Introduction
The current decline of insect abundance and diversity alerts ecologists
and the broad public worldwide (Hallmann et al. 2017, Sánchez-Bayo and
Wyckhuys 2019, Wagner et al. 2021). In particular, the loss of
pollinating insects has the potential to endanger the entire ecosystem
functioning at several trophic levels across ecosystems. Approximately
87% of all wild flowering plants depend on animal pollination (Ollerton
et al. 2011), therefore insect pollinators are essential for the
preservation of plant biodiversity (Fontaine et al. 2005, Biesmeijer et
al. 2006) and present an extraordinarily important economic factor
worldwide (Gallai et al. 2009).
The intensification of current agricultural practices is considered to
be one of the main driver for the loss of pollinator biodiversity and
abundances (Sánchez-Bayo and Wyckhuys 2019, Wagner et al. 2021). The
response of pollinators to land-use intensification should differ
between pollinator guilds, since taxa highly differ in their ecological
requirements and functional traits. Wild bees and hoverflies belong to
the main pollinator guilds in agricultural landscapes across different
habitats (Stanley and Stout 2013, Rader et al. 2020). Wild bees are
often considered to be habitat specialists due to their particular
nesting site requirements and their stationary foraging behaviour in
addition to their specialized resource uptake of nectar (Westrich 1996,
Johnson and Steiner 2010). Hoverflies, in contrast, are less specialized
in nectar uptake (Van Rijn and Wackers 2016), foraging across a wide
range of habitats and on much larger scales compared to wild bees
(Bankowska 1980, Power et al. 2016, Klaus et al. 2021). As a result,
hoverflies are regarded as generalists, which are less
susceptible to
land-use intensification than wild bees (Jaucker et al. 2009, Blaauw and
Isaacs 2014, Aguirre-Gutierrez et al. 2015). However, solid empirical
evidence is missing (e.g. Jauker et al. 2019) and a recent long-term
study reported a catastrophic decline of generalist hoverflies during
the past years in Central Europe (Hallmann et al. 2020). Despite recent
attempts, our understanding of how wild bees and hoverflies are affected
by different measures of land-use intensification is limited, which
hampers guidance for conservation measures and forecasting consequences
of pollinator losses (Senapathi et al. 2017, Rader et al. 2020).
Land use intensification leads to a higher coverage of arable fields
(Maskell et al. 2019). The current management regimes of these arable
fields include a high frequency of mechanical disturbance, the
application of pesticides and fertilizers. The resulting landscapes
barely offer value for pollinating insects as food resources or nesting
sites with the exception of short-flowering mass events (Riedinger et
al. 2014). As a result, pollinators are restricted to patches of
(semi-)natural habitats within the agricultural matrix. Therefore,
increasing amount of arable field coverage incorporates a reduction of
food supply and habitat loss, which hampers dispersal and
(re-)colonization of habitat patches. Consequently, this leads to a
decrease of pollinating insects like wild bees (Senapathi et al. 2017).
In contrast, some studies reported positive effects of arable field
cover on hoverflies in agriculture landscapes (Haenke et al. 2009,
Gabriel et al. 2010, Brandt et al. 2017). Though these mechanisms are
not fully understood, it can be expected that wild bees negatively and
hoverflies positively respond to arable field cover.
Moreover, land use intensification may cause a loss of landscape
heterogeneity (Maskell et al. 2019). The reduction of habitat diversity
at the landscape scale reduces the number of potential niches and food
resources, thus, landscape homogenization decreases species diversity
(Fahrig et al. 2011, Senapathi et al. 2017). Although landscape
heterogeneity and arable field cover may often negatively related to
each other (Tscharntke et al. 2012), high landscape heterogeneity may
compensate negative effects of arable field cover (Maskell et al. 2019).
However, it remains unclear how the effect of both parameters changes
with spatial scale and which is of greater importance for both
pollinator guilds (but see Maskell et al. 2019). Hoverflies may suffer
more from landscape homogenization, as they disperse across a wider
range of habitats compared to wild bees that forage nearby their nests.
Land-use intensification may reduce the habitat quality of pollinators.
Direct and indirect soil fertilization decreases overall plant species
diversity (Maskell et al. 2010, Borer et al. 2014), often accompanied
with a particular loss of forbs in grasslands (Maskell et al. 2010).
This decline in plant diversity is also found in the context of land
abandonment of unproductive habitats, such as dry grasslands, as a
consequence of land-use intensification and the (subsequent) cessation
of traditional land use practices (Habel et al. 2013). The decline of
plant diversity may have a negative effect on pollinator diversity,
since many pollinator species show a strong specialization towards
particular flower traits (Fenster et al. 2004, Fontaine et al. 2006,
Fornoff et al. 2017). However, rather than the taxonomic diversity of
plants per se , the functional diversity of flower traits should
positively affect pollinator diversity (Fontaine et al. 2006, Fornoff et
al. 2017). Moreover, particular flower traits that attract pollinators
in the landscape may increase the local pollinators. So far, detailed
analyses of flowering traits on pollinators are missing in the landscape
context, which is an essential part of how land-use intensification
affects local habitat quality for pollinators. Hereby, functional flower
diversity should have a stronger effect on wild bees, because they show
a stronger specialization to specific flower traits compared to
hoverflies (Johnson and Steiner 2000, Van Rijn and Wackers 2016).
Otherwise flower traits related to attractiveness should have a stronger
effect on hoverflies that migrate through the landscape.
In this study, we aim to reveal responses of two important pollinator
groups to different measures of land use intensification, in order to
get a better understanding of the underlying mechanisms of the current
pollinator loss and subsequent ecosystem functioning. As a study system,
we used isolated dry grassland patches that are embedded in an otherwise
intensively used agricultural landscape in NE Germany. We sampled bees
and hoverflies at 22 dry grassland patches within three sampling
campaigns using pan traps. Further, we quantified the local flowering
plant community at the time of sampling and estimated different measures
of local flower diversity and ‘attractiveness’. We determined arable
field cover and landscape heterogeneity on consecutive radii from 60m –
3000m around the dry grassland sites, in order to reveal the ‘scale of
effect’ (Jackson and Fahrig 2015), i.e. the spatial scale at which the
predictor has the strongest influence on the response variable.
We hypothesize that
1) the proportion of arable field cover surrounding the dry grassland
sites has a negative effect on wild bees (species richness and
abundance) and a positive effect on hoverflies,
2) landscape heterogeneity has a stronger positive effect on hoverflies
compared to wild bees,
3) the spatial scale at which arable field cover and landscape
heterogeneity affect the pollinator guilds, is smaller for wild bees
than for hoverflies,
4) functional flower diversity positively affects wild bees in
particular and flower traits that are associated with ‘attractiveness’
positively affect hoverflies in particular.