SINGLE FACTOR MORTALITY, THE ESSENCE OF BIOLOGICAL CONTROL, AND ITS VALIDATION, IN THE FIELD

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Abstract
A shift in the average number of an organism from an economic to a non-economic density, the goal of biological control, is usually brought about by the addition to the ecosystem of exotic "key" factors whose lethal action is more efficient and broader in scope than that of the mortality factors already operating.Such a shift in the homeostasis of a pest population can occur only when its protection from the lethal action of key factors is fortuitous. With fortuitous protection (in contrast to absolute protection) host-searching by parasite or predator populations is so unrestricted by physical barriers that no portion of the target population is continuously immune to attack.In many instances after the attainment of low-density homeostasis the controlling action of the added key factor does not appear to be either supplemental to or superimposed on the total pest mortality that had prevailed in the ecosystem but largely a substitution for that mortality through its displacement and greatly amplified replacement. Means for verifying the responsible key factor in a given locality following the importation and establishment of a possible key factor and observations on hierarchies of such factors are reviewed.