Ecology is the scientific study of the interactions between organisms and their environment. It was coined in 1866 by German biologist Ernst Haeckel and comes from the Greek words "oikos" meaning house and "logos" meaning study. There are six levels of ecological organization: individual, population, community, ecosystem, biome, and biosphere. Ecology has many branches including plant ecology, animal ecology, human ecology, applied ecology, medical ecology, ecophysiology, functional ecology, and fire ecology. Experts in ecology are called ecologists.