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In recent literature on the population dynamics of animals, there are at least two opposing opinions as to the maintenance of numbers. Some workers hold the view that there is a real regulation of numbers by feed-back mechanisms. Others are of the opinion that, in many cases at least, such mechanisms are not in operation and numbers fluctuate between relatively narrow limits due to the ever-changing influence of one or more (or a complex of) environmental factors. These concepts have been discussed at length in this Review by Solomon (123) and Nicholson (89) on the one hand, and Thompson (127) on the other. More data on this question can be found in Nicholson (88) and in Andrewartha & Birch (5), respectively. Arguments in favor of the former viewpoint can be found in recent papers of Wilbert (143, (64), in which models are given showing that the action of density-governing factors [definition in Nicholson (86)] is indispensable in explaining the numerical facts of populations, viz., constant mean density level and restricted fluctuation. 144) and KlompThe theory set forth by Nicholson (86, 88) implies that populations gen erally (exceptions are made for populations in fringe areas where the animals occur as a result of continuous immigration only), are really regulated, i.e., that increasing density, with or without a time lag, induces a higher mor tality rate or a lower reproductive rate or a higher rate of emigration, or a combination of two or three of these processesi decreasing density induces the reverse. This governing reaction operates at least over part of the density range.Field studies, laboratory experiments and models have suggested that, for the time being, we must recognize the following mechanisms of regulation:I. Regulation through interaction with organisms of another kind other than foodi such as the regulation of the density of prey-animals through the action of parasites, predators, or pathogens. [For literature see (56,123).] II. Regulation through interaction with organisms of the same kind. (A) Regulation through intraspecific competition: the organisms compete for some requisite which is in short supply. The literature is summarized in this paper. (i) The degree of depletion of the governing requisite and not its quantity produced is influenced by the competitors, e.g., competition for food among carrion feeders and fruit flies. (ii) Both the degree of depletion and the 1 The survey of the literature pertaining to this review was concluded in January 1963.2 My than ks are extended to Dr. K. Bakker of Leiden for the im provements he has proposed in the text. 17Annu. Rev. Entomol. 1964.9:17-40. Downloaded from www.annualreviews.org by University of Sussex on 10/08/12. For personal use only.Quick links to online content Further ANNUAL REVIEWS