Ecology, a term coined by the German biologist Ernst Haeckel in 1869, studies the environment's dependence and influence on living organisms. Sometimes, ecology refers to how stable the interactions are between living and nonliving things. For instance, ecologists might study how water pollution can disrupt the interactions of a stream's fish and plant life. Ecology is a branch of biology that's divided into two fields: animal ecology and plant ecology. For an ecologist, the environment is made up of living and nonliving, natural elements, such as chemicals, soil, temperature, moisture and light. Man-made structures and materials, such as machines, buildings, medicine, fertilizers and highways, also are part of the environment.