Final answer:
Higher trophic levels have higher concentrations of toxins due to the process of biomagnification, which results in the increasing concentration of toxic substances in organisms as one moves up the food chain.
Step-by-step explanation:
Higher trophic levels often have higher concentrations of toxins due to the process of biomagnification. Biomagnification is the increasing concentration of persistent, toxic substances in organisms at each successive trophic level. This occurs because these substances are fat soluble, not water soluble, and are stored in the fat reserves of each organism. As a result, apex predators often exhibit greatly increased levels of toxins in comparison to organisms lower in the food chain. Classical studies, such as those with the pesticide DDT or substances like PCBs and heavy metals, exemplify the biomagnification process in aquatic ecosystems and its impact on wildlife and human health.