Drivers of alien species richness in Romania’s protected areas

Abstract

Biological invasions represent a major threat to biodiversity, yet the vulnerability of protected areas (PAs) to alien species remains insufficiently quantified, particularly in Eastern Europe. Understanding whether PAs harbour more or fewer alien species than surrounding landscapes is critical for effective conservation planning within the EU’s Natura 2000 (N2K) network. We analysed the distribution of 683 alien species (185,199 occurrences) across 240,290 terrestrial 1 km² grid cells in Romania, spanning four biogeographic regions (Alpine, Continental, Pannonian, and Steppic and Black Sea). Grid cells with more than 48% area overlapping N2K sites were classified as inside PAs. We fitted zero-inflated negative binomial regression models controlling for wilderness, elevation, presence of roads, and biogeographic region to investigate drivers of alien species richness. An invasion risk index and species accumulation curves complemented the analysis. Preliminary results show that observed mean alien richness was lower inside PAs than outside, but this pattern reversed after adjusting for environmental covariates: grid cells inside PAs harboured 45% more alien species. The presence of roads was the dominant predictor of vulnerability to invasion, increasing the probability of a cell being invaded by 90%. Higher wilderness values were associated with a slight increase of alien species richness among suitable cells, while elevation reduced richness. The PA effect was significant in three of four biogeographic regions, ranging from a nearly threefold increase in alien richness in the Steppic region (IRR = 2.87), where PAs along the Danube and other rivers simultaneously serve as conservation areas and invasion pathways, to a modest effect in the Alpine region (IRR = 1.14). The PA effect varied markedly across taxa. Vertebrates showed the strongest response (IRR = 3.41), followed by invertebrates (IRR = 2.11), while plants exhibited the weakest effect (IRR = 1.21). This effect-size gradient mirrors introduction pathways, with deliberate introductions of fish, game birds, and mammals historically concentrated in or near wilder areas. Across N2K sites, alien and protected species richness were positively correlated, with the most biodiverse sites facing the greatest invasion pressure. The invasion risk index identified a subset of very high risk N2K sites, including Iron Gates, Almajului–Locvei Mountains, and Lower Siret Floodplain, characterised by high numbers of IAS of EU concern cf. EU Rule 1143/2014 (EU IAS) co-occurring with rich assemblages of protected species. Among EU IAS, Ailanthus altissima , Reynoutria japonica posed the greatest aggregate threat based on site occupancy and index scores. Semiaquatic species such as Nyctereutes procyonoides and Myocastor coypus showed disproportionately high occurrence concentrations inside PAs. Species accumulation curves showed no saturation across taxonomic groups, with recording rates increasing in recent decades. Our findings suggest that Romanian protected areas are hotspots where high conservation value coincides with elevated invasion pressure. The pronounced bioregional and taxonomic heterogeneity underscores that management strategies must be regionally calibrated, with particular urgency in the Steppic bioregion and along river corridors.