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\begin{document}

\title{Pollinator guilds respond contrastingly at different scales to landscape
parameters of land-use intensity}



\author[1]{Kolja Bergholz}%
\author[1]{Lara-Pauline Sittel}%
\author[1]{Michael Ristow}%
\author[1]{Florian Jeltsch}%
\author[2]{Lina Weiss}%
\affil[1]{University of Potsdam Institute of Biochemistry and Biology}%
\affil[2]{University of Potsdam}%


\vspace{-1em}



  
  \date{May 19, 2024}


\begingroup
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\selectlanguage{english}
\begin{abstract}
Land-use intensification is the main factor for the catastrophic decline
of insect pollinators. However, land-use intensification includes
multiple processes that act across various scales and should affect
pollinator guilds differently depending on their ecology. We aimed to
reveal how two main pollinator guilds, wild bees (specialists) and
hoverflies (generalists), respond to different land-use intensification
measures, i.e. arable field cover (AFC), landscape heterogeneity (LH)
and functional flower composition of local plant communities as a
measure of habitat quality. We sampled wild bees and hoverflies on 22
dry grassland sites within a highly intensified landscape (NE Germany)
within three campaigns using pan traps. We estimated AFC and LH on
consecutive radii (60-3000m) around the dry grassland sites and
estimated the local functional flower composition. Wild bee species
richness and abundance was positively affected by LH and negatively by
AFC at small scales (140-400m). In contrast, hoverflies were positively
affected by AFC and negatively by LH at larger scales (500-3000m), where
both landscape parameters were negatively correlated to each other. At
small spatial scales, though, LH had a positive effect on hoverflies
abundance. Functional flower diversity had no positive effect on
pollinators, but conspicuous flowers seem to attract abundance of both
guilds. In conclusion, landscape parameters contrarily affect two
pollinator guilds at different scales. The correlation of landscape
parameters may influence the observed relationships between landscape
parameters and pollinators. Hence, effects of land-use intensification
seems to be highly landscape-specific.%
\end{abstract}%



\sloppy


\textbf{Pollinator guilds respond contrastingly at different scales to
landscape parameters of land-use intensity}

Authors: Kolja Bergholz*\textsuperscript{, 1, 2}, Lara-Pauline
Sittel\textsuperscript{1, 3}, Michael Ristow\textsuperscript{1}, Florian
Jeltsch\textsuperscript{1, 2}, Lina Weiss\textsuperscript{1, 2, 4}

* corresponding author, bergholz@uni-potsdam.de, 0331 - 977 256231,
ORCID: 0000-0003-4443-3214

\textsuperscript{1} University Potsdam, Plant Ecology and Nature
Conservation, Am M\selectlanguage{ngerman}ühlenberg 3, 14467 Potsdam, Germany

\textsuperscript{2} Berlin-Brandenburg Institute of Advanced
Biodiversity Research (BBIB), Berlin, Germany

\textsuperscript{3} Museum of Natural History Potsdam, Breite Straße 11/
13, 14467 Potsdam, Germany

\textsuperscript{4} Bundesamt für Naturschutz (BfN) Alte Messe 6, 04103
Leipzig, Germany

\section*{Abstract}

{\label{abstract}}

Land-use intensification is the main factor for the catastrophic decline
of insect pollinators. However, land-use intensification includes
multiple processes that act across various scales and should affect
pollinator guilds differently depending on their ecology. We aimed to
reveal how two main pollinator guilds, wild bees (specialists) and
hoverflies (generalists), respond to different land-use intensification
measures, i.e. arable field cover (AFC), landscape heterogeneity (LH)
and functional flower composition of local plant communities as a
measure of habitat quality. We sampled wild bees and hoverflies on 22
dry grassland sites within a highly intensified landscape (NE Germany)
within three campaigns using pan traps. We estimated AFC and LH on
consecutive radii (60-3000m) around the dry grassland sites and
estimated the local functional flower composition. Wild bee species
richness and abundance was positively affected by LH and negatively by
AFC at small scales (140-400m). In contrast, hoverflies were positively
affected by AFC and negatively by LH at larger scales (500-3000m), where
both landscape parameters were negatively correlated to each other. At
small spatial scales, though, LH had a positive effect on hoverflies
abundance. Functional flower diversity had no positive effect on
pollinators, but conspicuous flowers seem to attract abundance of both
guilds. In conclusion, landscape parameters contrarily affect two
pollinator guilds at different scales. The correlation of landscape
parameters may influence the observed relationships between landscape
parameters and pollinators. Hence, effects of land-use intensification
seems to be highly landscape-specific.



