Alien aquatic plants do not have higher fragmentation rates than native species: a field study from the River Erft

Aquatic plants mostly disperse via vegetative means, and plant fragments are the most important vegetative dispersal units. Here we studied the fragment drift in the medium sized River Erft, in which the macrophyte community is dominated by alien aquatic plants. The plant fragments were sampled at six different sites in the drift of the river. We analyzed the drifting biomass, the number of fragments and the number of propagation units (based on the known minimum size of fragments needed for regeneration of each species) as potential propagation units. Overall, we sampled >13,000 drift units of various sizes, from short stem fragments 400 connected plants (Vallisneria spiralis), resulting in >500,000 propagation units. The total number of sampled drift units and propagation units were significantly higher for native than for alien plant species. The number of sampled drift units and biomass was correlated with the total plant mass (TPM, calculated from species abundances per river section and section lengths) of the species in the river. However, the highest numbers of drift and propagation units per unit TPM were found for the four natives Ceratophyllum demersum, Potamogeton pectinatus, Potamogeton crispus and Myriophyllum spicatum, contributing to more than 75 % of the total number of fragments sampled. Conversely, lowest numbers of drift units were found for alien plants Hygrophila polysperma and Hydrocotyle ranunculoides, and no drift units were collected for four aquatic plant species in the river (alien Myriophyllum aquaticum and natives Nuphar lutea, Potamogeton nodosus and Sparganium emersum).