Changes in phenology, the timing of life cycle events, are a prominent sign of climate change in many species. However, although we know phenology is changing, it is unclear how these changes affect population viability. Two prominent examples of phenological changes are advances in the average flight dates of butterflies, and flowering dates of plants. Overall, these tend to be changing at different rates; on average, plant flowering is advancing more with climate warming than butterfly flight. This difference leads to the hypothesis that butterflies will have fewer nectar resources available in future environments.
We tested the effects of changes in plant and butterfly phenology using Baltimore checkerspots (Euphydryas phaeton) and their interacting nectar species. We used historical collections (herbarium and museum specimens) to evaluate changes in phenology of Baltimore checkerspots and thirteen of their most common nectar species. We conducted field monitoring at three sites in each of two geographic regions (Maryland and Massachusetts) to estimate (1) the abundance of common nectar plant species, (2) total nectar provided by each plant species, and (3) time spent nectaring by Baltimore checkerspots. We used these data to estimate current vs. historical overlap of butterflies and nectar in both regions.
Butterflies and plant species were changing phenology in heterogeneous ways. Across its range in the northeastern US, changes in phenology would lead to slight decreases in nectar production during the Baltimore checkerspot flight season. Phenological mismatches were of greater concern in Maryland (13% reduction), where Baltimore checkerspots are a species of conservation concern, than in Massachusetts (5% increase), where Baltimore checkerspots are more common, and not in decline.
However, the changes attributable to phenological shifts were vastly smaller than regional differences in nectar availability. Nectar production was about two orders of magnitude higher at our sites in Massachusetts than our sites in Maryland. Butterflies in Massachusetts also spent three times more time nectaring in Massachusetts than in Maryland, suggesting that this difference in nectar availability led to differences in nectar acquisition by butterflies.
We discuss why regional differences in nectar availability were probably associated with land use and habitat management. From a conservation perspective, traditional concerns like maintaining disturbance regimes, conserving lands for conservation, and managing invasive species are probably much more important for population viability than climate-induced phenological mismatches.