Contrasting effects of a native and an alien N-fixing species on the diversity and composition of dune plant community assemblages

The establishment of plant species in coastal dune systems has received increasing attention in recent years due to their potential threats to the unique biotic and abiotic characteristics of these fragile ecosystems. Plants exhibiting invasive behavior, whether alien or native, merit particular focus in coastal dunes systems given their ability to form extensive, high-density populations. To address whether there are differences in the effects of a native and an alien species on dune systems, we examined the impact of two N-fixing species, one an alien invader (Ulex europaeus) and the other a native colonizer (Gunnera tinctoria), on taxonomic and functional plant diversity. In these coastal dune systems where the resident community contains a high (56 %) proportion of introduced species, invasion by the alien species Ulex europaeus significantly reduced the diversity of the resident community by decreasing species richness, evenness, and taxonomic complexity. In contrast, colonization by the native Gunnera tinctoria did not lead to significant reductions in species richness or evenness but did reduce overall species diversity and likely facilitated species turnover by altering species assemblages. We conclude that U. europaeus invasions exert significantly negative effects on resident coastal plant communities, whereas colonization by the native G. tinctoria has comparatively less impact. This study represents one of the few field experiments directly comparing the impacts of invasive alien species with those of range-expanding native species, providing evidence that alien species can exert greater ecological effects. These findings hold significant conservation and management implications, as they highlight the need to address the ecological consequences of overgrowing populations, whether native or alien, particularly in coastal dune systems.