Ecological isolation is based on the fact that population shows preference to one habitat over the other. This extensive forests become barriers to the dispersal of organims living in grasslands. The reverse that priaries being barriers to forest organisms is also true. The red tree mousephenocomys 1ongicaudu.s lives on fir needle trees and feeds on fir needle. It is understandable that for these mice not only priaries but the non-fir forests will also be a barrier for dispersal.

You may recall that in unit of this block we discussed extensively MacArthur's study of five species of warbler birds living in spruce trees. The five species of Dendroica are effectively isolated by ecological factors although they have a similar distribution. As a matter of fact, the ecological and food preference of the different species of warblers are very similar. Under such circumstances one would have expected a severe competition among them. But each species has carved a well defined niche and forages at a particular level in the trees. Differences in breeding dates and occupation of different habitats outside the breeding season are the contributing factors for the minimum competition and niche overlap among the species. There are other instances where potential mates keep away from each other because of their habitat preferences. In these cases there might even be a broad niche overlap and the individuals may exist in the same general area, but the distinctness of sub-species and species is maintained. Dice cited an example from United States where the two sub-species of mice Peromyscus maniculatus have overlapping niches and do not interbreed in nature, although there is interbreeding in the laboratory. Similarly another.study showed that the fresh water and salt water races of the water snake Natrix sipedon may come close together but may not interbreed because of their habitat preferences.