COMPETITION, CANNIBALISM AND INTRAGUILD PREDATION AMONG GROUND BEETLES (COLEOPTERA, CARABIDAE) - A LABORATORY STUDY

We used laboratory experiments to study the potential for predator-pre y and competitive interactions between two carabid beetles, the holarc tic Pterostichus adstrictus (Eschscholtz), and P. melanarius (Illiger) introduced to North America from Europe. These two species co-occur o ver much of northern North America and exhibit changing patterns of be tween-habitat distribution and abundance in western Canada. In our exp eriments, adult survival of P. adstrictus was ca. 35% lower in the pre sence of the larger-bodied P. melanarius. Because starvation of indivi dually isolated beetles did not elevate mortality for either species, we conclude that predation, and not interspecific competition for food , was responsible for reduced survival of P. adstrictus in the mixed-s pecies trials. Although beetle survival was not affected by reduced fo od, gravid P. melanarius lost mass when food was limited and showed po tential increased egg production with increased food. Larvae of both s pecies had high survivorship when held alone, but predation by both co nspecific and heterospecific adults significantly reduced larval survi val. All of these effects occurred in the presence of suitable alterna tive food but many were exacerbated when food levels were lowered expe rimentally. We conclude that intraguild predation and interspecific co mpetition for food could reduce survival and reproductive potential, t hus affecting the population dynamics of these two species in nature.