The study of the hazardous effects of substances on the aquatic and terrestrial environment is known as Ecotoxicology. Ecotoxicologists research the direct impacts of toxic substances on individual organisms and species in food webs, with the goal of forecasting consequences on animal populations, ecosystems, and human food supplies like fish and shellfish. Ecotoxicologists conduct ecotoxicity studies and risk assessments on new chemicals that may be used, disposed of, or otherwise reach the environment in order to better understand (and, ideally, forecast and prevent) unwanted outcomes in the natural environment. They are frequently involved in conducting in-depth monitoring studies of invertebrates and fish in polluted rivers and estuaries, examining species at multiple stages of the food chain. It may also be required to monitor organisms' physiological and biochemical responses after exposure to a pollutant, as these responses may indicate a hazardous effect. Sub-lethal impacts such as changes in behaviour, development, or reproduction may be just as critical for a species' survival as deadly consequences in many circumstances.