定義： A plain without trees (apart from those near rivers and lakes); it is similar to a prairie, although a prairie is generally considered as being dominated by tall grasses, while short grasses are said to be normal in the steppe. It may be semi-desert, or covered with grass or shrubs or both, depending on the season and latitude.
定義： An organism that is dependent at least for part of its life cycle on dead or decaying wood or on other organisms that are themselves dependent on this resource.
定義： A group of species that exploit the same food resources, and/or use the same feeding or foraging methods. http://en.wikipedia.org/wiki/Guild_(ecology)
