OlKOS 81: 266-278. Copenhagen 1998 Composition and trophic structure of detrital food web in ant nest mounds of Formica aquilonia and in the surrounding forest soil Jouni Laakso and Heikki Setala Laakso. J. and SetaIi, H. 1998. Composition and trophic structure of detrital food web in ant nest mounds of Fornlica oq~rilonia and in the surrounding forest soil. - Oikos 81: 266-278. Community composition and food web structure of soil decomposer biota in relation to various habitat properties were compared between upper parts of red wood ant (Fornlica aqtrilotlia) nest mounds and the adjacent forest soil. For a description of trophic structure of the decomposer community in the two habitats, soil decomposers were classified into 14 trophic groups. Classification of the taxa into three habitat preference categories resulted in a clear division of the fauna into either soil or nest specialists, relatively few taxa falling between these two groups. A large majority of the nest specialists belonged to a non-myrmecophilous soil decomposer fauna so far largely overlooked in studies on ant-invertebrate associations. Trophic organisation of the nest mound community differed clearly from that in the soil by having considerably larger biomass at the base of the food web, and less large predators - other than ants - at the top of the web. Contrary to forest soils. the clear dominance of bacterial feeding microfauna over the fungal feeding microfauna in the nest mounds suggests that most of the energy passing through the food web is channelled through a bacterial-based food-web compartment in the nest mounds. Relatively constant temperature and moisture in the nest surface, continuous energy input by the ants to the nests, and ant-induced reduction in predation pressure on macropredators are suggested to be responsible for the development of the typical decomposer community structure in the nest mounds. Thus. the food-web dynamics in ant nest mounds represent an interesting case in which the behaviour of an invertebrate species (i.e. the ant) has a potential to control the development of a system-level organisation. The high biomass of microbi-detritivorous animals, espe- cially earthworms, in the nest mounds suggests that the activities of the decomposer fauna may feed back to the structure of nest mound and indirectly alter the performance of the ant colony. J. Laakso and H. Setalii, Unic. of Jvriiskylii, Depr of Biological and Encirotznlenral Science, PO 5o.v 35, FIN-40351 J~rci'sk~A!i. Finland (jorla@.vzclfi). Nest building wood ants (Formicidae, Forntica) are known for their large colony size and long-lived nest mounds rich in organic material (Pokarzhevskij 198 1, Holldobler and Wilson 1990). These properties make the wood ant nests a suitable habitat not only for myrmecophilous invertebrates but also for an array of decomposer animals and microbes (see reviews by Kistner 1982, Holldobler and Wilson 1990). Al- though the high diversity and abundance of fauna associated with ant colonies are well recognised, the studies are scattered among various taxonomic and ecological studies of selected genera and higher taxa (Holldobler and Wilson 1990: 472). Consequently, at- tempts to explain the composition of ant nest fauna as a result of ecological processes, such as animal dispersion from the surrounding soil, or as an out- come of trophic interactions and abiotic constraints, are lacking. Accepted 16 July 1997 Copyright O OlKOS 1998 ISSN 0030-1299 . Printed in Ireland - all rights reserved