定義： The largest scale biogeographic division of the earth's surface based on the historic and evolutionary distribution patterns of plants and animals.
定義： Gravel is an environmental material which is composed of pieces of rock that are at least two millimeters (2mm) in its largest dimension and no more than 75 millimeters.
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)
