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Title: Massflowering crops increase richness of cavitynesting bees and wasps in modern agroecosystems

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GCB Bioenergy (2014) 6, 219–226, doi: 10.1111/gcbb.12080

Mass-flowering crops increase richness of cavity-nesting
bees and wasps in modern agro-ecosystems
€ T E R * , F R A N Z I S K A P E T E R * † , B I R G I T J A U K E R * , V O L K M A R W O L T E R S * and
*Department of Animal Ecology, Justus Liebig University, Heinrich-Buff-Ring 26-32, Giessen, D-35392, Germany,
†Deptartment of Conservation Ecology, Faculty of Biology, Philipps University, Karl-von-Frisch-Str. 8, Marburg, D-35032,

Considerable uncertainties exist on how increased biofuel cropping affects biodiversity. Regarding oilseed rape,
the most common biofuel crop in the EU, positive responses of flower-visiting insects to plentiful nectar and pollen seem apparent. However, previous investigations on this insect guild revealed conflicting results, potentially
because they focused on different taxonomic groups representing a narrow range of ecological traits and considered only short time periods. Here, using trap nests in landscapes with independent gradients in area of oilseed
rape and seminatural habitats, we assessed the whole community of cavity-nesting bees and wasps, including
early- and late-emerging species. Our study’s temporal resolution allowed determination of flowering and postflowering effects of oilseed rape on these species’ richness, abundance, and mortality. Species richness of cavitynesting bees and wasps significantly increased with oilseed rape, although nesting activity was considerably
higher after mass flowering. In addition to increasing richness independently of oilseed rape, the amount of
seminatural habitat in the landscape was the sole positive driver of insect abundance once the community’s
dominant species was accounted for as a covariate. Thus, growth of the co-occurring species’ community is not
stimulated by the resource pulse provided by oilseed rape early in the year, but by persistent resources provided by seminatural habitats after mass flowering. Early individuals of bivoltine species’ first generations accumulated in seminatural habitats when these habitats were scarce, but became increasingly diluted when habitat
availability increased. Once established, later foraging females generally benefited from the resource availability
of seminatural habitats when initializing the second generation. We conclude that mass-flowering crops, despite
covering only a short interval of the community’s main activity phase, benefit bee and wasp species richness.
However, seminatural habitats are crucial in maintaining viable communities of flower-visiting insects at the
landscape scale, mitigating potential negative effects of high land-use intensities in modern agro-ecosystems.
Keywords: agricultural landscapes, bioenergy, biofuel, canola, ecosystem service, environmental change, functional traits, oilseed rape, pollination, resource pulse

Received 27 March 2013; revised version received 27 March 2013 and accepted 28 March 2013

The rapid expansion of biomass and biofuel production
in agricultural systems will result in major land-use
changes at large spatial scales (Koh, 2007). In the EU,
oilseed rape is the most common oleaginous crop for
biofuel production (FAO, 2008) and the production area
has more than doubled within the past 20 years (Fig. 1).
However, significant uncertainties exist about the effects
of this extensive increase in biomass and biofuel cropping on biodiversity, especially at the regional scale (Eggers et al., 2009; Dauber et al., 2010). Here, to reduce
these uncertainties, we studied the diversity effects of
Correspondence: Tim Diek€
otter, tel. + 49-(0)641-9935701, fax + 49(0)641-9935709, e-mail: tim.diekoetter@uni-giessen.de

© 2013 Blackwell Publishing Ltd

the mass flowering of oilseed rape on the community of
trap-nesting bees and wasps in modern agro-ecosystems.
Considering the bounty of nectar and pollen that
mass-flowering crops supply, it seems plausible to
expect positive responses of flower-visiting insects to
oilseed rape at the landscape scale. Increased abundances of short-tongued social bumblebees with increasing area of oilseed rape early in the year seem to
confirm this assumption. This numerical increase in
worker bees, however, fails to translate into improved
sexual reproduction later in the year when food
resources are usually scarce in modern agro-ecosystems
(Westphal et al., 2003, 2009). Moreover, long-tongued
bumblebees were negatively affected by an increasing
area of oilseed rape. Once mass flowering had ceased,

