Main
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).