Variables and data
We used climate data from the Koninklijk Nederlands Meteorologisch
Instituut (KNMI 2016) from the period of 2000 to 2015 in bioclim format
(Fick & Hijmans 2017). A Principal Component Analysis (PCA) was used to
transform the 19 bioclim variables in five orthogonal PCA axes that
explained more than 90% of the variation. The resolution of the climate
variable was 100 m (100 m by 100 m) and all other variables were
transformed to the same resolution. The land use data consisted of 15
land use categories from different sources (see table S1 in supporting
information; Inter Provinciaal Overleg, 2016; Ministerie van Economische
Zaken (EZK), 2015; Centraal Bureau voor de Statistiek (CBS), 2012).
Together with the sum of the different land uses per grid cell as an
indicator for landscape heterogeneity, this resulted in 16 land use
variables. The 8 main aggregated soil types in the Netherlands (table
S2; grondsoortenkaart 2006) and the land use data were transformed to a
100 m resolution raster format with percentage cover values and they
were not strongly correlated (Spearman’s ρ < 0.7; Dormann et
al. 2013).
The bee occurrence data used in this study consist of opportunistic
observations of bees and flower visitation records from 2004 to 2019,
obtained from the European Invertebrate Survey Netherlands (EIS;
http://www.eis-nederland.nl/). Of the more than 300 bee species in
the Netherlands we selected 194 species, discarding the species without
flower visitation information and with less than fifteen observations.
Using the flower visitation data, the most visited plant species and
genus were determined. Bees were classified as either oligolectic
(includes both monolectic species which visit a single plant taxon and
oligolectic species which show a clear preference for a single plant
family), polylectic (collect pollen and nectar from various plant taxa)
(Rasmussen et al. 2020) and cleptoparasitic bees; these traits were
based on a database created for the Status and Trends of European
Pollinators (STEP) project and maintained by Stuart Roberts (Potts et
al. 2015). For all the 45 modelled oligolectic bees the most visited
plant was the plant that they are dependent on for their pollen in the
bee trait database (table S3). Since no quantitative interaction data
was available for cleptoparasitic bees and their host, we used the
occurrences of all the host bee species and genera in the Netherlands
listed in the literature (table S4; Peeters & Nieuwenhuijsen 2012). The
plant occurrences for the period 2004-2019 were obtained from Dutch
National Database of Flora and Fauna (NDFF 2021). The occurrences of the
interacting species were kept at a resolution of 100 m or aggregated to
a coarser resolution (500 m, 1 km, 5 km and 10 km) and they presented
presence/absence, i.e. binary data, similar to Godsoe et al. (2009).
Some of the cleptoparasitic bees had multiple hosts and the biotic
interaction variable map had a presence if any of the host species or
genera was present.