Population Stabilization and Competition Between the Ants Lasius flavus (F.) and L. niger (L.)

It is not possible at present to predict from observations on individual behaviour the effects produced on the population densities, but it is hoped that such predictions will be possible in the future when more examples of population interaction have been demonstrated and analysed in terms of individual behaviour.

Abstract

The term interspecific competition has been used with various meanings (Birch 1957) and this necessitates a brief review to clarify the terminology employed in this paper. The principal source of confusion is failure to discriminate between observations on individual behaviour and the effects of one population upon another. Park (1954) and Brian (1956a) have used the terms 'exploitation' and 'interference' to describe the two types of interaction at the individual level and their use in place of 'competition', where appropriate, would result in much greater clarity. Exploitation occurs where two species have a resource in common and interference is simply damage (excluding predation and parasitism) to the individuals of one species by individuals of another. It is not possible at present to predict from observations on individual behaviour the effects produced on the population densities, but it is hoped that such predictions will be possible in the future when more examples of population interaction have been demonstrated and analysed in terms of individual behaviour. The definition of interspecific competition used here is: interspecific competition is occurring if lower densities of the competitors are found than would be the case if each were present alone. Odum (1953, p. 166) uses the term in this sense, and 'amensalism' for an interaction which lowers the density of only one of a pair of interacting species. It is possible that each of two species having a predator or parasite in common would show a sustained increase in density if the other were experimentally removed. Competition would not have its conventional meaning if applied to this case, but it is convenient to have one term to cover all cases of the same type of population interaction. It is also necessary to have a term for any factors which can be demonstrated to counteract changes in population density caused by irregular environmental changes, they are called stabilizing factors here. Andrewartha & Birch (1954) claim that such factors are not important, since animal populations are unstable and undergo repeated local extinction, but this is obviously not the case with many observed populations including those in the present study. Williamson (1957) has defined interspecific competition as the result of two or more species sharing a 'controlling' (stabilizing) factor, but this is a less practical definition since one does not usually know what is stabilizing populations, and one cannot conclude that two species were sharing a stabilizing factor if an increase in density of either is produced by removal of the other. Crombie (1947) and Williamson (1957) have considered the possibility of stable interspecific competition. It has been stated that one competitor always replaces the other (e.g. Brian 1956a, p. 341, and Park 1954, p. 223), but this is not the case, and only follows from simple mathematical models based on false premises or in some simple laboratory experiments. The terminology is straightforward. Processes are not given different names when they do not proceed to completion, for example chemical reactions and genetic polymorphism. Polymorphism is more than an analogous phenomenon, since at least one possible