By which process do higher concentrations of a particular chemical, such as pesticide DDT, reach organisms higher up the food chain?

The process by which higher concentrations of a particular chemical, such as pesticide DDT, reach organisms higher up the food chain is known as biomagnification. This phenomenon occurs when substances like DDT accumulate in the tissues of organisms at each trophic level. When a lower trophic level organism, such as a small fish, absorbs DDT from its environment or from consuming contaminated smaller prey, it may retain and concentrate that chemical in its body. As larger predators consume these smaller fish, the concentration of DDT increases in their tissues due to the accumulation from multiple lower trophic levels. This results in top predators, such as birds of prey or large mammals, having significantly higher concentrations of the chemical compared to the organisms at lower levels in the food web.

In contrast, bioconcentration refers to the accumulation of substances directly from the environment into an organism's body without the food chain influences, while bioaccumulation focuses on how an organism accumulates a chemical over time from all sources, including water, air, and food, but does not encompass the transfer through multiple trophic levels. Biodegradation involves the breakdown of substances by microbial action and is unrelated to the concentration of chemicals in food chains.