The invasion of Bromus erectus alters species diversity of vascular plants and leafhoppers in calcareous grasslands

The most common mechanism of biological invasions is an increase in competition, which usually results in the loss of biodiversity. The invasion of Bromus erectus (Syn. Bromopsis erecta) in calcareous grasslands of western and central Europe is well-documented. Although, it is largely unknown to what extent this development affects biodiversity. In this study, we analysed the effects of B. erectus invasion on vascular plant and leafhopper assemblages of calcareous grasslands. At each of the 15 randomly selected sites, we compared one plot with stands of Bromus (presence) and one without (absence) (paired sampling design). The invasion of B. erectus affected vegetation structure as well as vascular plant and leafhopper assemblages. Despite similar abiotic site characteristics, Bromus plots had a higher turf height, vegetation density, and litter cover. Additionally, we recorded a much lower species richness in Bromus plots than in absence plots in all analysed groups of vascular plants. A similar pattern was found for leafhoppers. Absence plots exhibited a higher species richness of habitat and diet specialists than Bromus plots. The overall abundance of all leafhopper species was highest in presence plots, whereas that of diet specialists peaked in absence plots. We conclude that the ongoing invasion of B. erectus will lead to a strong structural homogenisation with negative effects on plant and insect diversity. One possible management tool might be a goal-driven rough grazing with sheep - especially in spring when B. erectus is much more palatable than in summer and autumn.