Declining fertility levels, and sharply higher immigration rates, have altered the relative importance of the components of population growth for the Central Ontario Zone. Recall that the growth rate for any geographical area is the result of three distinct components: 1) the rate of natural increase, 2) net domestic migration and 3) net foreign immigration. Cities, historically, have seldom reproduced their own populations, and have always relied on attracting in-migrants to sustain their population base and to achieve growth.
The current combination of growth components, however, is historically unique. In the case of the greater Toronto region, immigration now accounts for 70 to 75% of total population growth annually. Fertility rates, as noted, have declined. Net domestic migration for the region, the difference between in-migrants from other parts of the country and out-migrants, has fluctuated widely depending on local economic conditions, and at times has been negative. But throughout much of the recent period, it has contributed relatively little to regional growth.
One obvious result of this shift in the relative contribution of different components of population growth is that the growth rate is more uncertain. Natural increase is relatively predictable, and internal (domestic) migration rates is relatively constant - because they are largely determined by the age structure - although the geography of domestic migration does vary. Immigration is neither constant nor uniform, and thus is not easy to predict. It is subject, for example, to the whims of federal policy and special-interest groups, and to conditions abroad. Regional population forecasting, as a result, is now more difficult than it was in the past. Moreover, even if researchers agreed on an overall rate of growth expected for the Central Ontario Zone based simply on projecting past trends - an exercise fraught with errors - this rate will tell us little about the social composition, character or geography of that growth.