Bumblebee community homogenization after uphill shifts in montane areas of northern Spain.

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.