Effects of the timing of grazing on insect diversity and insect–plant interactions in mountain grasslands

Grazing is the common agricultural land-use in mountain regions. It is of high socioeconomic importance but also essential for conservation as extensive mountain pastures are hotspots of biodiversity. Climate change is causing earlier growing seasons, prompting earlier livestock turnout. The effects of grazing on biodiversity, however, may differ depending on the time of the year, yet our understanding of these effects is limited. Here, we evaluate how short-term effects of different livestock turnouts affect taxonomic, phylogenetic, and functional diversity of pollinators (wild bees and butterflies) and phytophagous insects (leafhoppers) as well as plant-insect interactions on eight mountain pastures in the northern Alps, Germany. At each pasture, we established three grazing treatments including an ungrazed control, early and late livestock turnout. We sampled wild bees and butterflies during two and leafhoppers during one growing season twice a year (summer onset and summer peak). To account for effects of grazing through changes in vegetation, we surveyed vegetation characteristics, such as the number of inflorescences and sward height. Early-grazing plots had lower wild bee and leafhopper diversity during summer onset, but this pattern shifted later in the season after grazing had stopped. During summer peak, wild bee diversity was higher at early-grazing plots than at late-grazing plots and structural equation modeling indicated that this could be partly explained by a higher number of inflorescences. Phylogenetic network diversity of wild bee- and leafhopper-plant networks was higher at late than at early-grazing plots. Our study shows that grazing in general, and also the timing of grazing, affects vegetation characteristics, insect diversity, and plant-insect interactions in mountain pastures. Effects of grazing on insect diversity were mostly positive, which supports the notion that extensive grazing is important to maintain insect diversity in mountain pastures below the timberline. Although negative effects of early livestock turnout treatments occurred, they disappeared and even turned positive later in the season. Thus, earlier livestock turnout does not appear to threaten insect diversity in mountain pastures, but further research is needed to understand long-term effects.