Migration - Population Dispersal
Dispersal is affected by the presence or absence of the barrier and vagility which means inherent power of movement also called dispersal powers. Many plants and lesser form of animals have greater dispersal power, although we all know that birds and insects are much known for their ability to move around.
Migration is a special type of population dispersal process often involving the mass movement of entire population. It occurs only in motile organisms and is best developed in arthropods and vertebrates like certain fishes, birds and mammals. Due to seasonal and diurnal migrations the organisms are able to occupy the regions which would be otherwise inhabitable during unfavourable conditions. In most cases migration of population may occur for food, shelter or reproduction and various ecological hazards such as temperature fluctuation, predation etc. Non-migratory populations in such unfavourable periods assume some form of dormancy or undergo considerable reduction in number, for example, frogs hibernate during winter season.