Elevational gradient and human effects on butterfly species richness in the French Alps

We examined how butterfly species richness is affected by human impact and elevation, and how species ranges are distributed along the elevational gradient (200-2700m) in the Isere Department (French Alps). A total of 35,724 butterfly observations gathered in summer (May-September) between 1995 and 2015 were analyzed. The number of estimated species per 100-m elevational band was fitted to the elevational gradient using a generalized additive model. Estimations were also performed on a 500mx500m grid at low altitude (200-500m) to test for the human impact on species richness using generalized least squares regression models. Each species elevational range was plotted against the elevational gradient. Butterfly richness along the elevational gradient first increased (200-500m) to reach a maximum of 150 species at 700m and then remained nearly constant till a sharp decrease after 1900m, suggesting that after some temperature threshold, only few specialized species can survive. At low elevation, urbanization and arable lands had a strongly negative impact on butterfly diversity, which was buffered by a positive effect of permanent crops. Butterfly diversity is exceptionally high (185 species) in this alpine department that represents less than 5% of the French territory and yet holds more than 70% of all the Rhopalocera species recorded in France. Both climate and habitat shape the distribution of species, with a negative effect of anthropization at low altitude and strong climatic constraints at high altitude.