\section*{Keywords}

{\label{keywords}}

\section*{wild bees, hoverflies, syrphids, landscape homogenization,
plant functional
trait}

{\label{wild-bees-hoverflies-syrphids-landscape-homogenization-plant-functional-trait}}

\section*{Introduction}

{\label{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 \emph{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.

\section*{Methods}

{\label{methods}}

\textbf{Study area}

This study was conducted in the north-eastern part of the federal state
of Brandenburg in Germany (AgroScapeLab,
http://www.zalf.de/de/struktur/eip/Seiten/AgroScapeLab.aspx, 52°52'N --
53°23'N, 13°20'E -- 14°12'E). The study area is located at the
transition zone of the west-European oceanic and the east-European
continental climate and is characterized by a temperate climate (8.6°C)
with an annual precipitation of 563 mm. The region is sparsely populated
and a typical Central European agricultural landscape, to a great extent
intensively used for agriculture (\textasciitilde{} two thirds of the
area, Fig. 1). The dominant crop types are wheat, barley, maize and rape
seed. The remaining area is mainly covered by forests and (mostly
intensively managed) grassland. Dry grasslands are found mainly on hills
and slopes or former military areas and make less than one percentage of
the land cover. They are remnants of the former extensive farming system
of sheep grazing, and today sheep or cattle grazing and mowing is used
to preserve some of the remaining patches. The sampled dry grassland
plant communities belong to the class \emph{Festuco-Brometea} with some
elements of the class \emph{Koelerio-Corynepheretea} , which developed
under the constant land use of humans as pastures for several hundred
years. Dry grassland patch sizes vary between 270m² and 100.000m², with
a median of 5600m².

\textbf{Sampling design}

At 22 dry grassland sites (Fig. 1), three pan traps (yellow, blue,
white; 19,6 x 15,4 cm with a 300ml 8\%-Formaldehyde-water-dish
wash-solution) were attached to sticks approximately 40cm above the
ground in a triangle 1m wide triangle. The traps were installed for
three sampling campaigns in 2017: May (15/5-18/5/2017), June
(12/6-15/6/2017) and August (15/8-18/8/2017), located at representative
sites of the patches. Each trap stayed for 48 hours in the field within
each sampling campaign. The weather conditions were sunny and dry
throughout sampling campaigns.

Specimens were needled and determined to species level using Amiet et
al. (1999-2017) and Scheuchl (2000-2006) and van Veen (2010) and Bot and
van de Meutter (2019) for the hoverflies.

\textbf{Flowering plant sampling}

During the three sampling campaigns, we recorded all flowering forb
species (henceforth plant species) nearby the pan traps, in order to
characterize the local plant community, respectively the nectar supply
for the pollinators. For this purpose, we placed a circle (r=5m) around
the traps with eight equally sized segments (`pie slices'). For each
segment, we recorded the presence of flowering forb species (excluding
wind-pollinated forbs \emph{Sanguisorba minor} and \emph{Plantago
lanceolata} ), in order to have a measure of local abundance for the
forbs (ranging from zero to eight). In orientation to Fornoff et al.
(2017), we gathered functional flower traits of the recorded forbs that
are considered to be relevant for pollinators from Biolflor Database
(Klotz et al. 2002) and Jäger (2016): UV radiation {[}a,b{]}, UV
reflectance {[}numeric 1-6{]}, color {[}categorical: yellow, red, blue,
white, rose, purple, violet{]}, flowering height {[}continuous{]} and
nectar access {[}categorical: open, half-open, hidden{]} (Supplementary
information, Appendix 1). We hypothesized that functional diversity of
these traits positively affect pollinator diversity (hypothesis 4).
Further, we predict that a higher share of yellow flowers attract a
higher number of pollinators, since this color is preferred by numerous
bee and hoverfly species (Leong and Thorpe 1999, Lunau 2014). A larger
flowering height is supposed to attract more pollinators, first, because
of the generally high conspicuousness of tall floral units, second,
because of the ''effective pollination'' hypothesis, according to which
tall stature resulting from strong apical dominance attracts greater
pollinator visitation, thus allowing larger pollen loads (Donelly et al.
1998).

