While there is strong evidence that climate change is driving an increased incidence of phenological shifts worldwide, key gaps remain in our understanding of the mechanisms that organisms use for phenological cuing, and the fitness consequences of these shifts. We used three approaches to examine the causes and consequences of phenological shifts. First, we conducted experimental manipulations of seasonal timing in order to observe the fitness consequences of phenological shifts relative to the surrounding community. Second, we examined broad patterns of phenological shifts that have been suggested in the literature (e.g., are phenological shifts greater at higher latitudes, or do they differ systematically by trophic position?) in order to better understand their proposed mechanistic bases. Third, we developed a model of phenological cue evolution in order to examine how selection could act to optimize a phenological cue strategy that integrates multiple sources of information from complex and dynamic environmental conditions, using real-world climatic data. We used this model to test hypotheses about which environmental cues (e.g., temperature, precipitation and photoperiod) would be favored under different regimes of inter- and intra-annual variation, and to assess the resilience of different cuing strategies to climate change.
Experiments manipulating seasonal timing reveal strong seasonal windows of opportunity and suggest a steep underlying fitness landscape associated with phenological shifts. These experiments suggest that even small phenological shifts relative to a surrounding community are likely to have large fitness consequences. In our examination of the literature, we find that most proposed patterns of phenological shifts lack clear mechanistic hypotheses and are inconsistent across taxa or systems. We suggest that understanding and predicting variation in phenological shifts will likely require a more mechanistic understanding of how different organisms integrate phenological cues. Finally, our model suggests that some broad patterns in the evolution of phenological cue strategies may be predictable based on inter-annual versus intra-annual variation in environmental conditions, but also suggests that hidden variation in underlying cuing strategies could contribute to observed variation in the direction and magnitude of phenological shifts under climate change. In the context of stationary environmental variation, multiple cue strategies can yield similar fitness outcomes, and show neutral coexistence. However, these same cue strategies often show strong fitness differences in the context of climate change. Taken together, these three approaches illustrate complementary ways to examine the mechanistic causes and fitness consequences of phenological shifts in a community context.