An indicator species is an organism that serves as a reflection of the surrounding environment, and can range from bacteria to complex organisms like animals. These indicator species can act as an early warning system for changes in the environment. In North America, examples of indicator species include amphibians like frogs and toads, and birds like spotted owls, red-cockaded woodpeckers, and marbled murrelets. Additionally, green frogs and gray tree frogs are used as indicators of environmental conditions in parks. Other examples of indicator species include lichen, mayflies, salmon, marsh periwinkles, E. coli, river otters, salamanders, and various organisms that rely on old-growth forests.