DOI: 10.5061/dryad.9w0vt4bjr
title: Neutral processes related to regional bee commonness and dispersal distances are important predictors of plant-pollinator networks along gradients of climate and landscape conditions

description: Understanding how niche-based and neutral processes contribute to the
 spatial variation in plant-pollinator interactions is central to designing
 effective pollination conservation schemes. Such schemes are needed to
 reverse declines of wild bees and other pollinating insects and to promote
 pollination services to wild and cultivated plants. We used data on wild
 bee interactions with plants belonging to the four tribes Loteae,
 Trifolieae, Anthemideae, and either spring- or summer-flowering
 Cichorieae, sampled systematically along a 682km latitudinal gradient to
 build models that allowed us to (a) predict occurrences of pairwise
 bee-flower interactions across 115 sampling locations, and (b) estimate
 the contribution of variables hypothesized to be related to niche-based
 assembly structuring processes (viz. annual mean temperature, landscape
 diversity, bee sociality, bee phenology, and flower preferences of bees)
 and neutral processes (viz. regional commonness and dispersal distance to
 conspecifics). While neutral processes were important predictors of
 plant-pollinator distributions, niche-based processes were reflected in
 the contrasting distributions of solitary bee and bumble bees along the
 temperature gradient, and in the influence of bee flower preferences on
 the distribution of bee species across plant types. In particular, bee
 flower preferences separated bees into three main groups, albeit with some
 overlap: visitors to spring-flowering Cichorieae; visitors to Anthemideae
 and summer-flowering Cichorieae; and visitors to Trifolieae and Loteae.
 Our findings suggest that both neutral and niche-based processes are
 significant contributors to the spatial distribution of plant-pollinator
 interactions so that conservation actions in our region should be directed
 towards areas: near high concentrations of known occurrences of regionally
 rare bees; in mild climatic conditions; and that are surrounded by
 heterogeneous landscapes. Given the observed niche-based differences, the
 proportion of functionally distinct plants in flower-mixes could be chosen
 to target bee species, or guilds, of conservation concern.
description: In 2017, we sampled
 flower-visiting bees at 115 sites in northern Germany, western Denmark,
 and southeastern Norway (Figure 1, Table 1). We sampled bees visiting plant species belonging
 to four tribes with morphologically distinct flowers and flowering times:
 White-petalled Anthemideae (Asteraceae) species; yellow-petalled
 Cichorieae (Asteraceae) species, which also differed in their flowering
 phenology, Cichorieae spring (<em>Taraxacum</em> spp.), or
 Cichorieae summer (e.g. <em>Hieracium</em> spp.); and two
 tribes of Fabaceae with zygomorphic flowers: Loteae species with yellow
 flowers and Trifolieae species with white to purple
 flowers. Species within
 these plant tribes attract a diverse community of bees (e.g. Rasmussen et
 al. 2016). The number of sampled plant types varied between sites, with
 one, two, and three types sampled at 92, 17, and 6 sites, respectively.
 Anthemideae were not sampled in Denmark as we were unable to find suitable
 locations. Site-pairs with between-site distances &lt; 1000m could
 potentially share pollinators from the same populations (Greenleaf et al.
 2007). We identified 76 clusters of sites within which between-site
 distances were &lt; 1000m and between-cluster-distances were always
 &gt;1000m, i.e. 76 statistically independent clusters. Of the 76
 clusters, 50 contained one site, 17 contained two, six contained three,
 two contained four, and one cluster contained five
 sites. At each site, we collected bees within an area
 of up to 50 by 50m, depending on the spatial distribution of the focal
 plant species. All bees were sampled by the first author. At each site,
 bee sampling was conducted once by collecting flower-visiting bees for 30
 minutes on each plant type, adding 30 seconds of handling time per sampled
 bee. Sampling took place when the air temperature was &gt; 15 °C and
 with little to no wind (Beaufort scale 0-3). Sampling followed onset of
 the flowering season, i.e., it started in northern Germany and ended in
 southeastern Norway (Table 1). All collected bees were stored in 96%
 ethanol prior to pinning and species identification. Voucher specimens are
 deposited in the entomological collection at the Norwegian Institute for
 Nature Research, Oslo. The .csv table contains columns
 for: "Occurrence" = Presence or absence of
 bee-plant interactions "Interactions" = Number
 of bee-plant interactions "SiteID" = Identifier for
 the location where sample was conducted "SamplingDate"  = Date sampling was
 conducted "Bee" = Bee species sampled on
 plant "PlantMrf" = Plant morph/tribe on
 which bees were sampled "RegionalCommonness" = regional
 commonness of bee species, estimated from GBIF
 occurrences "DistanceToSpecies" = distance to
 nearest known occurrence of bee species, estimated from GBIF
 occurrences "BIO1" = annual mean temperature,
 estimated from WorldClim dataset.
 "SH250m" = Shannon
 landscape diversity estimated at a 250m radius
 "SH1000m" = Shannon
 landscape diversity estimated at a 1000m radius
 "DCA1" = Bee floral
 preference score on detrended correspondence axis
 1 "DCA2" = Bee floral
 preference score on detrended correspondence axis 1 
 "DCA3" = Bee
 floral preference score on detrended correspondence axis 1
 "DCA4" = Bee floral
 preference score on detrended correspondence axis 1
 "FirstMonth" = Bee
 phenology estimated as the first month of activity
 "Bombus" =
 categorical variable noting if bee species is solitary or belongs to the
 eusocial genus Bombus "ClusterID" = Cluster identity
 defining which cluster of sites a study belongs to
 "Longitude" =
 longitude in degrees (WGS84) "Latitude"  = latitude in
 degrees (WGS84)