Discussion
Our analysis reveals that Lepidoptera in Finland most often use only one of the commonly assumed adaptive responses to climate change, as 47% of the studied species responded by either shifting their NRB northwards or by advancing their phenology. However, nearly as a large a proportion (41%) were unable to utilize either of the two strategies. Importantly, this un-responsiveness coincided with more negative population trends. In contrast, the 13% of species that responded by both shifting their NRB northwards and advancing their phenology showed, on average, the strongest positive population trends. This minority of species, able to capitalize on both responses, advanced their flight period by 3.8 days/decade and shifted their NRBs 124.5 kilometers further north between T1 and T2, on average. Although this study cannot provide evidence for de facto on underlying processes giving rise to the observed patterns, the results point to pervasive fitness benefits of combining in situ adjustments with range shifts.
We found only few trait effects on the responses and population trends (cf. Pöyry et al. 2009; Angert et al. 2011; Coulthardet al. 2019), which raises concerns related to the potential to identify species vulnerable based on their life history. However, we found that adult overwintering species tended to advance their phenology and species overwintering as pupae were less likely to shift their ranges further north, with some even contracting their range southwards. Earlier studies have hypothesized that species overwintering as adults are among the species mostly benefiting from increased spring temperatures, whereas species overwintering as pupae are likely to increase the number of generations produced per year (Virtanen & Neuvonen 1999; Teder 2020). The lack of effect of voltinism was surprising in light of the general trend towards an overall increase in voltinism in European Lepidoptera (Altermatt 2010; Pöyry et al.2011), which would assumedly affect both species phenology and range shifts. However, the effect of added generations may not offer actual benefits to all species, rather “tricking” some species into another generation too late in the season, which could cause a so-called lost-generation effect (cf. Pöyry et al. 2011).
Responding by advancing phenology was, overall, relatively rarely observed among the studied species, as a striking 73% of the species did not shift their phenology, or even delayed it. This result is surprising and in contrast to several reports that are based on similar data and show strong advances in Lepidoptera phenology (e.g., Roy & Sparks 2000; Stefanescu et al. 2003; Diamond et al. 2011). However, most of the previous studies have been conducted in temperate regions, and regional differences in abiotic conditions and rates of climate change may induce different responses (Renner & Zohner 2018). Several other species groups in Finland have, however, been reported to advance their phenology (e.g., Helama et al. 2020 for plants; Lehikoinen et al. 2019 for birds). It is possible that other environmental factors prevailing in northern latitudes, such as light conditions (Arietta et al. 2020; Hodgson et al. 2011) or variation in weather conditions in the early season, limit the possibility for advanced phenology of Lepidoptera. These findings are also in line with those by Fric et al. (2020) who observed less advancement and even delays in early flight periods of butterflies towards higher latitudes.
Although there was a positive connection between phenological response and population trends, the effect was not statistically significant. Radchuk et al. (2019) found that even though phenological advance is often stated as an adaptive strategy under climate change, this may not be the rule for all species, nor is phenological advance always enough to provide fitness benefits under ongoing rapid environmental change. Climate change also introduces more variable weather conditions (Rummukainen 2012; Vasseur et al. 2014), whereby environmental cues may become less reliable and advanced phenology may not offer the expected fitness benefits, but even cause declines in readily responding species. Additionally, declining populations may not only be less able to disperse and colonize new areas (fewer individuals that emigrate) but also have a lower potential for adjusting in situ, because of loss of genetic variability (Anderson 2016). Large declines in insect populations have recently been reported, and although these trends vary greatly between regions and taxa (Crossley et al. 2020; van Klinket al. 2020; Pilotto et al. 2020), our results also point to comprehensive population declines among Finnish Lepidoptera as 38.5% of the studied species showed negative population trends.
In contrast to advanced phenology alone, northwards shifts in NRB was associated with significantly stronger population trends. Our results also show that the studied species are more often capitalizing on range shifts than phenology shifts. This is in line with previous studies that documented strong range shifts among Lepidoptera (Parmesan et al.1999; Kharouba et al. 2009; Pöyry et al. 2009; Masonet al. 2015) and points to range shifts perhaps being a more readily available response for many species of Lepidoptera. Simultaneously, however, only less than half (45%) of the species studied here, had shifted their NRBs northwards. Habitat availability plays a crucial role when species are moving as a response to climate change (Platts et al. 2019), and other abiotic factors than rising temperatures are likely to affect the ability of species to shift their ranges (Spence & Tingley 2020). In Finland, decrease in the area and quality of suitable habitats is known to have substantial negative effects on butterflies (Kuussaari et al. 2007; Ekroos et al. 2010; Pöyry et al. 2018). This highlights the importance of considering species dispersal in land-use planning as it is one of the main pathways through which species can adapt to ongoing changes. Halting habitat decline and fostering the persistence and even reconstruction of large and connected habitat areas can help sustain large enough populations that can both colonize new area and harbor sufficient genetic and phenotypic variation to respond, in situ, to global changes. Policies like the European Union’s Biodiversity Strategy for 2030 that aims at protecting at least 30% of terrestrial and aquatic areas (European Commission 2020) could enable more species to combine the two viable strategies for maintaining within their thermal niche.
Our study highlights that combining advanced phenology and a northwards range shift provides the best potential for population viability. Among boreal Lepidoptera, however, only a small proportion of species are currently able to use both responses to form a winning strategy. Together with the large proportion of species that were not able to utilize either of the adaptive responses, this indicates that moths and butterflies in Finland are presently on a track towards becoming either winners or losers, and that this division is likely strongly affected by habitat availability and species’ abilities to make use of newly available habitat and adjust appropriately within their ranges.