An example which has not received sufficient ttention is competition in a heterogeneous environment. As has often been pointed out (Kluijver and Tinbergen 1953, Lack 1955, Hinde 1956)birds may emigrate or disperse from the most suitable areas where reproduction is successful into marginal habitats, Consider such a species
which will be Called A. Let B be a species that lives only in the area that is marginal for species A. Now, even if in an unlimited environment of
this type, species B would eliminate species A by competition, in the heterogeneous environment species B may be eliminated from its own preferred
habitat. For. if there is sufficient dispersal by
species A, it may maintain, partly by immigration,
such a high population in the marginal habitat that
species B is forced to decrease. This process is
probably very important in considering the en
vironmental distributions of birds and implies that'
small areas of habitat typical for one species may
not contain that species.