\textbf{Determination of landscape parameter}

Both landscape parameters, cover arable fields and landscape
heterogeneity, were determined on the basis of the biotope mapping of
the federal state Brandenburg
(lfu.brandenburg.de/lfu/de/aufgaben/natur/biotopschutz/biotopkartierung/).
The map distinguishes a large variety of habitats. However, for this
study, we used only the twelve main habitat categories: arable fields
(64\% cover within the study area), forests (13\%), grasslands (11\%),
swamps (2\%), built-up areas (3\%), standing waters (3\%), anthropogenic
immature soils (2\%), deciduous copse and avenues of trees (1\%), parks
and cemeteries (1\%), dwarf shrub heaths (\textless{}1\%), streaming
water including shores (\textless{}1\%) and special biotopes
(\textless{}1\%). For small parts of our study area in the federal state
Mecklenburg-Vorpommern, we conducted a biotope mapping by ourselves with
the help of aerial images. Arable field cover is defined as the
percentage of arable field cover around the traps for a specific radius.
Landscape heterogeneity is defined as the Shannon-Diversity of main
habitat types weighted by their coverage (Maskell et al. 2019). We
calculated both landscape predictors for continuous radii from 60m --
3000m. Cover of arable fields and landscape heterogeneity were shown to
have a scale-dependent correlation (Supplementary information, Appendix
2). At small scales (\textless{}500m) no significant clear correlation
was found, whereas at larger scales (\textgreater{}500m) both measures
tended to be negatively related to each other.

\textbf{Statistical analyses}

We analyzed the effect of landscape parameters (arable field cover and
landscape heterogeneity) and different measures of the local flowering
plant community on wild bee and hoverfly species richness and abundance,
i.e. number of caught individuals, of a respective site. \emph{Apis
mellifera} , the European honey bee, was excluded from all analyses.
Arable field cover, landscape heterogeneity and plant community
attributes were investigated in separate models, but the overall model
structure was the same. We used GLMMs with poisson distribution and a
log-link function for both response variables species richness and
abundance (\emph{glmer} , R-Package lme4, Bates et al. 2020). As
covariates, we included sampling campaign as categorical fixed-effect
and study site as random-effect to account for the nested design of the
study. About 92\% of all hoverfly individuals (n=214) were caught in the
third sampling campaign and 3/4 of the sites within the first two
campaigns did not contain any hoverflies, which is a typical pattern for
agrarian landscapes (Brandt et al. 2017). Due to this highly unequal
distribution across the sampling campaigns, we restricted our analyses
of of local plant attributes on hoverfly species richness and abundance
for the third sampling campaign. For these, we used GLMs with poisson
error for species richness and negative binomial for abundance without
any covariates, as we had no pseudo-replication in this dataset. We
assured that model assumptions (normality and over-/underdispersion of
residuals, heteroscedasticity, spatial autocorrelation of response
variables and model residuals and zero-inflation) were not violated with
R-package DHARMa (Hartig and Lohse 2020).

In order to reveal the effects of arable field cover and landscape
heterogeneity on pollinators (hypothesis 1 and 2), we determined the
scale of effect, i.e. at which spatial scale a landscape parameter has
the largest effect on the response variable (Jackson and Fahrig 2015,
hypothesis 3). For this purpose, we used the \emph{multifit} -function
of Huais (2018), which compares a series of models that differ solely in
the scale a landscape parameter was quantified (see the specific model
structure above). In our case, we compared models of arable field cover
and landscape heterogeneity that were quantified at radii between 60m
and 3000m. Landscape predictors were standardized (z-scaled) for each
radius in this analysis that parameter estimates are comparable
(Schielzeth 2010). The model with the lowest AIC was considered to be
the best model. Further, we assured that the natural distribution of dry
grasslands in clusters did not affect our results due to possible
pseudoreplication of the landscape parameters (Supplementary
information, Appendix 3). Additionally, we assessed whether the
predictors had a statistically clear effect \emph{sensu} Dushoff et al.
(2019).

