COMPETITIVE HIERARCHIES IN LABORATORY DROSOPHILA.

If competition within some guild were always for the same resource, and pursued in the same way, it would be a simple matter to rank species unambiguously on the basis of their competitive abilities. Competitive rank could be assigned according to how effectively any species preempted that resource in that way. As it happens, surprisingly simple systems may defy this sort of analysis. For example, F. E. Smith (unpubl.) investigated patterns of sapling distribution in a beech-maple woodland, where robust maple saplings were found predominantly under beech canopy trees, and robust beeches under maple. As both trees are well represented dominants in the climax forest it is not immediately clear how they should be ranked competitively, especially since we cannot simply say that one or the other species is superior in its primary struggle (competitive sapling growth) for a fundamental resource (space). The reason is that, contrary to what we might have first expected, the saplings perceive two fundamental resources (space under a beech canopy tree, and space under a maple) and one species is the superior competitor for one resource, while the second is superior at the other resource. That competing species may partition a resource is a classic observation (Gause, 1936), well integrated in modern ecological analysis (e.g., Schoener, 1968); what is novel in the present example is that the essence of the partitioning is entirely contingent upon the presence or absence of the respective competitor species, rather than somehow being an intrinsic subdivision of their common "environment." As the multiplicity of objects of competition of this sort can increase in proportion to the total number of competing species, one implication is that competitive ranking may be ambiguous. If species A outcompetes B and B outcompetes C, the object (or manner) of competition may be different for the two pairs, in which case we have no a priori way of predicting the outcome of, say, A competing against C. For this third pairwise interaction, the critical resource might be either of the resources which respectively determined the interaction of the other two pairs, or it might even be a third. So it may be quite consistent biologically that species A outcompetes B, B outcompetes C, and C outcompetes A. In this situation we would judge the competitive relation between the three species to be intransitive; and, in the context of a competitive ranking of the three species in some single dimension, their pairwise competitive relations are mathematically inconsistent. The degree of intransitivity of competitive relations can serve as an indirect measure of the dimensionality of the resource space the competitors perceive. The number of effective resources available to a competition community will determine the maximal number of species that can persist in a stationary state (Levin, 1970). Intransitivities may permit continued coexistence of competitors with stable oscillation of numbers (Gilpin, 1975). Thus it is of interest to know what is the extent of intransitivity in pairwise interactions between competitors. In this paper, I report the results of an attempt to assay these properties in a suite of Drosophila species in laboratory culture.