Weak effects of the exotic invasive Carpobrotus edulis on the structure and composition of Portuguese sand-dune communities

Sand dune ecosystems have a high conservation value worldwide, but they are highly threatened by exotic plant invasion. We investigated the impacts of the exotic invasive species Carpobrotus edulis on the composition and structure (spatial pattern, total cover, species diversity and species co-occurrence) of native sand dune communities in the western coast of Portugal. We studied eight sites following a north-south gradient; in each site we established 8-10 transects of 25 contiguous quadrats of one square meter. C. edulis had a significantly clumped pattern in five of the study sites, which, however, was not related to the spatial pattern of native species. The effects of climate on the community structure variables were on average three times stronger than those of C. edulis. This species also had small effects on the floristic composition of native species. Our results indicate that the success and impacts of C. edulis are habitat-dependent and context-specific. They also provide evidence of a strong resilience to the impacts of invasion in the studied sand dune ecosystems: C. edulis did not reach large abundances or exert negative impacts on native communities to the extent expected. These ecosystems provide a unique opportunity to increase our understanding on the origin of impacts by invasive species, and on how particular communities resist the impacts of an invader.