Floral resources are considered to have substantial direct influence over wild bee population sizes, but their effects on bee demography remain poorly understood. Most of the available evidence on how food resources influence bee populations comes from studies that use patterns of bee abundance to infer causal relationships between environmental conditions and population dynamics. To date, only a few studies have examined the relationship between flower availability and demographic success in order to mechanistically link this aspect of habitat quality with the health of bee populations. In this study, we manipulated the levels of forage available to bumblebee (Bombus vosnesesnkii) colonies and tracked the fates of marked individuals to assess how aspects of colony development responded to receiving a pulse of food resources lasting 3-4 weeks early in colony development. “Pulsed” colonies (n=7) had access to supplemental resources; “Control” colonies could only access ambient resources. Colonies were located in a nutritionally challenging agricultural setting in Davis, CA (USA). We assessed the effect of the pulse on worker production, worker survival and body size, and reproduction. We also examined whether the responses of colonies were different when placed in the field with one or two cohorts of workers (~10 vs. ~20 bees).
Results/Conclusions
Access to a resource pulse early in colony development had different effects on overall worker production depending on the initial size of the colony (one vs. two worker cohorts) upon placement in the field. Among colonies deployed with one worker cohort, pulsed colonies produced fewer workers overall than control colonies, but workers in pulsed colonies were higher quality, being larger-bodied and longer-lived. For these colonies, generalized additive models show body size was elevated beyond the window of time in which we would expect the supplemental resources to directly influence the body size of developing bees. Among colonies deployed with two worker cohorts, access to the pulse resulted in greater worker production, but workers were not significantly larger-bodied and, unexpectedly, had shorter lifespans. Access to the pulse, regardless of initial size in the field, always resulted in greater reproductive output. Collectively, these results (1) demonstrate that greater resource availability early in the cycle is important for colony development and increases reproductive success, and (2) highlight that colonies may use different strategies for coming by this success, investing resources in either the production of more workers or the production of higher quality workers.