There is evidence from a diversity of biomes that plants and animals are responding to climate change independently; however, few studies have explored how changes in the distributions, phenology, and morphological traits of one partner will affect its encounters with and dependence upon the other. In alpine ecosystems, bumblebees serve as keystone mutualists for plants, and thus are of special concern. Yet, we lack clear predictions or knowledge of how the foraging patterns of alpine bumblebees may be changing in response to warming. Here we show how data from historical studies of alpine bumblebees can provide testable hypotheses for changes in their foraging niches with climate change. Regional surveys of bumblebee host-plant use were made from 1966-69 at 17 sites over a 1000 m altitudinal gradient in the Colorado Rocky Mountains (Macior, 1974). Assuming that altitudinal variation in floral resources has decreased with global warming, alternative predictions for shifts in resource use by alpine bumblebees can be based on historical differences between alpine and subalpine species. Moreover, by applying this approach to foragers collecting different kinds of resources (pollen vs. nectar) we can identify potential drivers of bee responses to climate change.
Results/Conclusions
Historical surveys reveal altitudinal variation in pollen and nectar resources from subalpine to alpine habitats. Additionally, bumblebee species exhibited altitudinal differences in the extent of resource partitioning. Niche overlap was greater for bumblebees in alpine habitats than in subalpine habitats in the 1960s. Alpine and subalpine foragers did not differ in the breadth of plant species visited or used as pollen sources. However, bumblebees tended to collect nectar from fewer plant species in the alpine than in the subalpine. These results indicate that low-elevation pollinator communities exhibit greater resource partitioning of pollen resources and lower specialization in nectar resource use than high-elevation communities. By comparing patterns of current resource use to these trends, we can test whether alpine bumblebees have responded to warming by (a) shifting their foraging patterns to reduce niche overlap, as seen at lower elevations historically (adaptation); (b) narrowing their resource use to plants found at the highest altitudes, therefore increasing niche overlap (escape); or (c) tracking short term changes in floral resource availability from year-to-year (null). Future work will assess whether alpine bees exhibit adaptation, escape, or short-term tracking as a strategy for coping with encroachment of subalpine conditions into alpine habitats under global warming.