The occurrence and distribution of carabid beetles (Carabidae) on islands in the Baltic Sea: a review

The distribution and occurrence of carabid beetles have been studied extensively in a number of archipelagoes in the Baltic Sea, a ca. 377,000 km², fairly shallow and young sea in northern Europe. This work has revealed some surprising results related to colonisation success and maintenance of populations. Dispersal from the mainland and between islands appears to be relatively easy because inter-island distances are small (tens or hundreds of metres) and the salinity of the Baltic is low, which allows survival during drift. This is reflected in the higher than expected proportion of brachypterous species on small islands. A lack of energetically expensive wings, flight muscles and flight fuel may give brachypterous species an advantage - in terms of resource allocation for survival and fecundity - over macropterous species in the colonisation of an island. Once an island is reached, successful colonisation mainly depends on the presence of suitable habitat. The most abundantly collected carabids on these islands are habitat generalists, but some degree of habitat association exists. Yet, the composition of carabid assemblages differs substantially among islands, possibly a result of chance effects. There is evidence that species on smaller islands occur in a wider variety of vegetation types than species on larger islands. Both habitat diversity and area per se are implicated in the positive species-area relationships found, but the small island effect has been identified as being significant in most of the archipelagoes investigated. Pterostichus melanarius, a eurytopic, wing-polymorphic species abundantly collected on the mainland is, unexpectedly, absent from small islands. Possible reasons for this include; (1) interspecific predation and competition for food by the closely related but slightly larger P. niger, which is dominant on small islands, (2) the apparent inability of P. melanarius to reproduce on small islands, (3) its reduced dispersal power as compared to P. niger and (4) the possible absence of some essential resources from small islands. In conclusion, the conservation value of highly threatened cultural landscapes on Baltic Sea islands, such as species rich wooded meadows, is discussed as well as the possible negative effects of climate change on carabid beetles.