The impact on plant communities of an invasive alien herb, Oenothera drummondii, varies along the beach-coastal dune gradient

Abstract

One of the major threats to the diversity of coastal dunes is the expansion of invasive species, such as Oenothera drummondii subsp. drummondii (Onagraceae). In southwestern Spain, we studied the impact of this American invasive on community structure and composition along a beach-dune gradient (beach, foredune, and inland dunes). Differences in density, biomass, and the cover of O. drummondii, the cover of perennial and annuals/biennials species, and Shannon diversity index H’ and dominance λ were compared between invaded and uninvaded sectors. We observed that the intensity and impact of the invasion by O. drummondii varies along the beach-dune gradient. The abundance of the invasive plant increased inland and in consequence, its impact on species richness and composition was highest in inland dunes. Here, plant cover of O. drummondii represented 57.9% of total plant cover; species richness was reduced (with 3.3 fewer species per 2 × 2m plot), diversity H’ was lower while dominance λ was higher. At a broader scale, species richness in the invaded sector was 25% larger than in the uninvaded sector, because of the presence of ruderal species. Species composition also was modified after the invasion. The abundance of a keystone native species was largely reduced (Ammophila arenaria), and some natives became locally extinct (Otanthus maritimus, Eryngium maritimum, Medicago marina and Elymus farctus). We conclude that the high environmental severity of the beach and foredunes results in a reduced invasion and impact of O. drummondii, whereas the milder conditions of inland dunes promote its expansion. The shift in community structure and composition can have an increasing domino effect and thus monitoring, and mitigation actions are necessary. When doing so, the environmental heterogeneity of the beach-dune gradient should be considered, given its relevance in the invasion process.