Changes in phenology predict trends in relative abundance in Massachusetts butterfly species

James Michielini, Tufts, Elizabeth Crone, Department of Biology, Tufts University, Medford, MA and Erik B. Dopman, Biology, Tufts University, Medford, MA

Background/Question/Methods: Phenological shifts due to climate change have been widely documented across taxa but their consequences for population dynamics are less clear. We investigated the trends in phenology and relative abundance using 27 years of citizen science records of 89 butterfly species from the Massachusetts Butterfly Club. Shifts in phenology were calculated using quantile regression, allowing us robustly estimate changes in the onset of flight, the end of flight, and the total activity period. These estimates were correlated with shifts in relative abundance using list length analysis and counts from field trips. We then used structural equations models to evaluate the potential effects of range shifts and voltinism changes on both phenology and relative abundance to understand how these factors interact and what potential consequences persistent climate change may have on species’ viability or population dynamics.
Results/Conclusions: Elongated activity periods were most strongly correlated with relative abundance. Species that both emerged earlier in spring and persisted later in autumn showed significantly higher abundance trends across years relative to those that did not. Species with geographic ranges centered further south exhibited significantly larger phenological shifts and increased relative abundance, suggesting these trends may be driven in part by range shifts. Certain species at the northernmost extent of their range in the state Massachusetts exhibited the greatest shift in both phenology and abundance with an apparent addition of a generation, indicating they may have been shifting from vagrants to residents with the capacity of overwintering in the state. Our results emphasize the necessity to evaluate species’ phenology throughout the year and to understand the consequences for such climate-related changed on viability and population dynamics.