First record of the semi-aquatic invasive plant Crassula helmsii in the Iberian Peninsula and its link to potential dispersal drivers

Abstract

Aquatic neophytes are among the most harmful invasive species worldwide. Here we report the finding of the semi-aquatic invasive plant Crassula helmsii, naturalized in a freshwater reservoir that provides water to the large city of A Coruña, NW Spain. To better understand the extent and potential environmental correlates of C. helmsii invasion, we recorded the presence and frequency of C. helmsii in its emerged and floating mat forms at twelve sampling sites along the reservoir margins, and characterized the soil characteristics, plant community and environmental factors. Crassula helmsii is dispersed by fragmentation of the stems, potentially aided through shredding by local fauna such as the invasive crayfish Procambarus clarkii. We hypothesize that this could be a case of an invasional meltdown, when multiple invasive species facilitate one another and have synergistic effects on native ecosystems. The emerged form of C. helmsii was found in ten out of twelve sites with a mean cover value of 9.8 ± 10.4% (mean ± SD). We found no correlation between frequency of the emerged form of C. helmsii and abundance of Procambarus clarkii, but frequency of the emerged form of C. helmsii increased with higher soil nitrogen and vascular plant richness. We outline emergency actions for management of C. helmsii at this stage of the invasion, focusing on containment and avoiding spread to nearby water bodies.