We predicted that functional diversity of flowers and traits associated
with attractiveness have a positive effect on pollinators (hypothesis
4). Functional diversity was estimated with Rao's quadratic entropy
(FD\textsubscript{trait}) of the different traits (see above) and the
`attractiveness' with community weighted mean (CWM\textsubscript{trait})
(Fornoff et al. 2017). The frequency that a forb species occurred within
the eight segments of the vegetation survey was used as abundance
measure for the weighting of forb species. In a first step, we checked
for possible correlations between FD\textsubscript{trait,},
CWM\textsubscript{trait} and number of flowering forb species
(Supplementary information, Appendix 4). Due to multiple correlations,
we included only parameters that were not strongly related to each other
(\textbar{}r\textbar{}\textless{}0.6): number of flowering forbs,
FD\textsubscript{flowering height,}, FD\textsubscript{color,},
FD\textsubscript{nectar access,}, FD\textsubscript{UV reflectance,} and
CWM traits that should attract pollinators, CWM\textsubscript{color
yellow} (percentage of yellow flowering species) and
CWM\textsubscript{flowering height} in our full model. In order to find
the most likely parameter combination of local plant community
attributes that explain pollinator richness and abundance, we applied an
information theoretic approach and compared all possible submodels
derived from the full model with AIC\textsubscript{c} (Burnham and
Anderson 2002). Since several models performed equally well, we
performed model averaging over the best models (delta
AIC\textsubscript{c} \textless{}6, Harrison et al. 2018), in order to
get more reliable parameter estimates (Dormann et al. 2018). Local plant
community attributes were not correlated to the landscape predictors at
any scale. All analyses were carried out in R Version 4.1.0 (R Core Team
2021).

\textbf{Results}

We caught in total 1419 individuals of 79 wild bee species,
excluding\emph{Apis mellifera} (honey bee). Most bee individuals were
caught in May (n=611) followed by June (n= 498) and August (n=310).
Hoverflies were predominantly (92\%) caught in August with 214
individuals of 21 species in total.

We observed 121 flowering animal-pollinated plant species (Table S1,
Online Appendix 1). Flowering plant species richness ranged from one to
22 species near the pan traps with the highest number in June (mean±SD
=11.55±5.4) followed by August (9.05±4.75) and May (7.41±3.45).

We found contrasting effects of arable field cover and landscape
heterogeneity on both pollinator guilds (Fig. 2). Arable field cover
negatively affected wild bee species richness. This effect was
statistically clear (p\textless{}0.05) at small to intermediate spatial
scales (140-400m) and peaked around 200m. Hence, dry grasslands that
feature high proportion of arable fields in the surrounding show on
average less wild bee species. In contrast, hoverfly species richness
and abundance were positively affected by arable field cover at much
larger scales (500-3000m), supporting hypothesis 1.

Landscape heterogeneity positively affected wild bees (Fig. 2, 3).
Similar to the effect of arable field cover, the scale of effect for
species richness had a peak at intermediate spatial scales (580m).
Hoverfly species richness and abundance were negatively affected from
intermediate to large spatial scales (\textasciitilde{}500m -- 3000m).
On these scales arable field cover and landescape heterogeneity were
negatively related to eachother (Supplementary information, Appendix 2).
At small spatial scales (\textasciitilde{}120m), landscape heterogeneity
had a statistically clear positive effect on hoverfly abundance,
indicating that a heterogeneous environment in the vicinity of dry
grasslands increase the abundance of hoverflies. Overall, we found no
support that landscape heterogeneity particularly enhance hoverflies
compared to wild bees (hypothesis 2). Yet, our scale-crossing analyses
showed that wild bees were mostly affected on smaller spatial scale
compared to hoverflies (with the exception of the positive effect of
landscape heterogeneity on abundance) supporting hypothesis 3.

We predicted that functional flower diversity in particular positively
affects wild bees and flower traits associated with `attractiveness' has
a positive effect on hoverflies (hypothesis 4). Species richness of both
guilds was not affect by any parameter of the plant community, except a
negative effect of FD\textsubscript{nectar access} on hoverflies (Fig.
4). Abundance of wild bees and hoverflies was positively affected solely
by CWM\textsubscript{flower height}. However, we observed also negative
effects of plant community attributes on wild bee abundance
(FD\textsubscript{flower height}, FD\textsubscript{nectar access},
CWM\textsubscript{color yellow}) and hoverfly abundance
(FD\textsubscript{nectar access}). In summary, we found no evidence that
functional diversity enhance pollinators, but the effect of
CWM\textsubscript{flower height,} indicate that plant communities with
conspicious flowers may attract pollinators.

