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  The reciprocal effect in a system, whereby a change in one variable (A) influences change in others (B and C), which in turn influence change in A. The feedback influences may be either negative or positive.

With negative feedback the system\'s equilibrium is maintained: in an ecosystem, for example, an increase in species A\'s abundance may generate an increase in the number of predators (B) which feed on it; as a consequence, the availability of A is reduced, the number of B predators that can be sustained falls, and the system returns to its former state. Such a system is said to be morphostatic, and in a condition of dynamic equilibrium.

With positive feedback an increase in A stimulates an increase in B, which in turn stimulates a further increase in A, as in the multiplier process central to input-output models of economic growth: such a system is termed morphogenetic.

The example in the figure shows a system with both morphogenetic and morphostatic feedback loops. The left-hand loop (P-B-G-D) is morphostatic: population growth in a city induces disease through the greater volume and density of waste matter, which subsequently limits population growth. The right-hand loop (P-M-C-P) is morphogenetic: a growing city is modernized, attracts more inmigrants, and so grows even further. The two loops are linked via variable S: modernization allows greater control over disease and the negative checks are reduced — thereby making the morphogenetic loop dominant and advancing the rate of population growth. (RJJ)

{img src=show_image.php?name=bkhumgeofig23.gif }

feedback Urban population charge (Langton, 1972)

Reference Langton, J. 1972: Potentialities and problems of adopting a systems approach to the study of change in human geography. In C. Board et al., eds, Progress in geography, volume 4. London: Edward Arnold, 125-79.



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