INTRASPECIFIC AND INTERSPECIFIC COMPETITION IN ADULTS OF 2 ABUNDANT GRASSHOPPERS (ORTHOPTERA, ACRIDIDAE) FROM A SANDHILLS GRASSLAND

Recognizing the existence and importance of intra- and interspecific competition in natural populations of insect herbivores remains problematical. We performed field-cage experiments to detect the presence of both intra- and interspecific competition among adults of two common, grass-feeding grasshoppers, Ageneotettix deorum (Scudder) and Amphitornus coloradus (Thomas). Based on increased mortality and lowered fecundity at higher densities, strong evidence for intraspecific competition exists for both species. Interspecific competition was weaker than intraspecific competition in this study. When interspecific competition existed, it was asymmetric in its effect, with the larger species (A. coloradus) negatively impacting the smaller one (A. deorum). Although there was little evidence for interspecific effects on mortality, egg production was impacted in a size-dependent, asymmetric manner between the two species. Significant density-dependent depression of available food was observed for A. coloradus, the larger species. Experiments were run at high densities (10-30 individuals per square meter), levels only irregularly reached under natural conditions (either in sporadic patches or overall about every 10-12 yr). Because naturally occurring densities are typically below the competitive threshold, interspecific competition between these two species may occur only intermittently. Other forces may greatly impact adult mortality at this site, including predation.