Competitive ability of neotropical Drosophila predicted from larval development times

Interspecific competition is a major aspect of the ecology of insect communities exploiting ephemeral and fragmented resources. To generate testable predictions on the role of competition in such communities, information is required about the competitive abilities of the species involved. In this study we test whether larval development time, an important life-history trait, can be used to predict interspecific competitive ability in neotropical Drosophila species. Based on earlier work, we predicted a negative relationship between larval development time and competitive ability. Pairwise competition experiments were conducted with seven Drosophila species (in all possible combinations) that are known to coexist in central Panama and represent a wide range of larval development times. We used a novel approach to determine experimental densities, standardising total larval food requirements, and thereby increasing the resolution for detecting general traits involved in exploitation competition. The effects of competition were measured by comparing values for three fitness-related parameters (thorax length, larval survival, and total mass of emerged adults) against their values at two levels of intraspecific competition. The outcomes of the pairwise competition were significantly explained by the differences in larval development times: within species pairs, species with a longer development suffered more from interspecific competition than those with shorter development. It is argued that larval development time is a major determinant of competitive rank order among drosophilid species, allowing testable community-wide predictions. In addition, this relationship provides a basis for studying the role of life-history trade-offs in community-level processes. Given the generality of the Drosophila biology, development time is expected to play a similar role in other insect communities exploiting ephemeral and fragmented resources.