The phrase ecology is used often today and has a wide meaning. Ecology is referred to as a science, ecology as nature, ecology as an idea and ecology as a movement. In this chapter, ecology is used to refer to ecology as a science that studies the mutual interactions between organisms and their environment. For practical reasons, ecology is typically divided into plant ecology, animal ecology and human ecology. One of the first to describe the significance of ecological observations was renowned natural historian Charles Darwin, in his book On the Origin of the Species