Compost heaps — refuges and stepping-stones for alien arthropod species in northern Europe

Abstract.  Compost heaps house a species-rich fauna of arthropods. This survey reviews changes in the composition of the beetle fauna in compost heaps in the Nordic countries during the 20th century. A total of 34 alien beetle species have established in compost heaps; 12 of these are also established in natural habitats. A combination of long-distance human transport and short-distance natural spread are proposed as dominant factors, explaining the dispersal of the species. An eastern route of immigration is proposed for Baeocrara japonica , Lithocharis nigriceps , Bohemiella paradoxa , Acrotona pseudotenera and Oxytelus migrator , while a southern route of immigration is proposed for Perigona nigriceps , Acrotona parens , Falagrioma concinna , Trichiusa immigrata , Cartodere nodifer , Ahasverus advena , Oryctes nasicornis and Atheta triangulum. There is no relationship between the types of compost preferred by the different species and the dominant way of dispersal. Among the trophic guilds that species belong to, the predators are more dependent on human transport than natural dispersal. The trophic guild to which a species belongs has no effect on whether or not the species is established away from compost heaps. The generalized compost fauna and species associated with bark compost naturalize away from compost heaps more successfully than expected. It is hypothesized that the successful establishment of alien species in compost habitats results from increased frequency of introductions and warm, thermal stability in large compost heaps during the Nordic winter. The establishment of new species in compost habitats probably has a minor impact on native inhabitants.