The occurrence of relatively definite sequence of communities over a period of time in the same area is known as ecological succession. Lithosere is a type of xerosere originating on bare rock surfaces. The original substratum is deficient in water and lacks any organic matter having only minerals in disintegrated unweathered state. The pioneer vetegation is, therefore, lichens. Hydrosere originating in a pond starts with the colonization of some phytoplanktons which form the pioneer plant community.