Higher species diversity inside than outside protected areas in the European Union

The Natura 2000 (N2K) protected area (PA) network is a crucial tool to limit biodiversity loss in Europe. Despite covering a large area (18% EU's land area), its representativity across taxa and bioregions remains uncertain, and only a few studies assessing its effectiveness at conserving species diversity have considered confounding factors such as land cover. Here, we used Propensity Score Matching (PSM) to test if more species are contained inside N2K PAs than outside, while accounting for the confounding effects of biogeographical regions, terrain ruggedness, and land cover, the latter quantified as the proportion of artificial, forest, semi-natural, and agricultural areas. Using records of 1769 priority species from the Birds and Habitats Directives of the EU with a resolution of 100 km², we calculated alpha diversity within each 100 km² cell and gamma diversity within each biogeographical region for all species and for distinct groups of taxa: mammals, birds, amphibians, reptiles, arthropods, fishes, mollusks, vascular and non-vascular plants. Differences in alpha diversity between PAs and non-PAs cells were assessed by fitting a generalized linear mixed model for each taxon group. In addition, beta diversity among cells was assessed by comparing the total assemblage of species PAs and non-PAs across biogeographical regions using the Wilcoxon Rank Sum Test. We consistently found a higher number of plant species inside than outside N2K PAs across biogeographical regions and taxa. Among biogeographical regions, for almost all the taxa species richness was significantly higher inside PAs than outside except for the Boreal region. Beta diversity was in general significantly higher inside N2K PAs than outside. Similarly, gamma diversity showed the highest values within N2K PAs except for the Boreal region for arthropods, mammals, and vascular plants. These results suggest that N2K PAs do not have the same representativity across biogeographical regions and taxa.