ESTABLISHMENT AND INTERACTIONS OF CARABID POPULATIONS - AN EXPERIMENTWITH NATIVE AND INTRODUCED SPECIES

The ground beetle Pterostichus melanarius (Coleoptera, Carabidae), of European origin, is abundant and wide-spread in the northern United St ates and southern Canada. Three field enclosures were used in a four-y ear experiment in Alberta, Canada, to examine whether the species is a ble to establish in natural aspen-poplar forest, and to test the hypot hesis 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 inva de from nearby populations. Pterostichus melanarius had no negative ef fect on population size or body mass of the native carabid species. Ho wever, the two most abundant native species (Ptero-stichus adstrictus and P. pensylvanicus) appeared to be more active (measured as escape r ate) in compartments where they co-occurred with P. melanarius. Althou gh the three enclosures were just a few metres apart, one of them cons istently yielded the highest densities of all carabid species. This di fference was correlated with high abundance of earthworms and low litt er 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 environ ments where P. melanarius is more abundant.