Weak effects on plant diversity of two invasive Impatiens species

The study aimed to examine the effect of the invasion of the two congeneric species Impatiens glandulifera and Impatiens parviflora on species richness and composition across a wide range of communities in North-western Germany. We applied a space-for-time substitution approach, comparing invaded plots with adjacent, environmentally similar uninvaded plots, based on the assumption that the latter represent the situation prior to an invasion. Even though the dominance of the invasive species resulted in a lower Shannon diversity in the invaded plots, species richness was not (I. parviflora) or only weakly (I. glandulifera) reduced. Also the community composition of the invaded sites was only marginally different. Invaded and uninvaded plots in general had similar habitat conditions, but both Impatiens species occurred in slightly shadier sites compared to the uninvaded areas, and the plots invaded by I. parviflora tended to have higher nutrient concentrations. These results suggest that dense populations of invading species may often be found at particular microsites. The relatively low impact of Impatiens on the vegetation is most likely caused by the annual life strategy of the species: while I. glandulifera shows large fluctuations in numbers between years and has a later phenological development than most other species in its habitat, I. parviflora establishes preferably in dark and acidic, often disturbed woodlands where it competes with few other species.