\section*{Discussion}

{\label{discussion}}

The catastrophic decline of pollinators and other insects calls for a
thorough understanding of the underlying mechanisms to provide measures
for nature conservation and mitigate losses of ecosystem services. In
this study, we showed that two important pollinator guilds of Central
Europe responded differently to parameters of land-use intensification
and at different spatial scales. Further, our study indicates that
functional diversity of flower traits as a measure for habitat quality
seems to have no particular positive effect on pollinators, highlighting
the role of landscape processes to maintain pollinator diversity.

Arable field cover

Arable field cover had a negative effect on wild bee species richness
and abundance supporting previous studies that show negative effects of
land use intensification on local pollinator diversity in agricultural
landscapes (Senapathi et al. 2017). Hoverflies were, in contrast to wild
bees, positively affected by the cover of arable fields, indicating that
dry grasslands surrounded by a high share of arable fields, have a
higher hoverfly species richness and abundance. Although this positive
effect seems counterintuitive at first sight, similar observations were
made for agricultural fields and flower strips (Brandt et al. 2017,
Haenke et al. 2009, Gabriel et al. 2010). Since hoverflies are highly
mobile, they may be attracted by specific habitats, if the landscape
offers no food resources (Haenke et al. 2009). As a result, hoverflies
may accumulate on dry grasslands with a high proportion of arable field
cover. Under this consideration, the observed `positive' effect of
arable field cover on hoverfly diversity indicates simply a limitation
(and concentration) of food resources within the whole landscape (Haenke
et al. 2009). In summary, our study demonstrates that arable field cover
is an important predictor for pollinators that affect both guilds
contrastingly not only in agricultural ecosystems (e.g. Brandt et al.
2017), but also in (semi-) natural habitats that present `biodiversity
hotspots' in Central Europe (Habel et al. 2013).

Landscape heterogeneity

Landscape heterogeneity should have a positive effect on pollinators, as
heterogeneous landscapes provide more niches with a higher diversity of
food resources and nesting sites (Fahrig et al. 2011, Hopfenmüller et
al. 2014). We predicted that generalist hoverflies benefit more from
landscape heterogeneity compared to wild bees, as they forage across a
wider range of habitats (H2). In our study, wild bees were positively
affected by landscape heterogeneity at intermediate spatial scales
(340-780m). Similar, hoverfly abundance was positively affect at small
scales (100-140m). However, at large scales (\textless{}750m), landscape
heterogeneity had continuously a negative effect on hoverflies. We
expect that the observed negative relationship of hoverflies to
landscape heterogeneity is primarily driven by hoverfly responses to a
limitation of other resources in the landscape (see above), since both
landscape heterogeneity and arable field cover are negatively related to
each other at large scales. Hence, positive effects of landscape
heterogeneity may only be important for hoverflies, if landscape
heterogeneity is uncorrelated to arable field cover (as in our study for
small spatial scales). In conclusion, we found no support for the
hypothesis 2, which may be reasoned by specifics of our landscape,
though negative correlations between arable field cover and landscape
heterogeneity should be present in many areas worldwide (Tscharntke et
al. 2012).

\textbf{Scale-dependency}

We predicted that wild bees are affected on smaller scales compared to
hoverflies (H3). Landscape heterogeneity and arable field cover affected
wild bees at spatial scales that correspond to maximal foraging
distances from the nest of small bees (140m -- 350m, Wright et al.
2015). In contrast, hoverflies that `migrate' through the landscape were
affected at much larger spatial scales (\textgreater{}750m, with the
exception of the positive effect of landscape heterogeneity on hoverfly
abundance, see below). Taking together, these results support our
hypothesis and indicate that the scale of effect, i.e. at which spatial
scale has a landscape parameter the largest effect on a response
variable, depends on the foraging behavior of pollinators. However, we
detected other scales of effect than previous studies. For example,
Meyer et al. (2009) found the strongest (positive) effect of landscape
heterogeneity on hoverfly richness in calcareous grasslands on 250m,
while Földesi et al. 2015 observed that landscape heterogeneity
positively affected hoverfly species richness at smaller spatial scales
(300m) compared to wild bees (500m). These deviations demonstrate that
the scale of effect may be primarily driven by the landscape context
rather than the ecological traits of the species (Galán-Acedo et al.
2018), making it tricky to deduce the scale of effect for other
landscapes. Moreover, our study shows that even the direction of
landscape effects may change with spatial scale. As outlined above, we
assume that the negative effect of landscape heterogeneity on hoverfly
abundance arises due to a negative correlation with arable field cover.
Therefore, it seems likely that the scale of effect and even the
direction of landscape parameter effects is driven by correlations
between landscape parameters that are associated with different
processes and change with scale. Therefore, we see a strong need to
thoroughly analyze and report correlations of possible confounding
landscape predictor across scales, in order to better understand the
underlying mechanisms of how the scale of effect and direction of
landscape effects arise.

