Synergistic phenology and range shifts in boreal butterflies are controlled by species' overall response capacity

Jussi Mäkinen, Yanjie Xu, Susanna Koivusaari, Risto K. Heikkinen, Maria Hällfors

Species track climate warming through shifts in both phenology and geographic distributions, but the strength of association between these responses remains poorly understood. Although recent research points to a tendency of species to shift across either the temporal or spatial gradient, there is no comprehensive understanding of how generalizable these phenology and range shifts are and whether they are together mediated by species traits and could thus be predicted. We tested the hypothesis that coupling between phenology and range shifts emerges from the combined influence of species response capacity mediated by traits and phylogeny and environmental constraints.

Using opportunistic occurrence and systematic community-survey data for 70 Finnish butterfly species from 1999 to 2020, we derived probabilistic model predictions of the onset of adult flight time and northern range boundary. From model predictions we estimated species-specific phenology and northern range shifts and quantified how traits, phylogeny, and latitude contributed to variation in these responses.

Species generally advanced their adult flight periods and expanded their northern range boundaries northward. Across species, stronger phenological advances were associated with greater range expansion, indicating an overall synergistic relationship between temporal and spatial climate tracking. Environmental context generated a latitudinal trade-off: southern species showed stronger phenological advances whereas northern species exhibited stronger range shifts. Habitat breadth and thermal tolerance, primarily influenced range shifts and contributed to this trade-off. In contrast, traits associated with general response capacity, particularly mobility and flight-period length promoted both phenological and range responses simultaneously. Our results suggest that the relationship between phenology and range shifts is an emergent property arising from the balance between environmental constraints and intrinsic response capacity. Environmentally structured drivers tend to favour alternative climate-tracking strategies, whereas environmentally independent response-capacity traits promote simultaneous tracking across time and space.