Functional traits may predict resilience to climate change among native bees accessing non-floral sugars in a California Mediterranean habitat

Background/Question/Methods

Thousands of species of wild bees provide indispensible pollination services to wild and cultivated flowering plants worldwide, yet little is known about their resilience to habitat changes. Wild bees are typically less social than managed honeybees, are largely unmonitored, and depend upon the availability of natural resources to survive and reproduce. Changes in climate are inducing phenological shifts in both wild bee emergence and floral bloom times that threaten to isolate bees from their host plants, and plants from their pollinators. This trend is of particular concern in harsh, flash-bloom habitats known to be especially sensitive to climate change, such as alpine and Mediterranean ecosystems. The persistence of wild bee diversity through increasingly unpredictable bloom conditions in these areas may depend on species’ abilities to access non-floral resources until flowering commences. Our capacity to forecast shifts in bee community composition, and thus pollination services, may benefit from understanding patterns of differential resilience to sugar limitation using functional traits, which can be applied across diverse bee communities. Working in a Mediterranean habitat at Pinnacles National Park in California, we experimentally investigated foraging behaviors in a diverse community of wild bees accessing a non-floral sugar source in the form of scale-insect honeydew.

Results/Conclusions

When separated into functional groups (body size, nesting habits, sociality, etc), we found that ground-nesting and smaller, more flight-restricted bees were significantly most likely to use honeydew, suggesting these sugars are either most easily accessed or beneficial for these groups. Assessing functional trait patterns of this behavior at a finer temporal scale, however, revealed significant shifts, predicted by body size, in which species are dominating this resource over the course of the early spring when bloom availability is most dynamic. Compared to their abundances in the background bee community, larger bees (intertegular span > 2.5 mm) dominated honeydew sites very early in the season, while smaller bees (IT span < 1.2 mm) were three times more likely to access honeydew during late spring. This may indicate competition for, or niche partitioning of, a variety of sugar sources within the bee community when bloom is most limited. Body size also played a role in the shifting honeydew-use dynamics among ground, stem, and cavity-nesting bees at different points throughout the season.  Non-floral foraging abilities could influence shifting bee community composition as climate change exacerbates delayed bloom, decouples specialist bees from host plants, and unequally stresses different groups of wild bees.