Discussion
Data from our agricultural field experiment show the importance of increased crop diversity in cropping systems for arthropod diversity and abundances. Independent of the sampling methods employed, we found that higher crop diversity increased flower visitor diversity under high intensity management (hypothesis 1). On the other hand, crop identity played an important role as well, as some mixtures were visited more frequently than others (hypothesis 2). In our study, especially linseed was visited often, in both mixtures and in monocultures. This suggests that when a mass flowering crop is available, this crop is more important than other crops (in our case faba bean) or weeds, masking their effects. Consequently, plant-flower visitor networks that we created were dominated by linseed; when this crop was available, most interactions were measured, most flower visitor species were found and therefore Shannon’s diversity of interactions was high. Weeds were significantly reduced under high-input management, and in these plots linseed played an even more important role as on plots without treatment. Increasing weed abundances on fields where no flowering crop is grown (i.e. wheat monocultures) can maintain flower visitor populations and ensure pollination services (Bretagnolle & Gaba 2015). As an alternative to accepting tolerable levels of weed densities in the field (Nicholls & Altieri 2013), intercropping could be integrated into conventional farming systems. Our predictions indicate that the number of flower visits could increase from about 4,700 visits/ha in wheat monocultures up to 566,000 visits/ha in wheat-bean-linseed mixtures. In low input systems, the number of visits could be as high as 1.5 million visits/ha in wheat-faba bean-linseed mixtures.
Previous studies showed that ecosystem service delivery is positively influenced by the richness of service-providing organisms like flower visitors (Dainese et al. 2019). Although honeybees (mainly the European honeybee Apis mellifera L.) are kept worldwide to provide crop pollination, other insects (such as wild bees, flies, beetles, wasps) contribute a lot more to total pollination than previously thought (Rader et al. 2016; Page et al. 2021). Floral abundance and richness are important for crop pollination services from unmanaged flower visitors (Kremen et al. 2007; Garibaldi et al. 2014), thus wild pollinator communities should be supported by increasing the floral abundance and richness in their environment. Obviously, monocultures of mass flowering crops such as linseed or oilseed rape can serve as a food resource for particular species (Bombus ) and for a limited period of time, but floral abundance can be increased using crop mixtures, too (comparing mixtures to cereal monocultures). The advantage of crop mixtures compared to a mass-flowering monoculture is that the monoculture (e.g. oilseed rape) has to be treated with higher amounts of fertilizers and pesticides. In mixtures, these amounts have to be reduced, especially if legumes are present. This could potentially lead to a system change in agriculture, leading away from intensively treated monocultures to less intensively treated mixtures. Pollinators, as mobile organisms, react to diversification at different spatial scales. Diversification methods have already been shown to benefit flower visitors and pollination services by enhancing floral diversity at the local scale (Garibaldiet al. 2014; Isbell et al. 2017). At the landscape scale, an increasing distance from a natural habitat leads to lower wild bee richness, visitation numbers and fruit set (Garibaldi et al.2011). Therefore, integrating flower-rich agricultural fields into conventional farming can provide important and well-connected resources for flower visitors as well as other arthropods.
Using yellow pan traps we showed that crop identity affected arthropod diversity and abundances. On oilseed rape monocultures and mixtures (especially wheat-oilseed rape and wheat-faba bean-oilseed rape mixtures) arthropod abundances were high, while plots including linseed attracted less individuals. These results can be explained by the poor crop performance of oilseed rape, which suffered from the late sowing date and only grew sparsely on some plots. Thus, plots with oilseed rape contained more bare ground and pan traps were easily visible, leading to a higher attraction than in e.g. linseed plots. While pan traps are suggested to be an efficient method for large-scale agricultural systems and to reduce collector biases (Westphal et al. 2008), they are group-specific and the catching success depends on color (Moreiraet al. 2016). Thus, for our plot-based trials observations seem more appropriate. The observed subplot represents a sufficient part of the whole plot and we were able to generate plant-flower visitor networks from qualitative observation data (Nielsen et al. 2011). On the other hand, observations are very time-consuming and can lead to biases when conducted by multiple people (Westphal et al. 2008). Using high resolution camera traps could be an efficient and standardized method to assess flower visits on multiple plots on the same time. When using high technology cameras or camera traps with adequate resolution, flower visitors can be determined up to species or family level (Droissart et al. 2021) with low sampling effort.
In conclusion, we suggest to integrate intercropping into agricultural systems, as we showed that arthropod diversity benefits from increased crop diversity, especially under high intensity management. Moreover, it is important to find suitable crop mixtures (ideally including mass-flowering crops) which provide important resources to arthropods.