The boundary between adjacent ecosystems is known as an ecotone. For example, the intermediary zone between a grassland and a forest constitutes an ecotone that has characteristics of both ecosystems. The transition between the two ecosystems may be abrupt or, more commonly, gradual. Because of the overlap between ecosystems, an ecotone usually contains a larger variety of species than is to be found in either of the separate ecosystems and often includes species unique to the ecotone. This effect is known as the edge effect. Ecotones may be stable or variable. Over a period of time, for example, a forest may invade a grassland. Changes in precipitation are an important factor in the movement of ecotones.