€ T E R et al.
220 T . D I E K OT

Fig. 1 Rapeseed crop area harvested in the European Union
per year from 1993 to 2010. The time frame is defined by the
continuous data availability from all 27 member states, except
Greece, Malta, Portugal, and Cyprus, but may include semiofficial and estimated data. (source: FAOSTAT, 28 November

short-tongued bumblebees, in contrast to long-tongued
ones directly utilizing this resource early in the year,
increased nectar robbing on long-tubed plants, the legitimate resource of long-tongued flower visitors (Diek€
otter et al., 2010).
In contrast to social bumblebees producing only
worker bees early in the year, the direct production of
propagable females and males in solitary wild bees
argues for a positive effect of oilseed rape on the reproductive success. Such an effect has recently been shown
for the polylectic and phenologically early mason bee
Osmia rufa (Jauker et al., 2012). Considering this trait
specificity in pollinator responses, here, we were interested in the effect of mass-flowering oilseed rape at the
community level of cavity-nesting bees and wasps as
potential shifts in the structure of this community may
not only be of direct conservation concern but also constrain the buffering of ecosystem services (i.e., pollination or biological control) from environmental changes
(cf. Brittain et al., 2013).
According to analyses with high temporal resolution,
the overall positive effect of mass-flowering oilseed rape
on O. rufa’s reproductive success resulted from early
reproductive benefits that outweighed postflowering
disadvantages of increased spillover of parasites and
parasitoids in close proximity to oilseed rape fields (Jauker et al., 2012). Such negative postflowering effects may
be mitigated by flower-rich and continuous seminatural
habitats via diluting antagonist spillover or providing
habitat for hyperparasitoids (cf. Jauker et al., 2012).
Additional benefits of seminatural habitats might
include the provision of above-ground cavities or specific soil microhabitats for nesting (Cane et al., 2007;
Steffan-Dewenter & Schiele, 2008). However, seminatu-

ral habitats have become increasingly scarce in modern
agricultural landscapes (Potts et al., 2010). This shortage
might become more severe as the area of energy crops,
including mass-flowering crops increases with an
increasing demand for biofuel (Koh, 2007; FAO, 2008).
While the effects of seminatural habitats on cavitynesting bees and wasps in agro-ecosystems have been
investigated before (e.g., Holzschuh et al., 2010), we are
not aware of any study providing information on the
response in the structure of this complex community to
the increasing amount of mass-flowering crops in comparison to seminatural habitats. Specifically, we tested
whether (i) species richness; and (ii) abundance of cavity-nesting bees and wasps were associated with the
area of mass-flowering oilseed rape and seminatural
habitats. By separating the flowering and postflowering
phase of oilseed rape in our study, we were able to
attribute total community responses across the year specifically to species with an early or late phenology.
Because some species with an early phenology, i.e., the
first generations of bivoltine species, hatch from their
nests during the year, we also analyzed the relationship
between (iii) nests vacated in the field and mass-flowering crops or seminatural habitats. Finally, we analyzed
patterns of (iv) mortality in relation to availability of oilseed rape and seminatural habitats.

Materials and methods
Study sites
The study was carried out in the Nidda catchment in Central
Germany, an area dominated by farmland (~50%) and woodland (~30%) interspersed with settlements and seminatural
habitats (center: Echzell, 50°23′0″N, 8°53′0″E). In this area,
twelve spatially separate (minimum distance 5 km) habitat
elements that represent typical nesting sites for cavity-nesting
pollinators (i.e., shrubs, hedges, and forest edges) were selected
as study sites for trap nest location. Percentage area of oilseed
rape and percentage area of seminatural habitats were quantified around the study sites for eight radii (250, 500, 750, 1000,
1250, 1500, 1750, 2000 m; Table 1) based on an updated
ground-truthed digital map derived from high resolution
color-infrared aerial photographs (0.5 9 0.5 m) from 2005
using ArcMap 10 (ESRI, Redlands, CA, USA). No other massflowering crops were detected during the updating procedure.
Coverages of oilseed rape and seminatural habitats were uncorrelated at all scales (P 0.369). Seminatural habitats included
fallows, orchards, field margins, tree rows, hedges, shrubs, and
forest edges (width 10 m), which may provide nesting or foraging resources for the community of cavity-nesting bees and
wasps. In addition, the area of the study sites (320–
1 367 282 m²) and the distance between their trap nests and the
nearest oilseed rape field margin (0–480 m) were measured.
There were no significant correlations between these additional
variables and the percentages of oilseed rape or seminatural
© 2013 Blackwell Publishing Ltd, GCB Bioenergy, 6, 219–226

B E E A N D W A S P R I C H N E S S A N D M A S S - F L O W E R I N G C R O P S 221
Table 1 Percentage areas of oilseed rape and seminatural habitats in the landscape surrounding study sites for different radii
(scale: 250–2000 m). Given are mean values for eleven study
sites and minimum (min) and maximum (max) values
Area percentage
Oilseed rape (%)

Seminatural habitats (%)

Scale (m)














habitats (P 0.107), except between the distance to the nearest
oilseed rape field and the percentage of oilseed rape at the two
smallest scales for which, however, no variables were retained
in any model.

