i. The population of any area is not only the result of births and deaths, but can also be affected by in-migration and out-migration. The failure of the Demographic Transition Model to consider the impact of migration on population change is a serious omission.
Any examination of population dynamics at a regional or national scale-cannot ignore this very important demographic phenomenon.
ii. Recognising this deficiency in the theory, W. Zelinsky, in 1971, suggested a relationship not only between the demographic transition and economic development, but also mobility patterns.
iii. Zelinsky hypothesised that as economic development and hence the demographic transition progresses, a society changes from one, where mobility is severely limited, to one where mobility is a necessary and desirable component of society.
As this transition takes place, there will be inevitable changes in the function, frequency, duration, periodicity, distance and routing of movements.
These changes in mobility patterns are the direct result of changing economic and social conditions, population pressure, the broadening of information fields (due to the communications revolution) and associated improvements in transport technology.
iv. Zelinsky suggests that in stage one of the Demographic Transition Model, mobility is limited and dictated by local custom and movements associated with trade, religion and war.
v. As the population moves into stages two and three of the model, increased population pressure results in two major types of mobility.
First, movements of people to colonies new agricultural areas, as occurred from Europe to the USA in the last century, and secondly, a movement fr6m the countryside to cities, as happened in the UK.
These phases are characterised by agricultural and industrial innovations and are associated with massive urbanisation.
vi. By stage four, rural-to-urban mobility has slowed, but mobility between cities, or inter-urban mobility, has become significant as people move between urban areas in search of employment and opportunities.
International mobility is also important at this stage, and movements of labour as occurred from the Commonwealth countries to the UK in 1950s, is officially encouraged.
The other important type of international mobility to surface at this stage is international tourism, fuelled by increased incomes and cheap and efficient methods of transportation.
By stage five, nearly all movement is within or between cities, with a significant flow of people out of urban areas looking for a better quality of life (away from pollution of cities).