through the food chain is called biomagnification. This process can be observed with substances like DDT and PCBs, where higher trophic levels have higher concentrations of these toxins.
Step-by-step explanation:
The increased concentration of toxins as they move up through the food chain is called biomagnification. Biomagnification is the process where persistent, toxic substances accumulate in organisms at each trophic level, from primary producers to apex consumers. This phenomenon has been observed with substances like the pesticide DDT and polychlorinated biphenyls (PCBs), where higher trophic levels have higher concentrations of these toxins. For example, apex consumers like birds or fish may have significantly higher concentrations of toxins compared to primary producers like phytoplankton.