Trap nests
Two trap nests fixed on a wooden post 150 cm above the
ground were set up at the edge of each selected study site in
March 2008, 1 week before the flowering of oilseed rape. Each
trap nest consisted of two plastic tubes (length: 30 cm; radius:
5 cm) filled with internodes of common reed Phragmites australis Cav. (diameter: 2–10 mm). At the end of May 2008, after the
mass flowering had ceased, internodes with nests were partly
extracted from the plastic tube, individually marked with a red
permanent marker on the outside of the internode, and repositioned to enable discrimination between nests built during and
after mass flowering. The trap nests were collected in October
2008 and stored in a climate chamber (4 °C) until March 2009.
All solitary bees and wasps emerging from trap nests were
counted (‘abundance’) and determined to species level (‘species
richness’). Nests of bivoltine species’ early first generations
were vacated before the trap nests were returned to the laboratory and thus counted without further species determination or
abundance record (‘vacated nests’). Nests where eggs, larvae,
or pupae died before hatching were counted without further
species determination and abundance record. This nest number
in relation to the number of hatched nests per site constituted
the ‘mortality rate’.

Statistical analysis
Trap nests from one site were excluded from statistical analyses due to contamination by road and tire wear from a nearby
uphill motorway that prohibited proper colonization of their
reed internodes. For the remaining sites, data from both trap
nests were pooled per site. Because bee and wasp individuals
emerging from different brood cells of the same nest lack independence, we used the number of nests as sampling unit when

© 2013 Blackwell Publishing Ltd, GCB Bioenergy, 6, 219–226

testing for sampling effects on species richness (t = 1.7999,
df = 9, P = 0.105) and evaluating completeness of sampling.
Species-accumulation curves produced with R-function specaccum (method ‘random’) in the vegan package (Oksanen et al.,
2012) indicated that for various sites sampling was incomplete
(see Supporting Information). Consequently, we estimated species richness using the nonparametric estimator Chao 1 (Chao,
1984) with R-function estimateS in vegan (Oksanen et al., 2012):
SChao1 ¼ Sobs þ


where Sobs = the number of species per site; F1 = the number
of observed species represented by a single nest per site (singletons); and F2 = the number of observed species represented
by two nests per site (doubletons; Magurran, 2004).
As Osmia rufa exceeded accumulated abundances of all other
trap-nesting pollinators by 6.2 times on average, it was
excluded from all dependent variables, except (i) estimated
species richness. This allowed for a detailed analysis of otherwise blurred diversity responses in the co-occurring species.
Instead, the number of nests of O. rufa entered all models as an
additional explanatory variable. As this number of nests
strongly correlated with O. rufa’s abundance (t = 19.15, df = 9,
P < 0.001), i.e., the number of provisioned cells in these nests,
we thus accounted for potential competition of this dominant
species with the remaining pollinator community not only for
nesting but also for food resources. In addition, we correlated
O. rufa abundances across the year as well as during and after
mass flowering with those of all other bees and wasps in the
community using R-function cor.test. Following the exclusion of
O. rufa nests for calculation of dependent variables, we then
determined the number of (ii) emerging bee and wasp individuals for each site; (iii) vacated nests of bivoltine species’ first
generation; and (iv) mortality rates.
Prior to testing for landscape effects, each untransformed
response variable was correlated with the scale-dependent
landscape variables percentage area of oilseed rape and percentage area of seminatural habitat across all scales with Spearman Rank Correlations using R-function cor.test (cf. SteffanDewenter et al., 2002). For each response variable, landscape
variables were selected at the scale that yielded the highest rvalue for entering the full model.
Landscape variables at these selected scales were then used
for transforming dependent variables, applying R-function
boxcox in package MASS (Venables & Ripley, 2002) to meet
normality and homoscedasticity assumptions. In addition to
these explanatory variables of main interest, the model specified
in this transformation procedure also contained the three scaleindependent explanatory variables (1) distance to nearest oilseed rape field, (2) area of study site, and (3) the number of
nests of O. rufa. Mortality rate was transformed by taking the
arcsin of the square root of the value. Each of these transformed
dependent variables was then again correlated with the percentage areas of oilseed rape and seminatural habitats at all spatial
scales to determine the decisive spatial scale to enter full models
together with the three scale-independent explanatory variables
already used in the transformation procedure. Analyses were
conducted using generalized linear models (glm) and the

