Establishment and interactions of carabid populations: an experiment with native and introduced species

The ground beetle Pterostichus melanarius (Coleoptera, Carabidae), of European origin, is abundant and wide-spread in the northern United States and southern Canada Three field enclosures were used in a four-year experiment in Alberta, Canada, to examine whether the species is able to establish in natural aspen-poplar forest, and to test the hypothesis that it has an adverse effect on the native carabid assemblage After introduction to the enclosures P melanarius maintained breeding populations in them, establishing that the forest is suitable habitat for the species, and suggesting that the species will eventually invade from nearby populations Pterostichus melanarius had no negative effect on population size or body mass of the native carabid species However, the two most abundant native species (Ptero-stichus adstrictus and P pensvlvanicus) appeared to be more active (measured as escape rate) in compartments where they co-occurred with P melanarius Although the three enclosures were just a few metres apart, one of them consistently yielded the highest densities of all carabid species This difference was correlated with high abundance of earthworms and low litter accumulation Relatively low densities of P melanarius may explain the lack of negative interspecific interactions in forests, although such effects are suggested by data from urban and agricultural environments where P melanarius is more abundant