An indicator species is an organism—bacteria, plant, or animal—that reflects the condition of the environment around it. They’re often the first in their ecosystem to be affected by a particular environmental change, such as a warming climate, pollution, human development, and other environmental degradation. By monitoring changes in the behavior, physiology, or number of an indicator species, scientists can monitor the health of its whole environment.

Crayfish, for example, can indicate the quality of freshwater, because changes in water acidity are stressful to them. The health of corals can indicate trends such as seawater rise and sea temperature fluctuation, which in turn are signals of climate change. Peregrine falcons are an indicator of pesticides; DDT, for example, causes their