Competition, cannibalism and intraguild predation among ground beetles (Coleoptera: Carabidae): A laboratory study

We used laboratory experiments to study the potential for predator-prey and competitive interactions between two carabid beetles, the holarctic Pterostichus adstrictus (Eschscholtz), and P. melanarius (Illiger) introduced to North America from Europe. These two species co-occur over much of northern North America and exhibit changing patterns of between-habitat distribution and abundance in western Canada. In our experiments, adult survival of P. adstrictus was ca. 35% lower in the presence of the larger-bodied P. melanarius. Because starvation of individually 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-species trials. Although beetle survival was not affected by reduced food, gravid P. melanarius lost mass when food was limited and showed potential increased egg production with increased food. Larvae of both species had high survivorship when held alone, but predation by both conspecific and heterospecific adults significantly reduced larval survival. All of these effects occurred in the presence of suitable alternative food but many were exacerbated when food levels were lowered experimentally. We conclude that intraguild predation and interspecific competition for food could reduce survival and reproductive potential, thus affecting the population dynamics of these two species in nature.