In ecological theory, the carrying capacity (K) of a geographical region, with respect to a particular species, is the maximum population size that the region can support. It is assumed that the birth and death rates are density-dependent, the former declining and the latter increasing as the population size (N) increases and food per individual decreases. Then the population will reach a stable maximum, the two rates intersect, and N = K. When a species is introduced into a region, it will experience a high, unconstrained growth rate. As N approaches K, the growth rate will fall. Thus N follows an S-shaped curve that rises steeply at first and then reaches a plateau with N = K. If the trends in the birth and death rates are linear, then this curve is the logistic function, first described by Verhulst in 1845. The model assumes a closed population (no immigration or emigration), no importation of food, and no improvement in the efficiency of food production. These assumptions are restrictive in the context of nonmigratory human populations, but anthropologists have estimated the carrying capacity for isolated hunter-gatherer tribes. Even where the assumptions hold, it has been observed that many animal species, including homosapiens, restrict their fertility to
For the world population as a whole, migration is not an issue. Over the years, several authors have estimated the global carrying capacity based on maximum food production. In 1983, Bernard Gilland calculated a global carrying capacity of 7.5 billion. However, improvements in food production have permitted the world population to reach 6 billion in the year 2000 without any evidence of increase in mortality.
GERRY B. HILL
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