Larval Competition in Drosophila Melanogaster and D. Simulans

Abstract

The Gause principle of interspecies states that if 2 species are forced to coexist in an undiversified environment, one inevitably becomes extinct, and if 2 species do coexist they must occupy different ecological niches. An extensive literature has been written on the development of this principle and the confusion that has arisen from the problem of competitive exclusion and identical niches (cf. Elton and Miller 1954; Andrewartha and Birch 1954; Hutchinson 1957). Gilbert et al. (1952) and Hardin (1960) have discussed the circularity that is usually involved in attempts to prove or disprove the Gause principle. If one species eliminates another we say the principle is proved, but if they coexist we conclude that they differ ecologically and occupy different niches. Any meaningful concept of species or ecological niches recognizes that no 2 species are genetically or ecologically identical, that the axiom of inequality provides automatic confirmation of the principle as it is stated. To this extent the principle can neither be proved nor disproved (Hardin 1960).The Gause principle is useful only when it can be applied to a conceptual model which can be tested with empirical data. When we find that a prediction is not verifiable we modify the model, but there is no procedural rule to tell us which element in the model we should abandon or change. Elton and Miller (1954) attempted to resolve certain of the descriptive problems in the niche concept by considering arenas of potential between species. Hutchinson (1957) has more recently used set theory to provide a more precise definition of an ecological niche and the conditions of interspecies competition. According to Hutchinson's formulation, if we consider the array of environmental variables relative to a species S1, we can define an n-dimensional hypervolume which corresponds to the state of environment which would permit species S1 to exist indefinitely. For any species Si this hypervolume N1 is its niche. If N1 and N2 are 2 fundamental niches, they may have points in common which are said to intersect. Thus, N1 . N2 is the subset of points common to both and is referred to as the subset. An intersection subset, according to H-utchinson's terminology, corresponds to what Elton and Miller (1954) referred to as an arena of competition in the overlap of 2 niches. Identity of 2 fundamental niches would imply that N1 N2, or every point of N1 is a member of N2 and every point of N2 is a member of N1. For the reasons mentioned above with regard to the axiom of inequality, this condition is, as Hutchinson (1957) states, so unlikely that the case is of no empirical interest. We frequently find among closely related species, however, that the intersection subset of fundamental niches provides a wide range of overlap in ecological requirements. The data provided by Park (1954, 1955) for interactions between the flour beetles Tribolium confusum and T. castaneum are an extremely well-documented example. A detailed analysis of these data by Neyman, Park, and Scott (1958) has shown that the outcome of between these 2 species is predictable in certain environments and indeterminate in others. In other words, within the intersection subset for confusun and castaneum there are points at which the ecological requirements and increase potentials of the 2 species are similar that the outcome of is a statistical probability rather than a predictable certainty. The genus Drosophila is a particularly good source of material for the study of among closely related species. The genus contains more than 700 recognized species and is well represented in all zoogeographical regions of the world (Patterson and Stone 1952) ; investigations of the genetics and ecology of Drosophila. have shown that remarkable similarity exists among various sibling species which coexist in nature, in apparent contradiction to the Gause principle. Merrell (1951) studied in cage populations of Drosophila funebris and D, melanogaster which coexisted in the laboratory for almost 2 years. Analysis showed that a fresh yeast medium was more suitable for melanogaster whereas funebris was able to maintain itself in older food, that periodic renewal of the food and medium induced environmental fluctuations which alternately favored one species and then the other. Moore (1952) obtained somewhat similar results with cage populations of the more closely related species Drosophila melanogaster and D. simulans. He concluded that melanogaster is superior to simulans in at 25?C and simulans