Small-scale heterogeneity in the spatial distribution of carabid beetles in the southern Finnish taiga

Small-scale distribution of ground beetles (Coleoptera, Carabidae) was examined, based on catches among 300 pitfall traps, in a coniferous forest in southern Finland. The sample from the whole snow-free season comprised 2405 individuals of twenty-two species. Each of the most numerous species (Pterostichus oblongopunctatus (F.), Calathus micropterus (Dft.), Leistus terminatus (Hellw. in Pz.), Notiophilus biguttatus (F.) and Cychrus caraboides (L.)), was non-randomly distributed and formed aggregations within the site of c. 1.3 ha in area. Although the five species occurred in every microhabitat defined in the plot, four of them (N. biguttatus was an exception) were more abundantly found in certain microhabitat types than in the others. In addition to vegetation around the traps, the numbers of Formica ants in the traps correlated with the numbers of carabids caught, mostly negatively. The considerable variation in catches and species richness among single traps and among blocks of 16 traps tended to even out, when larger subsamples were taken from the total pool (blocks of 48 traps). The distribution patterns observed within the plot were compared to a reference data set from similar habitat in the same region. Species distributions among microhabitats were slightly different in the reference set and the predictive success was relatively poor, probably due to different scales of study in the two data sets. Variation in species distribution in the two spatial scales studied (within a habitat patch and among them) is supposedly due to different factors. Although no direct evidence is available from our study, area, we suggest that active microhabitat selection explains the small-scale distribution within the study plot, whereas dynamics of local populations, influenced by regional-scale differences in habitat composition, are the most likely explanation for the distribution patterns among habitat patches.