PDF Archive

Easily share your PDF documents with your contacts, on the Web and Social Networks.

Share a file Manage my documents Convert Recover PDF Search Help Contact



flowering.pdf


Preview of PDF document flowering.pdf

Page 1 2 3 4 5 6 7 8 9

Text preview


€ 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
2012)

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