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.