Results
We found, on average, no systematic shift in any direction across the
species responses (Fig. 3). Among the 289 species studied, 45.3% of
species expanded their NRB towards the north and 27% of species
advanced their phenology (Fig. 3). By contrast, 40.8% of species
contracted their NRB towards the south and 35.6% delayed their
phenology. This lack of a systematic directional shift was evidenced by
the fact that the estimates of the intercept-only PGLS -models were not
significantly different from zero. We also found no difference in the
estimates between the two main taxonomic groups, butterflies and moths
(Table S2).
Almost half of the species (46.7%) responded according to hypothesis 1,
i.e. they either shifted their NRB northwards or their phenology
earlier, but not both. A minority of the studied species (12.8%)
responded according to hypothesis 2, i.e. they shifted both their NRB
northwards and phenology earlier. Finally, 40.5% of the species showed
no adaptive response, i.e. they neither shifted their NRB northwards nor
phenology earlier (hypothesis 0). Instead, the NRBs remained stable or
shifted southwards and their phenology remained stable or delayed.
More than half of the studied species (61.5%) showed positive or stable
population trends, but on average there was no systematic trend for
neither positive nor negative population trends across the studied
species (Fig. 3). Nevertheless, population trends differed between
species that responded differently in NRB and phenology shifts. Species
that advanced their phenology showed more positive (although
insignificant) population trends over the study period than those that
delayed or did not change their phenology (Table 2 - model a). Species
that shifted their NRBs further north (>20 km) showed
significantly stronger positive population trends compared to other
species (Table 2 – model b; Fig. 4a). In addition, species that
advanced their phenology tended to move their NRB more towards the
north, but this effect was not significant (Table 2 – model c). The
positive effect of a northwards shift in NRB on population trends in
model b) was also mirrored in model d) which indicates that both
combined responses including a northwards shift, no matter how the
species reacted phenology-wise, showed stronger positive population
trends (Table 2 – model d). This effect was stronger for species that
also advanced their phenology. Thus, species able to utilize a combined
response as postulated by Hypothesis 2 (both northwards shift of NRB and
advance in phenology) showed significantly stronger population trends
(Table 2 – model e; Fig. 4b). An ability to utilize either of the
responses (as postulated by Hypothesis 1) showed, on average, lower but
also positive population trend, but this effect was not significant. The
species that were not able to utilize either of the presumed adaptive
responses (Hypothesis 0) showed the lowest, and on average negative
population trends (Table 2 – model e; Fig. 4b).
None of the four life-history traits tested showed a significant
connection with population trends. Overwintering stage was the
only trait that had an effect on shift in phenology and on NRB (Fig. S6;
Table S3). Species overwintering as adults were more likely to advance
their phenology while species that overwinter as pupae tended to retreat
their NRB towards the south. However, due to imbalance in the number of
species representing different host plant-use categories, we had
combined species feeding on lichen and fungi into the specialist group
(8 species; 2.7% of studied species: see Methods ). In an
additional PGLS analysis treating lichen and fungi feeders as a separate
group, the eight species that feed on lichen and fungi showed a
significant shift in their NRB further north (t= 1.97; p<0.05)
and also had more positive population trends (t= 3.93;
p<0.001; Fig. S7).