€ T E R et al.
222 T . D I E K OT
corrected Akaike information criterion (stepAICc) for stepwise
(direction = both) model selection in R 2.14.2 (R Development
Core Team, 2012). The model yielding the lowest AICc according to this procedure always had the lowest number of predictor variables and was selected as the final model. We used Rfunction lm on final models to retrieve adjusted R2 values.

Altogether, 1540 trap-nesting bees and wasps from 16
species were recorded. O. rufa was represented by 1208
individuals (78%). The remaining nine bee species contributed 247 individuals; six wasp species contributed
85 individuals. Neither across the year (t = 0.6034,
df = 8, P = 0.563) nor for the separate periods during
(t = 0.4999, df = 8, P = 0.631) nor after (t = 1.0756,
df = 8, P = 0.314) the mass flowering of oilseed rape
were abundances of O. rufa and the total of all other
trap-nesting bees and wasps significantly correlated. On
average, 66% 21 (standard deviation) of all individuals per site hatched from nests built during the mass
flowering of oilseed rape. However, early nests were
dominated by O. rufa. Once this dominant species was
excluded, 78% 31 individuals hatched from nests
built after the mass flowering of oilseed rape (332 individuals, Fig. 2).
From a total of 516 nests, hatching bees and wasps
were determined for 133 nests, 220 nests of bivoltine
species’ first generations were vacated during exposure
in the field without record, and bees or wasps died
before hatching in 163 nests. Based on the records from

133 nests with species identification, estimated species
richness of trap-nesting bees and wasps across the year
was significantly positively related to the percentage
area of oilseed rape at the 1750 m scale and seminatural
habitat at the 1500 m scale (Fig. 2a–b; Table 2). In absolute numbers, estimated species richness increased twofold from approximately three to six species with an
increase in oilseed rape from 4 to 15% and from 2.5 to 5
species with an increase in seminatural habitats from 2
to 14%. While both landscape variables were not significantly related to estimated species richness during the
mass flowering of oilseed rape, after the mass flowering
the percentage areas of oilseed rape and seminatural
habitats were significantly positively related to estimated species richness at the 2000 m and 1000 m scale,
respectively (Fig. 2c–d; Table 2). In absolute numbers,
observed species richness after the mass flowering of
oilseed rape increased from approximately two to five
species with an increase from 4 to 15% in oilseed rape
and from 2 to 17% in seminatural habitats.
Seminatural habitats were also positively related to
the abundance of trap-nesting pollinators across the
year at the 1250 m scale (Fig. 2e; Table 2) and after the
mass flowering of oilseed rape at the 1000 m scale
(Fig. 2f; Table 2).
The number of nests from which individuals of bivoltine first generations hatched in the field was significantly negatively related to the percentage area of
seminatural habitats at the 1750 m scale across the year
and during the mass flowering of oilseed rape (Fig. 2g–h;
Table 2).

Fig. 2 Percentage of individuals hatched during and after the mass flowering of oilseed rape per species. Species appear in descending order with regard to percentage of individuals hatched during mass flowering and in alphabetical order for species recorded only
after mass flowering. In addition to species’ names, their taxonomic assignment, abundance, and information on their phenology are
provided (? indicates insufficient data; III = March, IV = April, etc.; Bl€
uthgen, 1961; Westrich, 1989; Schmidt & Schmid-Egger, 1991).
© 2013 Blackwell Publishing Ltd, GCB Bioenergy, 6, 219–226

B E E A N D W A S P R I C H N E S S A N D M A S S - F L O W E R I N G C R O P S 223








Fig. 3 Relation between estimated species richness across the year (a, b) and after mass flowering of oilseed rape (c, d), abundance
of pollinators hatched across the year (e) and after mass flowering of oilseed rape (f), number of undetermined nests of bivoltine first
generations hatched in the field across the year (g) and during mass flowering of oilseed rape (h) with the percentage area of oilseed
rape and/or the percentage area of seminatural habitats. Because in final models for estimated species richness the percentage areas
of oilseed rape and seminatural habitats were retained as significant explanatory variables, residuals of regression models containing
only one of these variables were plotted against the unfitted variable when displaying the relationships between estimated species
richness and these significant landscape variables (a–d).