\textbf{Functional flower traits}

The outstanding diversity of morphological and coloration traits in
animal-pollinated flowers is one of the most recognized examples for
niche differentiation in animal communities in ecology. Hence,
functional diversity of flowers is considered to positively affect
pollinator diversity and \emph{vice versa} (e.g. Fontaine et al. 2006,
Fornoff et al. 2017). However, we found no evidence that neither
functional flower diversity nor species richness of flowering plants
positively affected both pollinator guilds. This is indeed surprising,
since we investigated a strong gradient from one to 22 flowering plants
in our study sites. Similarly, to our study, Fornoff et al. (2017)
neither found strong positive effects of functional flower diversity on
pollinator species richness in experimental plant communities of the
size 1m². These plant communities were set even in the same landscape
context and therefore local effects should appear more clearly, compared
to our study. Possibly, the flower diversity at a rather small scale (in
our case study 10m diameter circle) may play a minor role for pollinator
diversity, as they are highly mobile and may search nectar resources
over large distances of several hundred meters (see above). A valid
estimation of functional flower diversity at such scales is hard to
achieve, but may be indicated by the positive effect of landscape
heterogeneity on both pollinator guilds in our study (see above).
Alternatively, diversity of flower traits is negligible in our system,
as species are less specialised on particular flower traits than
expected. Only six out of the 80 caught wild bee species are listed as
oligolectic in our data set (Westrich 2019). While a previous study
within our study area observed a much higher share of oligolectic
species (Saure and Berger 2006: 28 out of 161), our pollinator community
seems to become less specialized as a consequence of possible
fragmentation and land-use intensification (Jauker et al. 2019). Hereby,
the observed negative effects of functional flower diversity on
pollinator abundance may indicate that particular flower types are not
accessible for the present pollinator species, as specialized species
disappeared. Under such circumstances, the quantity of few plant species
with high floral rewards rather than the diversity of flowers may
maintain pollinator diversity (Bergamo et al. 2020), as indicated by the
positive effect of CWM\textsubscript{flower height} on pollinator
abundance, since large plants produce more flowers and are more
attractive for pollinators (Donnelly et al. 1998).

\textbf{Landscape context and pollinator composition}

The specific landscape context may modulate the observed responses of
pollinators. Although we are confident that similar findings (and
correlations between landscape predictors) can be expected in other
regions, we would like to highlight that our study area belongs to the
most intensified landscapes in Europe (for comparison: 39 \% of the
total land area of the EU is cropland, around our study patches mean of
60\%) with a long history of intensive fertilization and pesticides
input. Therefore, we assume that the past land-use intensification
already had a tremendous effect on the species pool in the area and the
species composition in our study is only a small subset of the species
pool of some decades ago. The comparison with previous studies in our
study area (Hahn 2002, Saure and Berger 2006) indicates that on the one
hand oligolectic bee species disappeared (see above), but also common
hoverflies. We observed remarkably low ratios between hoverflies and
wild bee individuals compared to studies of similar study systems (our
study 1419 wild bees vs. 214 hoverflies, ratio = 0.15; Mudri-Stojni\selectlanguage{polish}ć \selectlanguage{english}et
al. 2012: ratio 0.83, Jauker et al. 2009: ratio: 0.82, Jauker et al.
2019: ratio 0.95). In particular, generalist aphidophagous species
(e.g.\emph{Eupeodes corollae} , \emph{Sphaerophoria scripta} ) that
occur in high densities (e.g. Hahn 2002 for our landscape, Bankowska
1980), are underrepresented in our study, which concurs with the decline
of common hoverfly species (Hallmann et al. 2021). These deviations are
most likely a consequence of the high land-use intensity in our study
region, which is the main factor for the current insect decline (Wagner
et al. 2021) and question the hypothesis that hoverflies are less
vulnearble than wild bees.

