Aim To quantify spatio‐temporal changes in species richness and assemblage composition of Danish butterflies over more than 100 years and to assess whether different functional groups of butterflies show different temporal trajectories. Location Denmark, Europe. Methods We applied species accumulation curves to estimate relative species richness changes from unequally sampled occurrence records, collected over more than a century (1900–2012), at local (i.e. 10 km× 10 km) and regional (country‐wide) spatial scales. Furthermore, we calculated similarity in species composition between 10 km× 10 km grid cells to evaluate changes in assemblage composition (i.e. biotic homogenization) of butterflies through space and time. Trends were evaluated for all butterflies as well as for three functional groups, using hierarchical clustering and species‐specific values of eight ecological traits. Results We document severe declines in butterfly diversity over the last century, with substantial regional‐scale extinctions leading to a net loss of 10% of all Danish butterfly species. Segregating species into functional groups showed that the highest rate of regional extinction occurred among sedentary habitat specialists overwintering in the egg stage, while the most severe local‐scale declines occurred among sedentary host plant specialists overwintering in the larval stage. In contrast, mobile generalist species with mature overwintering stages showed stable richness trends throughout the century. However, a pervasive spatio‐temporal biotic homogenization of butterfly assemblages was revealed for all functional groups. Main conclusions Our results suggest that observed changes in species richness were driven by a gradual replacement of ecological specialists by broadly adapted ecological generalists. In addition to drawing attention to a severe long‐term impoverishment of the Danish butterfly fauna, our results suggest that trajectories of butterfly species richness and assemblage composition over the last century are related to multiple functional traits, highlighting the need to consider different aspects of ecological specialization when assessing extinction risk.

Aim To quantify whether species distribution models (SDMs) can reliably forecast species distributions under observed climate change. In particular, to test whether the predictive ability of SDMs depends on species traits or the inclusion of land cover and soil type, and whether distributional changes at expanding range margins can be predicted accurately. Location FinlandMethods Using 10-km resolution butterfly atlas data from two periods, 1992-99 (t1) and 2002-09 (t2), with a significant between-period temperature increase, we modelled the effects of climatic warming on butterfly distributions with boosted regression trees (BRTs) and generalized additive models (GAMs). We evaluated model performance by using the split-sample approach with data from t1 ('nonindependent validation'), and then compared model projections based on data from t1 with species' observed distributions in t2 ('independent validation'). We compared climate-only SDMs to SDMs including land cover, soil type, or both. Finally, we related model performance to species traits and compared observed and predicted distributional shifts at northern range margins. ResultsSDMs showed fair to good model fits when modelling butterfly distributions under climate change. Model performance was lower with independent compared with non-independent validation and improved when land cover and soil type variables were included, compared with climate-only models. SDMs performed less well for highly mobile species and for species with long flight seasons and large ranges. When forecasting changes at northern range margins, correlations between observed and predicted range shifts were predominantly low.Main conclusions SDMs accurately describe current distributions of most species, yet their performance varies with species traits and the inclusion of land cover and soil type variables. Moreover, their ability to predict range shifts under climate change is limited, especially at the expanding edge. More tests with independent validations are needed to fully understand the predictive potential of SDMs across taxa and biomes.

The response of body size to increasing temperature constitutes a universal response to climate change that could strongly affect terrestrial ectotherms, but the magnitude and direction of such responses remain unknown in most species. The metabolic cost of increased temperature could reduce body size but long growing seasons could also increase body size as was recently shown in an Arctic spider species. Here, we present the longest known time series on body size variation in two High-Arctic butterfly species: Boloria chariclea and Colias hecla. We measured wing length of nearly 4500 individuals collected annually between 1996 and 2013 from Zackenberg, Greenland and found that wing length significantly decreased at a similar rate in both species in response to warmer summers. Body size is strongly related to dispersal capacity and fecundity and our results suggest that these Arctic species could face severe challenges in response to ongoing rapid climate change.

Current global warming is particularly pronounced in the Arctic and arthropods are expected to respond rapidly to these changes. Long-term studies of individual arthropod species from the Arctic are, however, virtually absent. We examined butterfly specimens collected from yellow pitfall traps over 14 years (1996–2009) at Zackenberg in high-arctic, north-east Greenland. Specimens were previously sorted to the family level. We identified them to the species level and examined long-term species-specific phenological responses to recent summer warming. Two species were rare in the samples (Polaris fritillary Bolo-ria polaris and Arctic blue Plebejus glandon) and statistical analyses of phenological responses were therefore restricted to the two most abundant species (Arctic fritillary, B. chariclea and Northern clouded yellow Colias hecla). Our analyses demonstrated a trend towards earlier flight seasons in B. chariclea, but not in C. hecla. The timing of onset, peak and end of the flight season in B. chariclea were closely related to snowmelt, July temperature and their interaction, whereas onset, peak and end of the flight season in C. hecla were only related to timing of snowmelt. The duration of the butterfly flight season was significantly positively related to the temporal overlap with floral resources in both butterfly species. We further demonstrate that yellow pitfall traps are a useful alternative to transect walks for butterfly recording in tundra habitats. More phenological studies of Arctic arthropods should be carried out at the species level and ideally be analysed in context with interacting species to assess how ongoing climate change will affect Arctic biodiversity in the near future.

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