Abstract
Agriculture has been one of the main drivers of global changes that led to the decline of biodiversity, such as land use changes, water pollution, eutrophication and soil degradation. In this context, it is urgent to develop sustainable ecological solutions to protect biodiversity while ensuring food security for populations. The study of field margins can help assess the impacts of agricultural practices on biodiversity, as they are directly affected by agricultural practices applied to adjacent cultivated fields, while providing refuge and dispersal corridors for many species. The objectives of this thesis were (i) to assess the taxonomic and functional response of field margin plant communities to climate, soil, landscape, and agricultural practices at different spatial scales (resolution and extent) and over time (a decade), and (ii) to better understand the relative contributions of practices and flora on beetle communities. This thesis relied on the 500-ENI network, which integrates agricultural data as well as annual surveys of flora and beetles in more than 500 field margins in France between 2013 and 2021. First, we showed that agricultural practices (herbicides, fertilization) had a negative impact on local plant richness, while climate and crop diversity affected richness at larger spatial resolutions (40 and 75 km). Species composition, in contrast, was influenced by climate at the local scale, and by fertilization and crop diversity at broader resolutions. Agricultural effects were more easily detected at a regional rather than national extent and varied according to the biogeographic region. Over a decade, we demonstrated that climate trends confirmed the expectations of ongoing climate change (increasing temperatures, decreasing soil moisture), while the intensity of agricultural practices did not changed over this period (various changes in practices, but no temporal trend). We were thus able to observe changes in the functional composition of plant communities in response to climate change, through selection of species adapted to high temperatures and drought. Due to functional trade-offs, these species were also more sensitive to intensive agricultural practices (conservative strategy and stress-tolerance). Finally, we found that the impacts of tillage and pesticide use on beetle communities were mostly indirect, through an effect on vegetation. Linking the 500-ENI network to a local vineyard network revealed that agricultural impacts differed according to beetle ecological group and diet. Agricultural disturbances were overall detrimental but were also favorable to some groups, such as predators. Spontaneous vegetation managed by grazing was the most beneficial practice for species diversity and food regimes. This thesis calls for reconsidering the importance of agricultural effects at a broader scale, but also for adapting the regulatory framework to regional specificities. The maintenance of intensive agriculture combined with current climate change represents a major risk to the functional diversity of floristic assemblages. Some management practices, such as no-till and organic farming, could help preserve more diverse plant and beetle communities and the ecosystem services they provide.