Ecotone is a zone of transition between two adjacent communities. In ecotone the density of most of the species is higher than that in neighbouring communities. These species are called edge species and this feature of ecotone as principle of edges. The species which is found in only a single geographical area and nowhere else is said to be endemic species. The rare species have small total number of individuals often due to limited geographical ranges or low population densities, Sibling species are true species which do not interbreed but are otherwise difficult to separate on the basis of morphological characters alone.