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.