Land use intensification results in abrupt transitions between extensively and intensively managed grasslands

Understanding whether land use intensification causes regime shifts (i.e. persistent, large changes in ecosystem states) is of key importance for management, particularly if these shifts occur abruptly as thresholds in land use intensity are crossed. Further, these threshold responses can show hysteretic dynamics, where the transition between states cannot simply be undone by reducing land use intensity to its initial level, and instead larger changes are required. However, quantitative evidence for threshold responses and their dynamics remain scarce.
Here we use a unique, long-term grassland database in Germany to identify threshold responses to land use intensity. We include data on the response of 16 ecosystem functions (including plant biomass, herbivory, litter decomposition, pathogen infection, water recharge, C and N fluxes and retention and enzymatic activity) and the diversity of 21 taxa (including above-ground and below-ground taxa such as vascular plants, lichens, bryophytes, arthropods, vertebrates, fungi, protists and bacteria). Then, we use increased variance as an early warning signal to evaluate if threshold responses are indicative of critical transitions between states. ... mehrFinally, we use temporal data to identify hysteretic dynamics associated with those transitions.
We show that 5 out of 10 above-ground taxa, fungal diversity, shoot biomass and soil N retention showed threshold responses to land use intensity, that is abrupt changes between extensively and intensively managed grasslands. Some of the thresholds were associated with increased variance in the response variables, suggesting a potential critical transition between ecosystem states. Time-series analysis indicated that ecosystem functions showed a hysteretic response to land use intensity, but diversity measures did not.
Our results suggest threshold responses to land use intensity are relatively common in the grasslands we studied. These thresholds show abrupt changes in the ecosystem when highly diverse extensively managed grasslands are managed slightly more intensively. Further, shifting back to the functioning seen in extensively managed grasslands may be challenging, as it requires larger reductions in land use intensity than those required to shift to the high intensity state. Identifying thresholds along land use gradients is critical to prevent ecosystem degradation and conserve biodiversity and ecosystem functions in semi-natural grasslands.