According to the Millennium Ecosystem Assessment, "An ecosystem is a dynamic complex of plant, animal, and microorganism communities and the nonliving environment interacting as a functional unit" (Millennium Ecosystem Assessment, 2005). Ecosystems cover a range of scales and characteristics, from the global to smaller habitats. In addition, they span "from those relatively undisturbed, such as natural forests, to landscapes with mixed patterns of human use, to ecosystems intensively managed and modified by humans, such as agricultural land and urban areas" (Millennium Ecosystem Assessment 2005).