Can Dominance Influence Stability in Competitive Interactions?

Abstract

Competition and specialization are apparent processes in natural communities (Connell 1983, Schoener 1983). Levins (1968) and MacArthur (1972) suggested that species may coexist by specializing as much as the environmental heterogeneity allows, thereby avoiding competitive displacement. Theoretical (May 1972, Siljak 1974, McMurtrie 1975) and field (McNaughton 1978, Rejmanek and Stary 1979, Yodzis 1980) studies indicate that, as the number of species increases, natural communities tend to be organized into functional subunits or 'guilds' of species, with most interactions taking place within guilds (May 1981). A guild has been defined as a group of species that exploit the same class of environmental resources in a similar way (Root 1967), the interspecific competition being high and acting as an agent of guild structure. Although no model yet shows the extent to which niche overlap in biological guilds hastens a species' extinction, theory (MacArthur and Wilson 1967, Tilman 1982) and experiments (Rothhaupt 1988, Bengtsson 1989) indicate that the coexistence of competing species may be prolonged by reducing competitive overlap. Furthermore, the coexistence may be prolonged if the resource requirements of competing species are not limited by the availability of environmental resources (Tilman 1982, Arthur 1987). Nevertheless, dominance, another apparent fact in natural communities (Preston 1948, Ricklefs 1973, Sugihara 1980), has been seldom related to species coexistence in interspecific competitive interactions. McNaughton and Wolf (1970) claimed that dominance is the appropriation of potential niche space of certain subordinate species by other dominant species and so can be manifested most clearly only within a trophic level. In this paper, I shall explore this fact. In the absence of environmental perturbations, I assume that the coexistence of competing species in bilogical guilds is a function of species dominance and resource limitation, as major causes of interaction strength between competitors, and the niche overlap as a major cause of connectance between competitors. The species dominance tends to increase with the number of subordinate species because of the effect of one competitor's abundance on the abundance of another competitor (Camargo unpubl.) A competing species i may be considered as dominant if its relative abundance (pi) is higher than (itS) and as subordinate if pi < (1/S). According to this personal perspective, the average dominance per species (d') is a logical measure of species dominance in interspecific competition, d' being calculated as: