Ecology is a branch of science that studies the ways in which plants and animals interact with one another and with their surroundings. Ernst Haeckel, a German zoologist, invented the word "ecology" in 1869. It comes from the Greek words oikos, which means "household," and logos, which means "discourse" or "study." In The Riddle of the Universe, Haeckel applied the term oekologie to the "relation of the animal both to its organic as well as its inorganic environment."

For many years, ecology was an obscure branch of biology. In the late twentieth century, however, as environmentalism became a popular movement, ecology moved to the forefront of public opinion and also rose to prominence as a discipline. Some of the late twentieth and early twenty-first centuries' thorniest problems—expanding populations, food scarcity, and environmental pollution—were and are essentially problems of ecology.

Ecologists study organisms in various kinds of environments...