Spatial gradients and grazing effects of a transhumant herd on plants and insect herbivores in Pyrenean subalpine grasslands

Mountains harbour a disproportionate amount of biodiversity that is explained by both biotic and abiotic factors. Understanding the ultimate factors that shape these gradients including the interaction between trophic levels is important to highlight management practices that may help maintain biodiversity. Here we report an experiment carried out in a Pyrenean valley where a transhumant cattle herd grazes every year subalpine grasslands in a 700 m altitudinal range (1300–2000 m). In 2018 we measured species richness and abundance of plants and two groups of herbivorous insects (butterflies and grasshoppers) in 20 plots, and then we established 10 exclusion plots to study the effect of grazing during 2019 and 2020. Our results show that there were differences among the relative weight of abiotic and biotic factors that shape biodiversity gradients. Overall, the elevation gradient strongly affected plant, butterfly and grasshopper richness, as well as insect abundances. Plant and butterfly richness’s were correlated via a cascading trophic effect while the slope played a very important role for grasshoppers. Livestock exclusion had a negative effect on plant richness through rapid growth of grass species but there were almost no changes in insect community composition after two years. The intra-annual effects of grazing showed early grazing to be positive for butterflies flying later during the season, as seen for a subset of plots within a narrow altitudinal range. In grazed plots, butterfly richness and abundance increased over the summer when the herd moved to alpine environments and exceeded those of non-grazed plots, indicating that a traditional transhumance management system can help sustain biodiversity in subalpine grasslands as a type of extensive grazing.