High-altitude multi-taskers: bumble bee food plant use broadens along an altitudinal productivity gradient

Abstract

We use an extensive historical data set on bumble bee host choice collected almost 50 years ago by L. W. Macior (Melanderia 15:1-59, 1974) to examine how resource partitioning by bumble bees varies over a 2,700-m altitudinal gradient at four hierarchical scales: individual, colony, species and community. Bumble bee behavior, resource overlap between castes, and plant-bumble bee networks change with altitude in accordance with tightening temporal constraints on flowering and colony growth in alpine habitats. Individual bees were more likely to collect pollen from multiple sources at high altitude. Between-caste foraging niche overlap increased with altitude. Similarly, alpine forager networks were more highly nested than either subalpine or montane networks due to increased asymmetric specialization. However, interspecific resource partitioning showed a more complex spatial pattern with low niche overlap at intermediate altitude (subalpine) compared to montane (disturbed) and alpine (unproductive) sites. Results suggest that spatial variation in interspecific resource partitioning is driven by a shift in the behavior of long-tongued bumble bees. Long-tongued bumble bees specialized in the subalpine but generalized in montane and alpine zones. Our reanalysis of Macior's data shows that bumble bee behavior varies substantially with altitude influencing plant-bumble bee interaction networks. Results imply that pollination services to alpine host plants will change dramatically as subalpine species with unique foraging strategies move upward under global warming.

Beta diversity and specialization in plant–pollinator networks along an elevational gradient

“…However, as far as we know, the finding that pollinator insects become more opportunistic at higher elevations and thus have greater trophic niche breadth has not been reported in prior studies. This result is consistent with the expected reduction in interspecific competition in unproductive environments at high elevations (Ebeling et al, ; Fründ et al, ; MacArthur & Pianka, ), which may lead to niche expansion of pollinator species (Hoiss et al, ; Miller‐Struttmann & Galen, ). The findings are also congruent with previous research in other pollinator taxa, such as hummingbirds, which have been found to be more generalist at high elevations because of both food niche expansion and environmental filtering (Maglianesi et al, ; but see Dalsgaard et al, for contrasting results).…”

“…This is actually consistent with physiology‐based theories, such as those proposed to explain Bergmann's rule (Classen et al, ). Still, the association between body size and resource use, dispersal capacity and pollination efficiency (Chown & Gaston, ) might also explain the positive relationship between elevation and insect body size at the community scale, especially when trophic resources decrease with elevation (Miller‐Struttmann & Galen, ; Ramos‐Jiliberto et al, ). Pollinator body‐size is highly positively correlated with foraging distance and proboscis length in many taxa (Agosta & Janzen, ; Byrne, Buchmann, & Spangler, ; Casey, May, & Morgan, ; Greenleaf, Williams, Winfree, & Kremen, ; Levy & Nufio, ).…”

Flight of the bumble bee: Buzzes predict pollination services

“…We focused on the two main bumble bee species ( Bombus balteatus and B . sylvicola ) that are year-round residents and predominate above the timberline at our field sites [36–38]. …”

Specialization of plant–pollinator interactions increases with temperature at Mt. Kilimanjaro

“…Third, climate shapes plant and pollinator richness, composition and phenology and has been directly linked to network properties, including specialization (Petanidou et al, ). Temperature determines the costs of foraging flights in ectothermic pollinators (Kovac, Stabentheiner, & Brodschneider, ) and may thus modulate resource usage strategies in a way that species broaden their dietary spectrum in energy‐limited habitats (Miller‐Struttmann & Galen, ). Restricted foraging times due to persistent mist and/or temperatures below a threshold in which foraging is possible, should equally result in more generalized foraging.…”