Resource Shifts in Bee Flies (Bombyliidae): Interactions among Species Determine Choice of Resources

Abstract

This study documents resource shifts over time in small bee flies (Diptera, Bombyliidae, Homeophthalmae) occurring in the California [USA] deserts that strongly implicate competition among these species. It was postulated on the basis of seasonal resource shifts in 1981 that 1 sp. in the genus, Geron, was displaced from flower species it prefers by similar bee flies in the genera Phthiria and Oligodranes. If the latter were somehow removed from Geron''s preferred resources, Geron would shift proportionately more onto these resources. Environmental conditions produced exactly this experiment naturally in 1982, when resources were superabundant relative to 1981 and the preferred resources of Phthiria and Oligodranes were abundant unusually late in the season. The predicted shifts occurred: Geron shifted, to a greater degree in 1982 than 1981, from composites with small florets and exposed inflorescences to Stephanomeria pauciflora; and a variety of less abundant but valuable species, when Phthiria and Oligodranes were less common on the latter. A method was used for exploiting more of the information available in multi-way contingency tables than has previously been used in ecological studies. Shifts indicate that the bee flies in these genera form linear competitive hierarchies very similar to those of other flower visitors.