In a paper titled “Constraints and competition in the evolution of flowering phenology”, Kochmer and Handel (1986) showed that families of animal-pollinated plants differed significantly from one another in their flowering times. This result highlighted the importance of phylogenetic constraints – rather than competitive interactions – in the evolution of flowering phenologies. Solitary bees, which are entirely dependent on flowers for reproduction, must have phenologies that correspond to the availability of flowers of their host-plants. Yet bee phenologies, too, show taxonomic signal – implying some level of phylogenetic constraint on phenological change. Nevertheless, within some solitary bee genera (e.g., Andrena, Colletes, Osmia), there have been evolutionary shifts in phenology, sometimes in association with shifts in the identity of the overwintering life-stage (e.g., prepupa vs. adult). In this talk, I use published data from various systems along with data from our field sites in the Colorado Rocky Mountains to explore the link between evolutionary change in phenology and change in floral-host associations. In doing so, I evaluate the roles of phylogenetic and seasonal constraints, competitive release, and parasite attack in shaping the evolution of bee phenology.
In several temperate-region bee lineages, shifts to autumnal phenology are associated with specialization on pollen of the plant family Asteraceae; floral associations of early-season species are more varied. Among Osmia bees at our study sites, late-season activity is also associated with greater risk of death due to failure to complete larval development before the onset of winter; thus, autumnal phenology can be costly. Furthermore, multiple lines of evidence suggest that Asteraceae pollen is of poor quality for bee development. The compensatory benefits of specialization on aster pollen may include escape from competition and (as our Osmia data suggest) a reduced risk of attack by cleptoparasites. These observations suggest that selection to take advantage of the late-season resource provided by the abundant and diverse plant family Asteraceae may have led to repeated evolutionary shifts in bee phenology. Understanding the selective forces and constraints that have acted on bee phenology in the past should help us forecast future population responses to altered seasonal patterns of resource availability.