Floral resource continuity boosts bumble bee colony performance relative to variable floral resources

Abstract

Centuries of landscape changes associated with agriculture have dramatically reduced the amount and increased the temporal variability of the floral resources that support key pollinating insects such as bumble bees. Adapting to these novel resource conditions is important to ensure the persistence of bumble bee species. While several species appear to be in decline in modern agricultural landscapes, others have thrived, suggesting adaptation to exploit highly variable floral resources. Bombus impatiens, the common eastern bumble bee, is a prime example of such a species. We designed an experiment to compare how free‐foraging colonies of B. impatiens performed adjacent to areas with either temporally continuous or variable (pulsed) patches of purple tansy (Phacelia tanacetifolia) plantings. We found that colonies in Phacelia landscapes grew faster, had gained more mass, and produced more gynes than did colonies in reference landscapes with no Phacelia. Comparing colony responses between pulsed and continuous flowering resources showed that total mass gain at the end of the experiment was greater with continuous flowering resources. In contrast, colony growth rate and total gyne production were comparable for colonies adjacent to Phacelia plantings that were continuous versus pulsed. While low in statistical replication, given the scale of the experimental manipulation, our experiment shows that although B. impatiens colonies can exploit periods of resource discontinuity and gain mass, these continuously available floral resources appear important for colony growth and benefit gyne production.

Landscape‐scale floral resource discontinuity decreases bumble bee occurrence and alters community composition

“…We expected the positive effects of temporal continuity would be stronger for species that have declined in areas of intensive agriculture (Figure 1E, Hemberger et al, 2021). In contrast, we expected stable species (Figure 1F) to be less sensitive to temporal continuity, in line with recent laboratory and field experimentation that has shown these species to be tolerant to variable resource conditions (Hemberger et al, 2022; Hemberger & Gratton, 2018; Schmid‐Hempel & Schmid‐Hempel, 1998). Moreover, common species have other traits that are likely to enhance adaptability to resource heterogeneity including a highly plastic body size (Austin & Dunlap, 2019) that is more resistant to starvation (Couvillon et al, 2010) and a wider diet breadth (Wood et al, 2019) than declining species.…”

“…The fact that common species have a high likelihood of occurrence despite low resource abundance and continuity suggests these species have the capacity to adapt to resource scarcity that declining species lack. Experiments with B. impatiens support this explanation: colonies can still grow and reproduce adequately even with highly variable resource availability in both laboratory (Hemberger et al, 2020) and field settings (Hemberger et al, 2022). Moreover, bumble bee species intolerant to discontinuous resource conditions in the landscape have limited diet breadths (Wood et al, 2019) and reduced plasticity in worker size (Austin & Dunlap, 2019), a trait related to starvation tolerance (Couvillon & Dornhaus, 2010).…”

“…However, due to the logistic challenges associated with measuring floral resources over large spatiotemporal extents, resource continuity is typically not accounted for when evaluating resource availability in landscapes (Malfi et al, 2022;Timberlake et al, 2019Timberlake et al, , 2021; but see N. M. Williams et al, 2012). Studies that have examined the effects of resource abundance and continuity show that bumble bee behavior (Hemberger & Gratton, 2018) and colony growth and reproduction (Hemberger et al, 2020(Hemberger et al, , 2022 are highest when resources are abundant and continuous. However, whether the interaction of abundance and temporal availability can explain bumble bee occurrence and community composition in natural communities is largely unknown (but see Nicholson et al, 2021).…”

Recent and future declines of a historically widespread pollinator linked to climate, land cover, and pesticides

“…Drought also negatively affected B. occidentalis and can be exacerbated by increasing temperatures. Drought impacts bumble bee populations through indirect effects on floral resource availability ( 34 ), such as increases in seasonal floral resource gaps versus continuous resource availability that reduce bumble bee colony performance ( 35 ). Besides changes in floral abundance or continuity, drought can also reduce the quality and quantity of pollen and nectar resources ( 36 ).…”

Response of pollinator taxa to fire is consistent with historic fire regimes in the Sierra Nevada and mediated through floral richness†

“…Floral richness was positively associated with nearly all pollinator communities and taxa (although not always significant), and the effect of floral richness was strongest (i.e., highest coefficient values) for bumblebees and the European honeybee. Multiple flowering plant species that bloom sequentially may be necessary to support bumblebees and honeybees that rely upon pollen and nectar in all active life stages, with colonies to support over an entire season (and beyond for honeybees storing honey for winter; Aldridge et al., 2011; Hemberger et al., 2022). We expected a weaker response from wasp species, but we found crabronid and Podalonia wasps were similar to other Hymenoptera species.…”