The role of a mound-building ecosystem engineer for a grassland butterfly
For the protection or re-establishment of species-rich grasslands, the limiting factors controlling species richness have increasingly become of scientific interest. This study aims to analyze the role of disturbance for the occurrence of a low-competitive herb, Centaurium erythraea Rafn, in mesotrophic, lowland grasslands in NW Germany. We sampled a total of 38 plots with presence of C. erythraea and 24 control (random) plots in semi-natural grasslands. As a proxy for disturbance, we estimated the cover of bare ground and Ellenberg flooding indicator species and measured the distance between the plot and the nearest path. Moreover, we counted the number of C. erythraea individuals within each plot. In the GLM analyses the distance from path was the only predictor; both the presence of the species and the number of individuals decreased with the distance from path. The grasslands at the path edges had the highest disturbance intensity. Here horse riding, military-vehicle traffic and regular sod-cutting directly create bare ground. In general, disturbance creating bare ground seems to be the key factor enabling germination and growth of C. erythraea in mesotrophic grasslands and other low-competitive short-lived species. Disturbance enhances the expression of the seed bank, favours the development of the shade-avoiding and low-growing rosettes and oppresses tall-growing competitors. Therefore we suggest grazing as the best management method.