An invasive plant species enhances biodiversity in overgrazed pastures but inhibits its recovery in protected areas

Anthropogenic environmental change exposes biological communities to concurrent stressors (e.g., changes in climate and land-use, overexploitation, biotic invasions) that frequently persist over prolonged periods. Predicting and mitigating the consequences of human action on nature therefore requires understanding how exposure to multiple interacting stressors alters biological communities over relevant (e.g., multi-decadal) time periods. Here, we explore the effects of overgrazing and plant species invasion on plant community diversity and ecosystem functioning (productivity), as well as the patterns of recovery of plant communities following cessation of grazing pressure. In a Mediterranean pasture system, we utilized a "natural" experiment involving long-term exclusion of grazers (for 15-25 years in parks) and also conducted short-term grazing-exclusion and invasive species removal experiments. Our results reveal that invasion by a grazing-resistant plant (prickly burnet) has net positive effects on plant diversity under overgrazing conditions but inhibits the recovery of biodiversity once grazing ceases. Furthermore, while the diversity-productivity relationship was found to be positive in pastures, the interactive effects of overgrazing and species invasion appear to disrupt ecosystem functioning and inhibit the recovery of pasture productivity. These findings highlight the potential for prolonged exposure to anthropogenic stressors, such as overgrazing, to cause potentially-irreversible changes in biological communities that, in turn, compromise ecosystem functioning and resilience. In such cases, sustainable ecosystem management may require direct intervention to boost biodiversity resilience against centennial overgrazing.