Plant-eating organisms, known as primary consumers , are the next link in the food chain. Decomposers such as bacteria and fungi make up the final link in the food chain. They break down dead tissues and cells, providing nutrients for a new generation of producers. A toxic substance is one that can cause harm to the environment or human life. Most are synthetic and include PCBs, pesticides, dioxins, and furans.
Some toxic substances can enter a food web and be transferred through it.
The uptake of any environmental substance by an organism is called bioconcentration . As contaminated organisms are eaten by others, the toxic substances are transferred up the levels in the food web and become more concentrated, sometimes to harmful levels. This process is called biomagnification . The species at the top level of the food web, including humans, are often subjected to higher concentrations of toxic substances than those at the bottom. Toxic substances reaching harmful levels is one sign that the aquatic ecosystem is unhealthy.