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The regulation of numbers in Tribolium confusum by means of selective migration Carl, Ernest A


A lengthy, and at times heated, debate on the regulation of animal numbers has proceeded in the literature for over half a century. A wide variety of causal agents - from sun spots to a shortage of trace elements - has been proposed to explain the observed densities, and a wide variety of mechanisms has also been proposed - from natural selection to chance. Only occasionally have any of these proposals been rigorously tested, and the survival of partially or totally conflicting hypotheses has been correspondingly high. I have attempted to test just one of these proposals: the polymorphism hypothesis of Chitty (Proc. Ecol. Soc. Australia 2:51-78, 1967.) According to this hypothesis, any natural population which lives in a highly favourable habitat can regulate its numbers through the action of two morphs, one characterized by a high fecundity and the other by a superior ability to hold its position in the environment. Population density is postulated to be a function of the relative frequency of the morphs and to change in a predictable way. I have conducted five experiments to investigate the existence of, and the mode of interaction between, these presumed morphs in Tribolium confusum, the questions asked being: 1) Is the density achieved by open populations (i.e. those from which emigration is allowed) different from that in closed populations (i.e. those from which emigration is prevented)? 2) Is the mechanism of regulation in open populations different from that in closed populations? 3) Is the tendency to migrate a constant property of individuals? 4) Is the density achieved by populations founded by migrants different from that of populations founded by non-migrants? 5) Is the density achieved by open populations (with migration by self-selection) different from that achieved by closed populations from which an equal number of animals are removed at random? I found the answers to all these questions to be 'yes', and the differences in each case to be in the direction predicted by the polymorphism hypothesis. I suggest that the polymorphism hypothesis is useful for predicting future densities of populations from which emigration is occurring, but is not useful for predicting densities in populations 1) from which animals cannot escape or 2) in which mortality caused by extrinsic factors is so great that, despite high fecundity, the populations are unable to produce a migrating surplus. I argue that 1) and 2) are rare in nature, or at least have been studied rather seldom.

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