© 2013 Blackwell Publishing Ltd, GCB Bioenergy, 6, 219–226

€ T E R et al.
224 T . D I E K OT
Table 2 Generalized linear model results for four dependent variables. Explanatory variables, the corresponding spatial scale
(radius) and the adjusted R² are given for the most parsimonious model. Models were reduced using a stepwise (direction = both)
selection with AICc as a criterion to omit terms. For mortality rate, there was no significant final model





Radius (m)





Estimated species
richness (Chao1)


Oilseed rape
Seminatural habitat
Oilseed rape
Seminatural habitat
Seminatural habitat
Seminatural habitat
Seminatural habitat
Seminatural habitat







Vacated nests


There were no significant effects of any of our explanatory variables on the mortality rate (i.e., percentage of
nests where individuals died before hatching).

Here, we showed that species richness of the community of cavity-nesting bees and wasps comprising earlyand late-emerging uni- and bivoltine species increased
with the area of oilseed rape. Unlike the dominant wild
bee species O. rufa, the abundance of the community’s
subdominant species was not related to oilseed rape.
Congruent with previous studies, richness of bees and
wasps also increased with the area of seminatural habitats, as did abundance. This overall abundance effect of
seminatural habitats was dominated by the postflowering period, when most nests were built. This indicates
that mainly late univoltine bees and wasps and early
bivoltine species’ second generations benefited. The
decreasing number of vacated nests with a decreasing
proportion of seminatural habitats, in contrast, suggests
a dilution effect for early univoltine species and bivoltine species’ first generations.
In contrast to the positive effect on O. rufa (Jauker
et al., 2012), a polylectic and univoltine solitary wild bee
species with an early phenology utilizing oilseed rape
(Holzschuh et al., in press), the total abundance of the
remaining community of cavity-nesting bees and wasps
was not related to mass-flowering oilseed rape. Considering that the majority (78%) of individuals of these
remaining species in the community emerged from
nests that were built after the mass flowering of oilseed
rape had ceased, this seems less surprising. Given this
relatively later phenology of most recorded species,
however, the observed increase in species richness of
cavity-nesting pollinators with oilseed rape is unexpected. An explanation may be that, similar to the early
but univoltine O. rufa (Jauker et al., 2012), the bounty of

Adjusted R2
(whole model)

pollen and nectar resources provided by mass-flowering
oilseed rape benefited the first generation of bivoltine
species like O. caerulescens. These reproductive benefits
in landscapes with high proportions of oilseed rape
might then translate into enhanced abundances in this
species’ second generation, which may render species
usually difficult to detect more likely to occur in trap
nests. Yet, these increased abundances in subordinate
species may be too low to be detected statistically.
Alternatively, the increase may be compensated by
abundance declines in subdominant species so that –
besides on the dominant species O. rufa (cf. Jauker et al.,
2012) – an abundance effect of oilseed rape on the
remainder of the community of cavity-nesting bees and
wasps does not become apparent (cf. Arnan et al., 2011).
The possible mechanism causing the observed positive effect of oilseed rape on species richness may be a
competition cascade. Similar to ant communities (Arnan
et al., 2011), benefits of the dominant species from massflowering oilseed rape may result in increased competition (e.g., nesting resources, cf. Steffan-Dewenter &
Schiele, 2008) with the subdominant species. This in
turn may lead to a competitive release for the speciesrich subordinate compartment of the pollinator community. Because reductions in species number and/or
abundance of subdominant species are compensated by
an increase in the subordinate compartment, such a cascading effect would also explain the lack of correlative
patterns between abundances of O. rufa and the total of
the remaining cavity-nesting bee and wasp species (subdominant and subordinate species combined) at any
time. Considering previous results on O. rufa (Jauker
et al., 2012), however, adverse effects on bee and wasp
diversity could have also been expected from parasitoids benefiting from mass-flowering oilseed. Yet, due
to relatively small sample sizes, mortality of cells, which
was neither related to oilseed rape nor seminatural habitats, could not be differentiated into different processes,
© 2013 Blackwell Publishing Ltd, GCB Bioenergy, 6, 219–226

