Ecological impacts of the non-native macrophyte Crassula helmsii on Freshwater Macroinvertebrate Assemblages in Dartmoor National Park, UK

Abstract

Invasive species are a major threat to biodiversity globally. In freshwater ecosystems, invasive macrophytes are one of the most significant hazards. This study evaluated the impacts of the non-native macrophyte Crassula helmsii on macroinvertebrate assemblages in ponds in Dartmoor National Park, UK. Sampled ponds differed in the extent of invasion, being uninvaded, partially invaded and dominated by Crassula. Samples were taken from macrophyte stands using a hand net and taxa identified to the lowest taxonomic level possible in the laboratory. Assemblages were compared using univariate and multivariate approaches. Taxonomic richness and Shannon-Wiener Index did not differ significantly between invasion categories. However, abundance and evenness were significantly different where Crassula was predominant. Despite considerable variation between assemblages in individual ponds, assemblage composition differed significantly between invasion categories. Ponds where Crassula dominated harboured greater average abundances of non-native macroinvertebrates, of which Physella acuta constituted a substantial proportion. Although Crassula-dominated sites appear to support as many invertebrate taxa as those dominated by native vegetation, the identity of many taxa differs. In particular, Crassula appears to facilitate the spread of some scrapers and detrital shredders. Allelopathy, effects on water circulation and increased periphytic growth are likely the main causes of the impacts observed.