Year-to-year variation in carabid beetle (Coleoptera, Carabidae) assemblages on the Aland Islands, south-west Finland

Aims (1) To investigate changes in structure of carabid assemblages between sites on the main island of Angstromland and five small nearby islands between 1982 and 1999. (2) To determine whether island differences observed in one year is mirrored in another one. (3) To examine changes in abundance levels of individual carabid species on these islands since 1982. (4) To compare the proportion of flightless species on the main island of Angstromland with those on the small islands. Location Islands in the Baltic Sea, Finland; the main island of Angstromland (90,000 ha) and five small, nearby islands (8-29 ha). Methods Carabid beetles were collected, using pitfall traps, during 4 years (1982, 1987, 1988 and 1999). Sampling took place in moist, spruce-dominated, lush forest patches. Results Overall, 12,127 individuals representing forty-six carabid species were collected; forty-one species (8580 individuals) on the main island of Angstromland, and thirty-one species (3547 individuals) on the small islands. Carabid assemblage structure, measured using ordination techniques, was consistently and significantly different between the main island and the small island sites. Carabid assemblage structure and abundances of individual species did change significantly between years, possibly because of a combination of climatological and habitat changes in the Angstromland archipelago. The six most abundant species collected on the small islands, and the three most abundant species overall, were all short-winged. Furthermore, the small islands had a higher proportion and number of short-winged species compared with the main island of Angstromland. Main conclusions Carabid assemblage structure was significantly different between the main island and the small islands in all years sampled, indicating that single-year studies are sufficient to demonstrate differences in carabid assemblages on Baltic islands. Small islands host a non-random proportion of species pools available on the main island of Angstromland and mainland Finland. It appears that dispersal to islands in the Baltic Sea is easy for carabid beetles of all wing forms, but that short-winged species are more successful colonizers.