Biodiversity is not homogenously distributed over the globe, and ecosystems differ strongly in the number of species they provide. With this special issue we highlight the ecology and endangerment of one of the most diverse ecosystem of Europe: the European grassland ecosystems. The selected 16 contributions describe interactions from below-ground to the atmosphere and focus on (1) effects of abiotic and biotic on species diversity, (2) the impact of various factors along spatial and temporal gradients, (3) the relevance of falling abandoned and eutrophication—including countervailing management strategies like encroachment; and (4) intraspecific effects based on physiology, genetics and intraspecific plasticity. The contributions cover fungi, plants, and invertebrates and highlight effects taking place at the level of ecosystem, species community, species, populations, and also within individuals (physiology and genetics).