Contrasting response of vascular plant and bryophyte species assemblages to a soil-disturbing ecosystem engineer in calcareous grasslands

Species-rich grasslands are highly threatened by land-use change, including fertilization, land-use intensification and abandonment. These changes evoke a loss of structural heterogeneity. Aside from specific management measures to increase structural heterogeneity, small-scale patches of different vegetation within grasslands are promoted naturally by soil-disturbing ecosystem engineers. The aim of this study is to analyse the importance of Lasius flavus nest mounds for increasing structural and functional diversity in vascular plant and bryophyte assemblages within Central European calcareous grasslands.Our study clearly revealed that the mound-building activity of L. flavus ants in calcareous grasslands alters soil conditions and vegetation structure. Ant mound soil samples had higher potassium and phosphorus levels than control samples in the matrix vegetation. Ant mounds, in general, and the south-facing sides, in particular, represented highly disturbed microsites with open and low-growing vegetation containing high proportions of bare ground. In contrast, the north-facing sides were characterized by transient vegetation between the dense matrix and the open south-facing sides. However, the effects of soil disturbance by ants were different for vascular plant and bryophyte species assemblages. In vascular plants, differences among the three sample groups were less pronounced; in bryophytes, nearly all parameters differed.All in all, the occurrence of L. flavus increases vegetation heterogeneity within grasslands and, therefore, plays an important role in supporting species richness of both plants and animals in calcareous grasslands. This holds true in particular for fallow sites, where ant mounds often represent the only sites of open vegetation.