Colonization success of carabid beetles on Baltic islands

Aims (1) To test whether there is a significant increase in carabid species richness with an increase in island size and, if so, if it is due to island area per se or habitat diversity. (2) To investigate whether scattered islands accumulate species quicker than islands close to each other; per island size. (3) To investigate changes in the proportions of carabid wing morphs between the Finnish mainland and islands in the Baltic Sea. Location Islands in the south-western archipelago of Finland, in the Baltic Sea. Methods Carabid beetles were collected using pitfall traps (diameter, 65 mm; volume, 170 mL), half-filled with an ethylene-glycol-water mixture, from 22 May to 20 September 1993 on 24 islands. Island size varied between 0.5 and c. 7000 ha, and each island had between one and four habitat types sampled. Results A total of 61 carabid species were captured on these islands. Pterostichus niger was numerically dominant on 15 of the 24 islands and made up 34.5% of the total catch. The islands had a significantly higher proportion of brachypterous species compared to the Finnish mainland. The islands also accumulated species at a much slower rate (z = 0.06) than that generally observed in the literature, and, for carabids, a mainly predacious group, habitat diversity had little predictive power in explaining species richness. Islands close to each other (a few hundred metres apart) accumulated species at a slower rate than did scattered islands, as island size increased. Main conclusions Although carabids disperse relatively easily to remote islands (perhaps a result of low Baltic Sea salinity and short interisland distances), colonization success appears to depend on a multitude of factors, including availability of suitable habitat on these islands, competitive superiority, survival ability during dispersal and island arrival sequence.