Arthropod biodiversity after forest fires: winners and losers in the winter fire regime of the southern Alps

Since prehistoric times, natural and man made fires have been important factors of natural disturbance in many forest ecosystems, like those on the southern slopes of the Alps. Their effect on scarce, endangered or stenotopic species and on the diversity of invertebrate species assemblages which depend on a mosaic of successional habitat stages, is controversially discussed. In southern Switzerland, in a region affected by regular winter fires, we investigated the effect of the fire frequency on a large spectrum of taxonomic groups. We focussed on total biodiversity, taxonomic groups specific to certain habitat types, and on scarce and endangered species. Overall species richness was significantly higher in plots with repeated fires than in the unburnt control sites. Plots with only one fire in the last 30 yr harboured intermediate species numbers. Fire frequency had a significantly positive effect on species richness of the guilds of interior forest species and forest edge specialists. Species of open landscape, open forests and interior forests were not influenced by fire frequency. A positive effect of fire on species richness was observed for ground beetles (Carabidae), hoverflies (Syrphidae), bees and wasps (Hymenoptera aculeata, without Formicidae), and spiders (Araneae). True bugs (Heteroptera), lacewings (Neuroptera) and the saproxylic beetle families Cerambycidae, Buprestidae and Lucanidae showed positive trends, but no statistically significant effects of fire on species numbers or/and abundances. Negative effects of fire on species numbers or/and abundances were found only for isopods and weevils (Curculionidae). A compromise for forest management is suggested, which considers the risk of damage by fire to people and goods, while avoiding the risk of damage to biodiversity by imitating the effects of sporadic fires and providing a mosaic forest with open gaps of different successional stages.