Successional changes of ant assemblages: from virgin and ditched bogs to forests

We studied ant assembly changes after ditching of bogs with nest and pitfal l sampling in the southern Finnish taiga. The study sites clustered in dend rograms to hierarchical sets: virgin bogs and young ditchings, older ditchi ngs, and forests. Species richness was low on virgin bogs and young ditchin gs, and increased with the age of ditching. The number of species was highe st in clearcut, and decreased in spruce forests with increasing density of wood ants. Three bog specialists, Formica uralensis, F. picea and Myrmica s cabrinodis, were found only on bogs. Nation-wide draining of bogs implies s evere decreases in their population densities. As a corollary, the poorly k nown but potentially healthy populations of the obligate social parasite of M. scabrinodis, Myrmica karavajevi, may go extinct in extensive regions, b ecause of its need of high nest densities of the host species. The effects of habitat attributes on the local number of species were overshadowed by t op-dominant, polydomous wood ants. A wood with practically no F. aquilonia harboured 11 other ant species, whereas in high-density wood-ant forest onl y two other species were located. Pre-emption by the slave-maker F. sanguin ea may in several ways slow down the spread of wood ants to ditched bogs.