High-elevation range limit of an annual herb is neither caused nor reinforced by declining pollinator service

1. Pollination failure has been proposed to be an important determinant of plant species’ range limits, if pollinator activity declines along an environmental gradient, directly limiting plant populations, or if plant populations decline along an environmental gradient and subsequently fail to attract sufficient visitation. Both mechanisms predict reduced pollinator visitation, increased pollen limitation, and decreased seed production towards range limits, and the first additionally predicts declining pollinator abundance independent of any particular plant species. However, many self-compatible species have some capacity for autonomous self-fertilization, which may buffer reproductive success from declining pollinator visitation if inbreeding depression is mild. Thus pollinator-mediated limits may also predict selection for reduced reliance on pollinators towards range limits. 2. We tested these predictions towards the high-elevation limit of the self-compatible, bumble bee (Bombus) p...