Mechanical management decreases arthropod biomass by changing vegetation structure in fallow fields of high conservation value.

Arthropods play key roles in ecosystems as pollinators or as food resources for many birds. The decline in arthropods in farmland due to agricultural intensification is related to negative population trends in farmland birds. Semi‐natural areas such as fallow land are valuable habitats for arthropod communities in farmland, but the potential of these areas to boost biodiversity greatly depends on their management.We used a field experiment to explore the mechanisms behind the effects of mechanical management on arthropod communities in high conservation value fallow land. We used GLMMs to explore changes in arthropod abundance after treatment application, and pSEM to discriminate direct effects of treatments from indirect effects mediated by changes in vegetation structure.Tillage had stronger negative effects than vegetation shredding on total arthropod, spider and bee abundance, which were mediated by a reduction in vegetation height, green cover and flowering. Coleoptera biomass did not vary between treatments.The differences between treatments disappeared from 2 to 3 months after treatment application for total arthropod and spider biomass, but not for Orthoptera and bees.Synthesis and applications. Low‐intensity mechanical management of fallows has short‐term negative effects on arthropod abundance. For conservation purposes, fallow management should combine untreated fallow fields (or strips within fields) to boost arthropod communities, with low‐intensity management to create suitable breeding habitats for steppe birds. [ABSTRACT FROM AUTHOR]