Grasshopper re-surveys unravel threats to wet grasslands: drought, nitrogen deposition, and abandonment

Wet semi-natural grasslands are biodiversity-rich ecosystems that are increasingly threatened by climate change and land-use alteration. However, long-term empirical evidence on how invertebrate communities respond to these pressures remains scarce. Using a large-scale re-survey, we assessed changes in grasshopper communities in wet semi-natural grasslands across Germany over the past 20–35 years. We compared historical and recent species communities by re-surveying 107 grassland patches originally sampled between the late 1980s and early 2000s. Community responses were evaluated using species richness, functional groups based on mobility and habitat specificity, the Community Temperature Index (CTI), the Community Farmland Index (CFI), and Simpson dissimilarity ( SI ). Total species richness increased over time, driven by strong gains in highly mobile generalist species. By contrast, species currently classified as threatened and low-mobility habitat specialists both declined markedly. Communities became increasingly dominated by warm-adapted species, as indicated by rising CTI values, while decreasing Simpson dissimilarity revealed pronounced biotic homogenization. Mixed-effects models identified drought, atmospheric nitrogen deposition, and increasing cover of abandoned grasslands as major drivers of species losses. Our results demonstrate that gains in species richness can mask ongoing functional degradation of wet grasslands. Grasshoppers proved to be sensitive bioindicators of climate- and land-use-driven degradation pathways. To counteract these trends, maintaining high groundwater levels and continued low-intensity management—such as grazing with low stocking rates or conservation mowing with uncut refuge strips—are crucial to prevent further degradation and to sustain ecosystem functioning in wet grasslands.