Sometimes, there occurs a change in population size or the structure of a community which occurs between the boundaries of two habitats. This phenomenon is known as the edge effect. It is observed that sometimes the population density in some species in ecotone is larger than any community and this is defined as the edge effect. The edge species are the species that occur abundantly or primarily in the edge zone. For example, in the terrestrial ecosystem, the birds show edge effects and their population density is larger in the ecotone which is the area between the desert and forest.