Agricultural practices shape beetle communities in field margins directly and through indirect effects driven by plants

The presence of field margins in agricultural landscapes increases plant and animal diversity, providing food resources and shelter. Margins also increase ecosystem services’ delivery by hosting a diverse insect fauna. Beetles, in particular, are important contributors, acting as auxiliaries, decomposers and pollinators. However, these habitats are threatened by agricultural intensification, representing potential traps for biodiversity due to pesticide and fertilizer drift. In this work we aim at understanding the unintended effects of agricultural practices on beetle diversity, abundance and composition in agricultural field margins, at a national scale. More precisely, we investigated which agricultural practices directly impact beetles and which ones indirectly impact them through their effects on plants. Here we used data from a national standardized monitoring network in metropolitan France where both plants and beetles were identified to the species level in more than 300 different field margins, representing the main crops (cereals, vineyards and market gardening) and a gradient of agricultural intensification practices at the national level. We ran structural equation modeling (SEM) to disentangle the direct and indirect effects of management on beetles. We showed that intensive agricultural practices such as high pesticide pressure and fertilization are mostly associated with a decrease in beetle diversity and abundance, as well as in the proportion of floral visitors, while increasing the proportion of pests in each community. We also showed that many of these impacts are mediated by plant diversity and traits. These results suggest that managing composition and structure of field margin vegetation, as well as reducing agricultural intensification both at the regional and at the field-margin level, are key to guarantee high beetle diversity and optimal ecosystem services such as pollination and pest regulation in the field margins.