\textbf{Conclusion}

In conclusion, we observed contrasting and scale-dependent responses of
wild bees and hoverflies to measures of land-use intensification, with
no particular effects of local flower diversity of plants. As a
consequence, pollination service on dry grasslands should change with
the surrounding landscape. In homogeneous landscapes with a high share
of arable field cover, insect pollination should occur less frequent,
due to lower numbers of wild bee individuals and species. Although
hoverflies concentrate particularly in these landscapes at dry
grasslands they cannot compensate absence of wild bees, as they are less
specialized to particular flower types and appear only in late summer in
reasonable amounts (see also Brandt et al. 2017). Moreover, the
comparison with historical data indicate that species composition and
abundance of both guilds already shrunk in our study area, presumably
due to high land-use intensification. In order to achieve a better
understanding of how land-use intensification affects pollinators, we
advocate to a) acknowledge that landscape effects may differ between
landscapes and b) analyze therefore possibly confounding landscape
parameters across scales.

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\textbf{Figure legends}

\textbf{Fig. 1} Overview of the study area and sampling design. a) Most
of the study area is used for agriculture fields (grey). Forests (dark
green) make up to 13\%. Grasslands (light green) are mainly intensified,
wet grasslands or fallows. b) For each sampling site the cover of arable
fields and landscape heterogeneity was calculated for different radii
(60m - 3000m) around the site. c) dry grasslands are found mainly on
smaller hills or slopes, often surrounded by arable fields.

\textbf{Fig. 2} Wild bee (a) and hoverfly (b) responses to arable field
cover (black) and landscape heterogeneity (red) across multiple scales
(60m-3000m). The graphs show the parameter estimates of the models for
both predictors on both response variables species richness (solid line)
and abundance (dashed line) for each specific radius. Thick lines refer
to models, in which the parameter had a statistical clear
(p\textless{}0.05) effect on the response variable. The triangles refer
to the scale of effect, i.e. the scale at which the landscape predictor
has the largest effect (lowest AIC, see Fig. 3).

\textbf{Fig. 3} Effect of arable field cover (a, b) and landscape
heterogeneity (c, d) on wild bee (a, c) and hoverfly (b, d) species
richness (SR). The graphs show the relationships at the scale of effect,
i.e. the radius at which the landscape parameter has the largest effect
on the response variable (compare Fig. 1). The different colors refer to
the three sampling campaigns. The dotted lines in the hoverfly plots
refer to analyses (GLM) that included only the third sampling campaign.

\textbf{Fig. 4} Effects of local plant community attributes on wild bees
(a) and hoverflies (b). The figure shows parameter estimates and
confidence intervals of the averaged models for species richness (black)
and abundance (grey). The local plant community was recorded during
pollinator samplings within a circle (r=5m) around the traps. Please
note that for hoverflies, only the third sampling campaign was analyzed
(see Methods). SR= Species richness; FD = Functional diversity; CWM =
community weighted mean.

\section*{Declarations}

{\label{declarations}}

\section*{}

{\label{section-1}}

\textbf{Data accessibility --} We will store all collected data at the
open research database of ZALF
(\emph{http://open-research-data-zalf.ext.zalf.de/default.aspx}), which
is a master data repositories of Web of Science. Data will be freely
available as soon as the manuscript is published. DOI will be
10.4228/ZALF + Suffix.

\textbf{Conflict of interest} -- The authors declare no conflict of
interest.

\textbf{Author contributions} -- The study was conceived by KB, LW, MR
and LS. Experimental design and data collection was carried out by LS
and KB. Specimens were determined by LS (wild bees) and MR (hoverflies).
KB analyzed the data and worte the first draft of the manuscript. All
co-authors discussed the findings and contributed during the writing
process.

\textbf{Acknowledgments}

We sincerely thank the ZALF research station in Dedelow for the
accomodation and the nice athmosphere. Further, we thank Jarro Heinsohn
for help during the data collection. The work was funded by the German
Federal Ministry of Education and Research BMBF within the Collaborative
Project ``Bridging in Biodiversity Science-BIBS'' (funding number
01LC1501).\selectlanguage{english}
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