Threatened European Butterflies Concentrate in Areas of Strong Climatic Change and Atmospheric Deposition Pressure

Land use change, as a result of many local-scale decisions scaling up to large spatial extents, is considered the main threat to European butterflies. The impact of large-scale pressures, such as atmospheric nitrogen deposition or climate change, is less understood or less documented, respectively. However, it is acknowledged that they might reinforce the pressure on already threatened species. To evaluate the additional threat exerted by these pressures we compared their geographical pattern to those of threatened butterflies across Europe. We therefore derived range maps of 383 butterfly species and used two species-specific threat assessments derived from national and European Red Lists. We then used Spearman rank-correlations and beta-regressions to compare two metrics of species threat per 10 x 10 km raster cells with geographical patterns of cumulative nitrogen depositions from 1980 to 2015, as well as the magnitude of change in precipitation sums and temperature means between the decades 1979-1988 and 2004-2013. We found that threatened species tend to concentrate in areas with high nitrogen depositions and pronounced summer temperature changes. In particular, parts of central and eastern Europe were both hotspots of threatened butterflies and hotspots of climatic pressure. This spatial coincidence of the distribution of threatened butterfly species with large-scale patterns of nitrogen depositions and recent climate warming indicates an already considerable risk of regional to continental extinctions that will likely increase further in the future as climate change will most likely intensify. Consequences for area-based conservation measures are discussed.