Bird and hoverfly communities are impacted by vegetation heterogeneity in wood-pastures in the Swiss Jura

Abstract

Wood-pastures are a type of agroforestry system consisting of assemblages of cattle-grazed pastures and scattered trees. They maintain biodiversity because of their environmental heterogeneity due to irregular tree cover that promotes diversity of resources and habitats. Agricultural intensification threatens the fragile balance of these traditional systems. Thus, we aimed to assess which factors associated with trees, shrubs and pasture heterogeneity within wood-pastures influence the community composition and diversity of different groups of animal species. We selected 45 wood-pasture sites of 20 ha along a gradient of tree density in the Swiss Jura mountains. In each site, we used remote sensing data and agricultural inventories to characterize the tree, shrub and herb layers’ structural and compositional heterogeneity. In study sites we also conducted survey of birds and hoverflies (Syrphidae) and evaluated the relationships between species composition and diversity with environmental variables. Tree density and tree spatial distribution were the main drivers of differences in community specific composition for the two groups of species. For hoverflies, botanically rich pastures and elevation were also driving differences in species composition between communities. Increased tree density showed a generally positive effect on birds and Syrphidae diversity indices. Moreover, for birds, scattered trees’ spatial distribution and a high percentage of deciduous trees also promoted species diversity. In this study, we highlight the importance of trees in pastures for bird and hoverfly diversity, as well as the necessity for trees to be heterogeneously distributed across the pastures and composed of a mix of deciduous and coniferous species.