Introduction
Biodiversity is threatened by a multitude of anthropogenic factors,
including ongoing climate change (IPCC 2014; Díaz et al. 2019).
Under environmental change, species have two avenues for escaping
decline and consequent extinction: adapting in situ through plastic or
evolutionary responses, or moving to areas where conditions are more
favorable (Davis et al. 2005). Phenology and range shifts are the
most conspicuous responses of species to rapid environmental change, and
they have been referred to as the fingerprints of climate change
(Parmesan & Yohe 2003; Root et al. 2003). The spatial
distribution of a species and the temporal manifestation of life-history
events reflect its realized niche (Socolar et al. 2017). As
shifts in distribution and phenology represent the mechanisms through
which a spatial or temporal change in the utilization of the niche can
be observed (Amano et al. 2014), they are expected to offer
fitness benefits as means of adaptation under climate change. Species
can adapt to climatic change by advancing their early-season phenology
in accordance with warmer temperatures (Parmesan & Yohe 2003). Such
shifts in phenology have often been shown to be related with positive
population trends and increased demographic stability (Cleland et
al. 2007; Møller et al. 2008; Saino et al. 2011; Frankset al. 2018). Range shifting towards cooler areas is the other
main strategy for species to adapt to warmer climatic conditions, both
under contemporary and past climate changes (Parmesan et al.1999; Davis & Shaw 2001; Donoghue 2008; Spence & Tingley 2020).
Species that are able to shift their geographic distribution to track
climate change and thus remain within their climatic niches are less
likely to suffer population declines and local extinctions (Cooperet al. 2011; Devictor et al. 2012; Urban 2015).
The most straightforward expectation is that species would use either
range or phenology shift to respond to changing climatic conditions
(Hypothesis 1; Fig. 1a). Mobile species would not have as strong a need
to advance their phenology as they can track suitable conditions in
space. Less mobile species, however, will experience stronger selection
pressure to adapt in situ and adjust their phenology in order to
maintain suitable thermal conditions during critical life-history events
(Amano et al. 2014; Socolar et al. 2017). Indications for
such complementary thermal niche tracking via space or time have been
found for both plants and birds (Amano et al. 2014; Socolaret al. 2017). Nevertheless, these strategies are not necessarily
mutually exclusive and can provide enhancing benefits that together
buffer species against climate warming and increased variation in
extreme events. In such a case, species would benefit from usingboth range and phenology shift (Hypothesis 2; Fig. 1a). The
ability to adapt in one dimension of niche utilization is likely to be
correlated with high responsiveness to variability in others, not least
through positive feedback loops that increase population persistence
(Willis et al. 2010). Phenological timing is perhaps the most
important aspect of life-history that affects species distributions
(Chuine 2010), as it defines where and how successfully individuals of a
population can proliferate. Hypothesis 2 is thus based on the assumption
that adaptive in situ responses in phenology increase the fitness of
individuals, leading to higher survival rates and more offspring
(Cleland et al. 2007). Evidence suggests that stable or positive
population trends, that is, no change or increase in abundance, are a
prerequisite for species to expand their ranges (Mair et al.2014) as emigration is higher from larger populations (Pärn et
al. 2012; Glorvigen et al. 2013). Also, the probability of
successful establishment increases with the summed contribution of
individuals from neighboring source populations (Hanski et al.1995; Hanski & Ovaskainen 2003).
In this study, we assess how 289 species of Lepidoptera in Finland have
responded to nearly 30 years of climate change. Lepidoptera have been
shown to be responsive to climate change, as exemplified by the observed
range shifts towards higher latitudes (Parmesan et al. 1999;
Pöyry et al. 2009; Chen et al. 2011) and altitudes
(Konvicka et al. 2003; Wilson et al. 2005), as well as
phenology shifts (e.g., advance in the date of first appearance [Roy
& Sparks 2000; Stefanescu et al. 2003; Diamond et al.2011] and increased voltinism [Altermatt 2010; Pöyry et al.2011]). Previous studies have shown that the distribution of
butterflies in Finland is mainly determined by climatic factors (Luotoet al. 2006) and that the phenology of moths tends to be
controlled by temperature (Valtonen et al. 2014). During the last
few decades, the mean temperature in Finland has risen by 0.2-0.4
°C/decade (Mikkonen et al. 2014; Fig. S1), with springs starting
earlier and the timing of phenological events being advanced for many
species (Helama et al. 2020; Hällfors et al. 2020).
Therefore, we expect Lepidoptera to respond to changing climatic
conditions by either shifting their ranges or adjusting their phenology
in situ, or both. However, it is unclear how these different strategies
influence species’ population trends. Here we ask whether Lepidoptera
have advanced their mean flight period (=adaptive phenology shift) or
shifted their northern range boundary (NRB) northwards (=adaptive range
shift). Using these response estimates, we test two main hypotheses: do
the same species tend to 1) either advance their mean flight
period or shift their NRB northwards, or 2) both advance
their mean flight period and shift their NRBs northwards (Fig.
1b). To gain understanding on the potential causes and effects of the
response combinations, we further relate the responses to life-history
traits and population trends (Fig. 1).