The growth of the non-native Crassula helmsii (Crassulaceae) increases the rarity scores of aquatic macrophyte assemblages in south-eastern England

The impact of invasive species on native species is often overlooked. Anecdotal and unmeasured evidence often gains more notice because more empirical research is not available. This study examines the impact of the aquatic invasive species Crassula helmsii (T. Kirk) Cockayne across a range of waterbody and landscape types in south-eastern England. Plant species lists were compiled for both invaded and uninvaded sites. Scoring systems using both national and county level indices were used to give a measurement of species rarity. The results showed both how invasion has not caused reductions in native species diversity, and also how species assemblages have been altered, often favouring rarer species. Explanations for these findings are discussed. Limitations of the findings, including translation to other species and to other geographical areas, are also discussed.