Widespread alterations in species distribution and abundance as a result of global environmental change include upwards and polewards shifts driven by local extinctions in the south or at lower elevations and colonizations of newly available habitat elements in the north or at higher elevations. Although cumulative changes on patterns of community composition are also expected, studies following a community-level approach are still scarce. Here, we estimate changes in abundance and distribution of bumblebee (Bombus spp.) species over two decades along an elevational gradient to test whether these changes entailed concomitant alterations on patterns of community composition. Bumblebee species showed an overall trend to shift uphill their upper- or lower-elevational boundaries, resulting in narrower elevational ranges from one period to another, coincident with a regional warming of ca. 0.9 °C. Changes in elevational ranges were, however, mainly related to retractions of the lower limit of species distribution, rather than to variations in their upper elevational limit. Species turnover was associated with colonization and extinction events and also with variability in the relative abundance of short-, medium- and long-tongued species along the elevational gradient. Extinctions were especially relevant at medium elevations, while only communities at higher elevations had a positive net outcome between colonization and extinction events. The combination of these effects resulted in the homogenization of bumblebee assemblages, especially between medium and upper elevations. The changes reported in our study strongly match with predictions of global change driving elevational shifts in species distribution and provide the first evidence of elevational changes in bumblebees at both species and community level.

Aim Our aim was to determine the role of environmental variables in explaining occurrence and abundance patterns of bumblebee (Bombus) species in a mountain region. We also used a historical dataset to compare historical and recent habitat suitability predictions for forecasting variations in species' responses to regional climate warming.Location The Cantabrian Range (Iberian Peninsula, south-western Europe). vulnerability assessments for conservation-focused management planning.

for research. Most schemes sample sites that are self-selected by contributors and therefore tend 20 to cover locations that are rich in butterflies. To provide a more representative assessment of 21 butterfly populations, the Wider Countryside Butterfly Survey (WCBS) was developed with a 22 stratified-random sample of survey sites across the UK. We compare butterfly trends from the 23 WCBS locations against those measured from traditional butterfly transects which are typically 24 located in areas of good quality semi-natural habitats. Across the 26 species analysed, there was a 25 significant positive relationship between trends measured from the two schemes between 2009 and 26 2013, the period when both schemes were operating fully. There was a tendency (17 out of 26 27 species analysed) for these changes to be greater within WCBS compared to traditional BMS 28 transects, although this effect was not consistent across comparisons between pairs of consecutive 29 years. When assessing these individual year-to-year changes, there was however a significant 30 correlation between the two schemes in all cases. Over relatively short time periods, weather 31 patterns are likely to dominate butterfly population fluctuations and lead to comparable trends 32 across monitoring schemes. Over longer time periods, differences in land management may affect 33 habitat condition differently for protected areas versus the wider countryside and it is therefore 34 important to maintain comprehensive butterfly monitoring programmes to detect and interpret 35 such effects. 36 3

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