Agricultural intensification has profoundly affected European farmlands in the last decades, and their associated biodiversity has undergone a widespread decline. Although largely considered by farmers to be a threat to crop productivity, weeds and arthropods are key factors in farmland ecosystems and provide multiple ecosystem services. We analyze variations between three arable habitats—cereals, plowed and fallow fields—in the biomass of weeds, seeds and arthropods in an extensive rotation system of Central Spain. We also investigate the effect of the vegetation structure in the overall biomass of arthropods and that of the most abundant orders found, Coleoptera and Orthoptera, which constitute key resources for many declining farmland birds. Our results show that fallows had the greatest biomass of weeds and seeds, whereas no differences were found between cereal and plowed fields. Although vegetation structure was related to the biomass of total arthropods, as well as that of coleopterans and orthopterans of a large size, arthropod biomass did not vary between the three arable habitats. Overall, higher weed cover and denser vegetation favored higher biomass of arthropods. Our results suggest a process of ecological homogenization, where arthropod populations may be suffering a widespread decline across habitat types, likely due to high tillage frequency or agrochemical use, despite the extensive rotation system and the mosaic landscape of our study site. Short‐ and mid‐term fallows might function as a source habitat for weeds and arthropods, although this role might be compromised by their progressive disappearance and isolation within the landscape.