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 Sanguisorba minor and 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).