Résumé
Burning has traditionally been used in the southern French Alps to maintain open lands for grazing. In the context of land use abandonment, prescribed burning may be the only realistic practical tool available to oppose encroachment by woody plants in numerous mountainous landscapes. Although only recently developed as a modern tool for wildfire prevention and management of pastoral grasslands, this practice is now becoming widespread, which raises the issue of its impact on the fauna in general, and on endangered species in particular. We studied the impact of a prescribed fire on the survival and small-scale movements of one of the rarest snakes in Europe, the Orsini's viper Vipera ursinii, in order to evaluate its potential threat to population sustainability. We evaluated the body condition of snakes and the quality of their habitat, more precisely the abundance of grasshoppers as the main food resource, and the vegetation cover as shelter. About 3.6 ha of a 8.8 ha study site was burnt in autumn 2003. Snake population parameters were estimated using multi-state capture-mark-recapture methods. We demonstrate that burning reduced the survival of the vipers by more than half during the first year following the fire. Contrary to what was expected, there was no evidence of emigration from the burned to the unburned area immediately after the fire. Despite the decrease in grasshopper density and vegetation cover in the burned area, there was no evidence of mid-term mortality in snakes that could result from an increased predation rate or from the reduced body condition of snakes. As such high mortality, mainly or entirely due to the direct effect of fire, is likely to have large repercussions on population sustainability, we suggest several improvements of prescribed fire protocols to minimize their impact and strongly emphasize the need for a prescribed fire policy that takes into account specific recommendations for threatened species like Orsini's viper. The case of the Orsini's viper is very typical of conservation problems in open mountain lands, and we believe that our research will help determine the most suitable approach for long-term conservation of biodiversity in such semi-natural ecosystems.