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Arthropods and especially flower visiting insects provide a range of important ecosystem services. Since more than one third of the global food production comes from crops that depend on pollinators (Kleinet al. 2007), increasing cropping system diversity in space or time may help to balance environmental sustainability and agricultural production. Modern agricultural landscapes are often dominated by large expanses of crop monocultures (Eurostat 2018), where food or habitat resources for flower visiting arthropods are generally scarce (Nicholls & Altieri 2013). Some mass flowering crops, such as oilseed rape (canola) or sunflower, can support some flower visitor species, but only for limited time periods (Westphal et al. 2003) and without providing safe sites for reproduction and hence population growth. Herbicide application removes weeds efficiently from agricultural fields and thus leads to clean landscapes where only a fraction of species can survive due to habitat and resource losses (Nicholls & Altieri 2013). Input of other agrochemicals, such as fertilizers, insecticides and fungicides increase yield, but often at the expense of overall agrobiodiversity, potentially contributing substantially to recent insect declines (Benton et al. 2002; Tscharntke et al.2005; Dicks et al. 2021). There are, however, countermeasures focusing on the concept of sustainable intensification, a process (or system) where yields are increased without harmful environmental impacts. Integrated pest management and conservation agriculture (including diversified crop rotations) have been practiced for a long time already with mixed biodiversity benefits (Lichtenberg et al. 2017; Dainese et al. 2019; Beillouin et al. 2021). One approach to support biodiversity in agriculture is intercropping (Martin-Guay et al.2018; Wuest et al. 2021), where two or more crop species are grown on the same piece of land. As large expanses of cropland worldwide are dominated by cereal monocultures, growing mixtures of cereals with another crop (e.g. legumes) may have positive impacts on the quality of the matrix (Perfecto et al. 2009) in which natural habitats are embedded.
Intercropping has been shown to increase flower visitor or natural enemy abundance and diversity (Norris et al. 2018; Brandmeier et al. 2021) while at the same time enhancing yield stability and productivity (Yu et al. 2015; Raseduzzaman & Jensen 2017; Liet al. 2020) or reducing needs for chemical fertilizers when cereals are intercropped with legumes (Hauggaard-Nielsen et al.2008). Despite these benefits, intercropping has remained surprisingly unpopular in industrialized countries, though it is widely used in low-input tropical agroecosystems (Hauggaard-Nielsen et al. 2009) and in traditional smallholder farming systems in the Global South (Brooker et al. 2015). When implemented at larger scales in the landscape, intercropping may be an important measure to support arthropod populations by increasing the availability of food and nesting resources.
Flower visitor richness and species composition depend on the local plant community (Rotchés-Ribalta et al. 2018), and it was shown that not only crop diversity, but also crop identity affects arthropods (Meyer et al., 2019). Additional community attributes such as interaction network complexity can help to understand relationships between crop diversity and ecosystem functioning, such as pollination success or crop yield (Saunders & Rader 2019). Besides crop diversity and identity, a large body of literature has examined effects of management intensity (e.g. organic vs. conventional farming) on flower visitors. However, only few studies so far compared monocultures and mixtures under high vs. low input management (Brandmeier et al.2021).
In our experiment, we focus on biodiversity benefits from intercropping. We experimentally manipulated management intensity (high vs. low), crop diversity within the cropping system (crop monocultures, two- and three-species mixtures and, as control plots, plots where no crops were sown. These plots in particular were overgrown by weeds, depending on the application of herbicides as part of the management treatment) and crop identity (wheat, faba bean, linseed, oilseed rape) in a series of intercropping trials to test effects of increasing crop diversity and different crop identities in monocultures and mixtures on arthropod and especially flower visitor diversity and community structure. We expect higher arthropod diversity and higher arthropod abundance with increasing crop diversity (hypothesis 1). We assume that some crops are more appealing than others, due to floral resource provisioning and differences in growth habit, leading to higher diversity and abundances in these plots and therefore more complex plant-flower visitor networks (hypothesis 2).