The effect of forest ski-pistes on butterfly assemblages in the Alps.

1. The abandonment of man-made pastures below the tree line is favouring natural reforestation in the European Alps. As such, the creation of forest ski-pistes might, paradoxically, be beneficial to butterflies. This study is the first to focus on the effects of forest ski-pistes on butterfly assemblages (Lepidoptera, Rhopalocera). 2. We sampled both butterflies and flowers on ski-pistes, in adjacent forests and in the nearest pastures at three sites in the Aosta Valley (north-western Italian Alps). 3. Transect counts showed that butterflies were more abundant on ski-pistes (and forests) than in pastures. However, species richness and diversity were lower on skipistes than on nearby pastures. Flower abundance was higher, but flower richness and diversity were lower on ski-pistes than in pastures. Multivariate regression trees indicated that butterfly abundance was positively associated with flower abundance and butterfly diversity was positively associated with flower diversity. 4. We conclude that: (i) butterfly abundance on ski-pistes was higher because in mid-summer these grassland strips were more abundant in flowers than nearby pastures and (ii) butterfly diversity was lower both because of the lower flower diversity and because of the higher isolation and⁄ or smaller area of ski-pistes by comparison with pastures. 5. In the Alps, several grassland butterfly species are of conservation concern. Ski-pistes are probably sub-optimal habitats for butterflies. Nevertheless, within a natural reforestation framework, they may be useful to butterfly conservation, on condition that they are completely re-vegetated and that grass cover of ski-pistes are favoured and maintained to facilitate their colonisation.