Ecotones are the transition zones between two distinct communities or ecosystems. These edge habitats typically have a greater diversity of species than the neighboring communities. Many species of plants and animals require a heterogeneous mixture of habitat, and thus flourish in these areas. This is known by ecologists as the "edge effect." The species diversity tends to increase with greater distinction between the two communities. For example, ecologists often find a higher number of species in the ecotone between a mature forest and the adjoining grassland than between two types of forests. Wildlife biologists recognize the importance of ecotones and often promote these edge habitats through selective cutting or prescribed burning. For example, ruffed grouse require both dense and open forests as well as areas with low-growing herbaceous plants. Other examples of ecotones include riparian habitats along stream banks, lake shores, marsh edges, ocean beaches, and forest meadows. All of these areas are subject to the negative impact of human intrusion such as dams on rivers, marshes drained for housing developments, roads along the edges of forests, and oil spills along coast lines.