Contrasting patterns of richness, abundance, and turnover in mountain bumble bees and their floral hosts

Abstract

Environmental gradients generate and maintain biodiversity on Earth. Mountain slopes are among the most pronounced terrestrial environmental gradients, and the elevational structure of species and their interactions can provide unique insight into the processes that govern community assembly and function in mountain ecosystems. We recorded bumble bee–flower interactions over 3 years along a 1400‐m elevational gradient in the German Alps. Using nonlinear modeling techniques, we analyzed elevational patterns at the levels of abundance, species richness, species β‐diversity, and interaction β‐diversity. Though floral richness exhibited a midelevation peak, bumble bee richness increased with elevation before leveling off at the highest sites, demonstrating the exceptional adaptation of these bees to cold temperatures and short growing seasons. In terms of abundance, though, bumble bees exhibited divergent species‐level responses to elevation, with a clear separation between species preferring low versus high elevations. Overall interaction β‐diversity was mainly caused by strong turnover in the floral community, which exhibited a well‐defined threshold of β‐diversity rate at the tree line ecotone. Interaction β‐diversity increased sharply at the upper extreme of the elevation gradient (1800–2000 m), an interval over which we also saw steep decline in floral richness and abundance. Turnover of bumble bees along the elevation gradient was modest, with the highest rate of β‐diversity occurring over the interval from low‐ to mid‐elevation sites. The contrast between the relative robustness bumble bee communities and sensitivity of plant communities to the elevational gradient in our study suggests that the strongest effects of climate change on mountain bumble bees may be indirect effects mediated by the responses of their floral hosts, though bumble bee species that specialize in high‐elevation habitats may also experience significant direct effects of warming.

High species turnover and unique plant–pollinator interactions make a hyperdiverse mountain

“…Olympus, specifically unimodal clinal patterns for bumblebees and hoverflies due to their ability to cope with high‐altitude low temperatures (McCabe & Cobb, 2021; Peters et al, 2016); this is in line with their α‐diversity patterns found in previous studies on mountains around the world (Table 1, lines 10–14, 18–19). Studies on elevational patterns of β‐diversity of pollinators have always shown a linear increase, usually deriving from the high turnover component (Table 1, lines 2, 6, 8, 13; but see line 4 for the prevalence of nestedness component); besides, a meta‐analysis considering different organisms and ecosystems also showed prevalence of the turnover component (Soininen et al, 2018). In addition, studies on β‐diversity of interactions were also found high turnover component (Encinas‐Viso et al, 2022; Simanonok & Burkle, 2014; Sponsler et al, 2022). We predict that this will be also the case in Mt.…”

Cuticular hydrocarbons of alpine bumble bees (Hymenoptera: Bombus) are species-specific, but show little evidence of elevation-related climate adaptation

“…Albeit CHC profiles and climate-associated chemical traits did not change systematically along the elevational gradient in B. pascuorum, we detected a decline in trait variance: Individuals collected in high elevations-especially above the tree line-differed less in the proportion of saturated components than individuals from low-or mid-elevations. A similar reduction of trait variation with elevation, and with the tree line as potential threshold, was detected in other functional traits, including bumble bee tongue length (Sponsler et al, 2022b) and wild bee body size (Classen et al, 2017). Pronounced environmental filtering at habitats with less variable microclimatic conditions (Hoiss et al, 2012) might cause such declines.…”

“…Alpine bumble bees (Apidae: Bombus) are the most efficient and widespread pollinators in temperate mountains (Bingham and Orthner, 1998;Gorenflo et al, 2017). Within the genus Bombus, species differ substantially in their preferred elevational niche (Rasmont et al, 2015;Minachilis et al, 2020;Sponsler et al, 2022a), with few species being restricted to the cool and exposed conditions above the tree line. Within a species, populations and individuals can face substantial differences in their climatic environments along mountain slopes or during foraging flights under the typically fluctuating weather conditions of the mountains (Sponsler et al, 2022a), occasionally, when the cloud cover shifts abruptly, within minutes.…”

“…Within the genus Bombus, species differ substantially in their preferred elevational niche (Rasmont et al, 2015;Minachilis et al, 2020;Sponsler et al, 2022a), with few species being restricted to the cool and exposed conditions above the tree line. Within a species, populations and individuals can face substantial differences in their climatic environments along mountain slopes or during foraging flights under the typically fluctuating weather conditions of the mountains (Sponsler et al, 2022a), occasionally, when the cloud cover shifts abruptly, within minutes. Despite their broad temperature tolerance (Peters et al, 2016), many bumble bee species are sensitive to climate change.…”

Temperature and livestock grazing trigger transcriptome responses in bumblebees along an elevational gradient

“…On a genus level, they encompass wide geographic ranges, with diverse hotspots in both cool mountainous regions and high temperate latitudes ( Williams, 1998 ), but also with several species in the Neotropics and Southeast Asia ( Michener, 2007 ). In temperate mountains like the Alps, many, but not all, species have broad elevational distributions and cope with climatically heterogeneous environments ( Sponsler et al., 2022 ).…”