Bark beetle outbreaks enhance native bee biodiversity in alpine forest

Bark beetle outbreaks alter forest structure with direct and indirect effects on plants and animals in forest ecosystems. We tested how tree mortality from bark beetles impacts bee foraging habitats and populations in an alpine spruce forest. Bees were collected across the growing season for two years and collections were used to analyze patterns in species abundances and diversity. Three important findings emerged: (1) forest stands that were post-outbreak had higher floral density and more floral species during peak bloom, respectively, than non-affected stands; (2) bee captures were highest early-season (June) and were not strongly affected by bark beetle outbreak; however, mean number of bee species and Shannon–Weiner diversity were significantly higher in post-outbreak stands and this effect was pronounced early in the growing season. Corresponding analysis of β-diversity indicated higher accumulation of bee biodiversity in post-outbreak stands and a turnover in the ratio of Bombus: Osmia; (3) bee captures were linked to variation in foraging habitat, but number of bee species and diversity were more strongly predicted by forest structure. Our results provide evidence of increased alpine bee biodiversity in stands that have experienced bark beetle outbreaks, and evidence for increased availability of floral resources. We conclude that large-scale disturbance from bark beetle outbreaks may drive shifts in pollinator community composition through cascading effects on floral resources, mediated via mortality of overstory trees.