Introduction
The abundance and accessibility of floral resources (hereafter, ‘FR’) has been identified as the primary factor limiting wild bee populations globally (Roulston and Goodell 2011). Specifically, if wild bee populations are to persist, there must be sufficient provision of FR over both time and space. However, extensive conversion of natural habitat to arable land to support the growing human population is resulting in the removal of many of the naturally occurring FR on which wild bee populations rely (Kremen et al. 2002, Brosi et al. 2008, Murray et al. 2009). Even if crops themselves provide FR, they do so for only a portion of the growing season, which may be insufficient to support bee populations throughout their activity periods. An abundance of research looking at spatial provisioning of floral resources has generally found that increasing either heterogeneity or abundance of FR will result in increased population sizes or visitation rates of wild bees (synthesized in Kennedy et al. 2013). However, a few recent studies have found the opposite, with certain FR-providing habitats actually distracting bees from visiting crop fields (Nicholson et al. 2019), or causing a dilution of pollinators across landscapes when floral resources are less limited (Kovács-Hostyánszki et al. 2013, Holzschuh et al. 2016).
While the influence of spatial arrangement of FR on bees foraging in agricultural landscapes has been well established, the influence of FR availability over time has been relatively understudied. Much of the existing research on the latter topic has found that in landscapes providing a consistent source of FR over time, wild bees respond positively in terms of their abundance (Mandelik et al. 2012, Mallinger et al. 2016, Martins et al. 2018), density in crops (Kovács-Hostyánszki et al. 2013), colony growth (Westphal et al. 2009, Crone and Williams 2016), and sexual reproduction (Rundlöf et al. 2014). However, most studies examining the effect of temporal and spatial arrangement of FR on bees focus on responses of abundant, social taxa such as honey bees (Lau et al. 2019) and bumble bees (Timberlake et al. 2019), or examine the responses of broad functional groups of bees, often by grouping solitary bees together (Le Féon et al. 2013, Kovács-Hostyánszki et al. 2013). An increase in bee population size or density in landscapes with high FR can only be observed within one season if bees produce multiple broods per season, or if there is immigration from adjacent landscapes. For wild bee species that have limited flight distances and produce a single brood annually—as is the case for most species in temperate regions—we would expect population sizes to remain stable when FR abundance is consistent or increases over a season, and to decrease in response to periods in a season when resources become scarce. Given the differences in brood production, foraging periods, and foraging ranges among bee taxa, fluctuations in FR should produce a diversity of responses (Ogilvie and Forrest 2017). We therefore expect that the spatial and temporal scale of FR that most influences bee population size should be specific to the taxonomic group of bees that is examined. Understanding the responses of specific bee taxa to seasonal FR in agricultural landscapes is important for development of conservation and management strategies that can both enhance pollination services and preserve bee functional diversity.
The objective of this study was to examine the relationship between visitation rates of bees and the amount of FR in agricultural landscapes over one season, to determine at which within-year temporal scale and landscape spatial scale the abundance of FR predicts local bee abundance. We examined visitation rates of the most common genera of bees and the corresponding amount of FR in surrounding agricultural landscapes in four sequential time periods over one season, to assess the relative support for the following hypotheses for each genus (presented in Fig. 1):
H1: Bee visits are only influenced by local FR – for bee populations that are limited by something other than FR (e.g., nesting habitat, pesticides, natural enemies), population sizes should not be correlated with the amount of FR in the broader landscape; instead, we expect that the present abundance of local FR (i.e., within the area in which bee visits are measured) will best predict local visitation rates.
H2: Bee visits are influenced by the present abundance of landscape FR – for bee populations that are influenced by the availability of FR, but that have relatively short foraging periods within a season (overlapping only one time period in which FR were measured), we expect that the present abundance of FR within the landscape will best predict bee visits, and that either (a)bees are limited by FR, so that in landscapes with higher FR abundance more bee visits will be observed; or that (b) bees are not limited by FR, but instead are “diluted” (dispersed) across landscapes with higher FR abundance, in which case fewer bee visits will be observed at a local flower patch.
H3: Bee visits are influenced by the previous abundance of landscape FR – for bee populations that are influenced by FR and have long foraging periods within a season (overlapping multiple time periods in which FR were measured), we expect that (a) for bees producing a single brood per season, both the abundance of FR in the landscape when foraging begins and any decreases in the abundance of FR later in the season will best predict bee visits; or (b) for bees producing multiple broods per season, the cumulative abundance of landscape FR from when foraging begins will best predict bee visits.