Newt decline in Western Europe: highlights from relative distribution changes within guilds

The recent increase in the number of monitoring schemes has formed the basis for high quality distribution atlases. This provides the opportunity of estimating global and specific decline patterns across regional and national borders. In this framework, this study focused on four sympatric newt species—including the great crested newt (Triturus cristatus), an Annex 2 European Habitats Directive species, over six geographic areas (five countries) in Western Europe. A relative comparison of distribution maps across time is used here and is based on more than twelve thousands occupied grid cells. It benefits from the definition of a guild, as these species are simultaneously detectable in wetlands. T. cristatus and the alpine newt (Mesotriton alpestris) were the most and the least threatened newt species, respectively, whereas the palmate (Lissotriton helveticus) and smooth newt (Lissotriton vulgaris) had an intermediate decline level at both coarse and fine grain resolutions. However, regional variations across Europe and scale effects were also found. On one hand, these results show that T. cristatus is not only regionally threatened but suffers from a global decline in Western Europe. On another hand, the results indicate that patterns of decline are not uniform within Europe and that species often considered as common and not threatened are, in fact, declining more than others. Finally, the proposed methodology, i.e. using guilds to assess relative decline, would be useful as a complement to other standardized methods in correctly advising conservation managers and policy makers, particularly for species with more subtle declines.