B E E A N D W A S P R I C H N E S S A N D M A S S - F L O W E R I N G C R O P S 225
such as abortion or parasitation. Future research is
required before a concluding evaluation of mass-flowering oilseed rape modulating population dynamics of
cavity-nesting pollinators will be possible.
As a consequence of the predominantly later phenology of the species recorded in our trap nests, species
richness of cavity-nesting bees and wasps was positively associated with the proportion of seminatural
habitats that provide a diverse set of alternative floral
resources once the mass flowering has ceased (Mandelik
et al., 2012). By providing diverse and more permanent
foraging as well as nesting and overwintering resources,
seminatural habitats are known to facilitate not only
species richness (Steffan-Dewenter et al., 2002; Le F eon
et al., 2010) but also bee abundances (Steffan-Dewenter
et al., 2002; Jauker et al., 2012). Congruently, in the present study, the abundance of cavity-nesting bees and
wasps was positively related to seminatural habitats.
Analyses at higher temporal resolution revealed that
this relationship was only apparent after the mass
flowering of oilseed rape. Thus, being masked in the
whole-year analysis, the lack of such an abundance
effect during mass flowering indicates the importance
of higher temporal resolution in understanding the
modulation of annual community dynamics by pulsed
and permanent resource availability.
In addition to univoltine (e.g., Heriades truncorum, Hylaeus difformis) or partially bivoltine (e.g., Megachile centuncularis, Discoelius zonalis) species with a late
phenology, especially the second generation of early
bivoltine species, such as Ancistrocerus nigricornis, A. trifasciatus, or Osmia caerulescens may have benefited in
their abundance from resources offered by increasing
amounts of seminatural habitats after the mass flowering. As suggested by coarse taxon identification based
on brood cell residues (i.e., clay cell walls and cocoon
fragments), these latter species’ first generations might
be responsible for the observed negative relationship
between seminatural habitats and early vacated nests.
The first nest-building individuals of the year might distribute themselves across the increasingly available
seminatural habitats, while they are concentrated to few
locations when these elements providing valuable nesting sites are scarce. A similar dilution of pollinators was
reported in grasslands with increasing foraging
resources (i.e., mass-flowering crops; Holzschuh et al.,
2011). Once the first generation is established, this trend
seems reversed, as foraging females initializing the second generation may benefit from increasing availability
of resources in seminatural habitats. Potentially, this
population growth of the second generation exceeds the
dilution effect of the first generation, thus resulting in
the positive response of recorded late abundances to
seminatural habitats. To substantiate this assumption,
© 2013 Blackwell Publishing Ltd, GCB Bioenergy, 6, 219–226

however, a higher temporal resolution is needed in the
assessment of species-specific nesting activity throughout the year.
Our results may contribute to a more complete evaluation of how increased biofuel cropping may affect the
diversity of flower-visiting insects and potentially the
services (i.e., pollination and biocontrol) they provide in
modern agro-ecosystems. The increase in species richness of cavity-nesting bees and wasps with oilseed
rape observed in this study and positive responses of
pollinator abundance and reproductive success previously shown suggest that overall the community of
flower-visiting insects benefit from mass-flowering
oilseed rape. Yet, insect phenology seems to be a very
important trait in affecting the response to oilseed rape.
Disadvantages or transient benefits of this mass-flowering
crop previously shown for long- or short-tongued bumblebees, respectively, further confirm the trait specificity
in this response. With regard to the most important general outcome of the invariable value of seminatural habitats for bee and wasp diversity, we strongly
recommend securing high proportions of seminatural
habitats enabling early positive effects of mass-flowering biofuel crops on pollinators and biocontrol agents to
be carried on throughout the year and temporarily
dynamic disservices to be buffered against in intensively used modern agro-ecosystems.

We thank farmers for their cooperation and Christoph Scherber
for providing the stepAICc R-code.

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Supporting Information
Additional Supporting Information may be found in the
online version of this article:
Figure S1. Species accumulation curves for trap-nesting
bees and wasps for ten out of eleven analyzed study sites.
For one site no individuals emerged from any nest.

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