Is dispersal density-dependent in carabid beetles? A field experiment withHarpalus rufipes (Degeer) and Pterostichus niger (Schaller) (Col., Carabidae)

In order to investigate whether density affects dispersal, carabid beetles of two species, Harpalus rufipes (Degeer) and Pterostichus niger (Schaller) , were released separately in six and two field enclosures, respectively. T he enclosures were 4 m(2) each and located in an oat field. The former spec ies was released in seven densities, each with 12 replicates and the latter in five densities each with four replicates. Beetles leaving the centre of the enclosures were captured in pitfall traps along a 1 m wide strip of ba re soil. Logistic models were fitted to the data. There were no significant effects of the density treatment variable on P. niger. For H. rufipes the best model was obtained by specifying the density treatment as a nominal va riable. The deviance accounted for by this model was highly significant. Th ere were, however, no consistent trends for either species with respect to the density treatment. The inclusion of climatic variables, the number of n ew beetles introduced at the onset of each trial, or the date of the trial did not improve the fit of any of the models. It is concluded that density did not significantly increase dispersal in the two species. It thus appear s that a possible increase in behavioural interactions (i.e. interference c ompetition) did not increase the dispersal in adults of the two species exa mined in this experiment.