Complexities of Phenological Shifts for Plant-Pollinator Interactions and Ways Forward.

Changing climatic conditions can lead to diminished overlap in the timing of flowering and pollinator foraging, potentially resulting in the weakening or loss of plant-pollinator interactions and reducing the fitness of both partners. However, several complexities of phenological shifts limit our ability to predict their consequences for plant-pollinator mutualisms. First, phenological shifts reflect the responses of individuals but are often summarized at the community, species, or population level, potentially obscuring variation that has important implications for interactions within and between species. Second, metrics of phenological asynchrony in pollination, such as temporal overlap between flowering and pollinator foraging, may not accurately characterize changes in interaction strength or fitness costs and benefits and thus are not true metrics of mismatch. Third, our focus has been on shifts in individual life-history events, such as flowering, rather than entire life cycles, despite the physiological integration of seasonal life-history stages (phenophases) that may be under different selection pressures. We suggest that we can advance our understanding of phenological shifts and their consequences for plants and pollinators by studying individual phenological variation in both partners across natural or experimental environmental gradients, measuring interaction rates and their fitness implications in addition to synchrony or overlap, and taking an integrated life cycle approach that can reveal trade-offs. Together, these approaches can yield temporally explicit fitness landscapes for plant and pollinator phenologies and improve our understanding of the consequences of climate change-induced phenological shifts.