Recent climate change is altering the abundance and phenology of organisms, and these often species-specific responses may scale up to alter communities. It is important to understand how climate variation influences assemblages of resource organisms because those changes may then affect their consumers. Plant flowering is overwhelmingly responsive to climate variation and often species-specific. This suggests that assemblages of flowers used by different pollinators may have divergent responses to climate variation. Flowering assemblages may change in overall abundance, richness, diversity, and turnover—all metrics that should be important for pollinator foraging and abundance. However, few studies have dissected the potentially divergent responses of different pollinator floral guilds to climate variation. Furthermore, within-season changes in floral resources that should be important for pollinator foraging may be masked when only examining aggregated seasonal metrics. Using long-term (1974-2017) records of flowering and climate at the Rocky Mountain Biological Laboratory, we examine the response of floral resources to time and climate variation from a pollinator perspective. We asked if (1) seasonal and within-season floral resource metrics varied in their relationship to time and climate variables, and (2) if different pollinator guilds of flowers (long-tongued bees, short-tongued bees, flies, lepidopterans, and hummingbirds) varied in their relationship to time and climate variables.
Results/Conclusions
Our examination of the long-term responses of floral resources to climate variation revealed two broad results. First, in general, within-season floral resource metrics showed strong patterns over time and with climate variables, whereas aggregated seasonal floral resource metrics did not. In particular, within seasons, the number of days below a low floral abundance threshold have mostly increased over four decades, along with the number of days above a low floral richness threshold. Such trends over time may have conflicting effects on pollinator populations. Furthermore, both the number of days below a low abundance threshold and days above the low richness threshold were strongly predicted by snowmelt—both increased with earlier snowmelt dates. Thus, by only examining aggregated seasonal metrics we may overlook patterns consequential to resource consumers. Second, we found differences in how the different pollinator floral resource guilds respond through time and with climate variables, especially at within-season scales. This suggests that pollinator taxa are experiencing different resource landscapes with climate variation which could ultimately reshape pollinator communities.