Competition and Coexistence with A Guild of Herbivorous Insects

Resource exploitation and competition were measured for 13 herbivorous stem—boring insects inhabiting a climax prairie in Illinois. Nine species overlapped > 70% in their exploitation of plant species, stem sizes, and locations within stems, but competition was detected between only two species. In this case the competition resulted from aggression rather than exploitation for limited resources, and exclusion may be delayed or prevented by specific behaviors or refuges. To quantify how closely the guild members were packed upon the available resources, probabilities of coccurrences between species were calculated using resource abundances, population sizes, and overlaps in resource exploitation. The species packing averaged over the entire guild was 30%, but some species had several potential competitors and probabilities of cooccurrences were > 60%. This species structure offers the conditions for diffuse competition, but competition seldom occurred because stems could support several individuals and resources were generally nonlimiting. Although the potential for competition is high and species probably compete occasionally, the high resource overlap and species richness that competition has not been a major organizing force within this guild. When factors known to influence competition are considered, it seems possible that competition has been too infrequent, mild, or nondirectional to cause strong selection for competitive avoidance or displacement. This guild demonstrates that overlap values may not equal competition coefficients and that high overlap may exist because competition is rare. This guild is probably not unique in lacking competitive organization; herbivorous insects may be among the least likely groups to exhibit the patterns predicted by competition theory.