Understanding diversity in flower visitor assemblages helps us improve pollination of crops and support better biodiversity conservation outcomes. Much recent research has focused on drivers of crop-visitor diversity operating over spatial scales from fields to landscapes, such as pesticide and habitat management, while drivers operating over larger scales of continents and biogeographic realms are virtually unknown. Flower and visitor traits influence attraction of pollinators to flowers, and evolve in the context of associations that can be ancient or recent. Plants that have been adopted into agriculture have been moved widely around the world and thereby exposed to new flower visitors. Remarkably little is known of the consequence of these historical patterns for present day crop-visiting bee diversity. We analyze data from 317 studies of 27 crops worldwide and find that crops are visited by fewer bee genera outside their region of origin and outside their family’s region of origin. Thus, recent human history and the deeper evolutionary history of crops and bees appear to be important determinants of flower-visitor diversity at large scales that constrain the levels of visitor diversity that can be influenced by field- and landscape-scale interventions.

Having selected a candidate list of plant families we conducted a literature review of bee visitation to the crop species in each family using Web of Science, CAB Abstracts, and Google Scholar (first 100 hits only) with the following search terms: (genus AND species AND visit*) OR (genus AND species AND pollinat*), where genus and species included all synonyms for each crop. These searches returned over 10,000 articles, so a hierarchy of filters was applied to focus on the most relevant studies. The first filter was applied to titles of papers to remove obviously irrelevant material (e.g. studies comparing effectiveness of different crop varieties as pollinizers). A second filter applied to title and abstract for relevance (e.g. any mention of flower visitors or pollinators). Two studies on small, isolated islands were excluded due to possible island biogeography effects. A third filter was applied to ensure appropriate taxonomic resolution of flower visiting bees, selecting those studies that identified all bees visiting crops to at least genus level. Where these data were not included in the published article but it seemed likely the data had been collected we contacted authors and requested these data.

The following data were then extracted from each study:

1) number and identity of bee genera observed visiting each crop,

2) biogeographic realm and latitude of location(s) where crop visitors were observed (centroid of locations when there were multiple within a study),

3) number of locations where crop visitors were observed.

The biogeographic histories of crop species and plant families were determined on the basis of a literature search using Google Scholar with the following search terms: (genus AND species AND orig*) OR (genus AND species AND domesticat*), and (family AND orig*) or (family AND evolution). We attempted to attribute a single origin to each crop and family according to the most recent published studies, though some crops and families were given multiple origins in cases where origins are currently controversial or there were multiple, independent domestications in different biogeographic realms (Table S4). The descriptions are the most current found for family and crop origin based on best available data.