The responses of pollinators to climate change could include changes in phenology of migratory pollinators and in the routes or destinations for their migration, changes in the phenology and distribution of non-migratory species, and changes in the host plants they visit for nectar and pollen.  Plants face similar challenges with regard to changes in their distributions, their reproductive phenology, and interactions with both co-flowering species and pollinators (competition, facilitation, etc.).  Unless pollinators and their host plants are responding similarly to changing environmental cues that affect their phenology, their historical patterns of interaction, both mutualistic and competitive, are likely to change.  Long-term data are essential to investigating which if any of these potential outcomes are occurring.  A 36-year record of abundance and phenology of flowering of 90+ wildflower species, surveys of the altitudinal distribution of bumble bees in the 1970s and the past few years, and data from a long-term Malaise trap sampling program, all near the Rocky Mountain Biological Laboratory (West Elk mountains, Colorado) are used for this investigation.

Results/Conclusions

Although the flowering phenology of all species examined to date is affected by a single environmental event, disappearance of the winter snowpack (range 22 April -19 June since 1975), either their responses to that single cue are not uniform, or different species respond to additional cues in addition to snowmelt (e.g., growing degree days).  Thus the community of co-flowering species varies temporally and quantitatively among years; differential sensitivity to frost damage is an example of an environmental variable that generates the quantitative variation among years, and is in turn affected by date of snowmelt.  Arrival dates of migratory Broad-tailed Hummingbirds are significantly correlated with the amount of snow remaining on 30 April, and with the day of first flowering of Erythronium grandiflorum (glacier lily), the first flower that they visit at this site in the spring.  Altitudinal distributions of at least some bumble bee species and of the flowers they feed on are also changing, with one bee species (Bombus nevadensis) occurring about 600m higher than it did 30 years ago and one wildflower (Mertensia cilata) disappearing from lower altitudes where it was historically common.  As these communities of plants and pollinators respond to environmental changes with changes in phenology and distribution, new interactions will be created and old ones will be lost.