Bark beetle outbreak enhances biodiversity and foraging habitat of native bees in alpine landscapes of the southern Rocky Mountains

Authors: DAVIS, THOMAS - Colorado State University; RHOADES, PAUL - Idaho State Department Of Agriculture; MANN, ANDREW - Colorado State University; Griswold, Terry

Submitted to: Scientific Reports
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 9/15/2020
Publication Date: 10/2/2020
Citation: Davis, T.S., Rhoades, P.R., Mann, A.J., Griswold, T.L. 2020. Bark beetle outbreak enhances biodiversity and foraging habitat of native bees in alpine landscapes of the southern Rocky Mountains. Scientific Reports. 10: Article 16400. https://doi.org/10.1038/s41598-020-73273-z.
DOI: https://doi.org/10.1038/s41598-020-73273-z

Interpretive Summary: Large-scale outbreaks of bark beetles change forests dramatically and they may affect the native bees that live in the forests. This study looked at how dead trees in an alpine spruce forest impacted the bee community. It found that forests with more dead trees had more flowers and more kinds of flowers for native bees. When there were the most bees (in June) amount of dead trees had no effect on bee abundance but there was greater diversity in kinds of bees. Alpine bee diversity increased after bark beetle outbreaks suggesting that these outbreaks may be important for conservation of pollination in forests.

Technical Abstract: Landscape-scale bark beetle outbreaks alter forest structure and may have direct or indirect effects on plants and animals in forest ecosystems. Using alpine spruce forest and a native bee community as a study system, we tested how tree mortality from bark beetles impacts bee foraging habitats and populations. Three important findings emerged: (1) the primary structural difference between stands affected by outbreak was the proportion of basal area mortality, and tree mortality had a positive linear relationship with floral resources. On average, stands that were post-outbreak had 62% higher floral density and 68% higher species richness during peak bloom, respectively, than non-affected stands. (2) Bee abundances were highest early-season (June) and were not strongly affected by bark beetle outbreak; however, bee species richness and a-diversity were significantly higher in post-outbreak stands and this effect was pronounced early in the growing season. Corresponding analysis of ß-diversity indicated higher accumulation of bee biodiversity in post-outbreak stands and a turnover in the ratio of Bombus spp.: Osmia spp. (3) Bee abundances at study sites were linked to variation in foraging habitat, but bee richness and diversity were more strongly predicted by forest structure. Our results provide evidence of increased alpine bee biodiversity in post-outbreak stands as well as increased availability of floral resources. We conclude that large-scale disturbance from bark beetle outbreaks has consequences for native pollinator communities and may be important for the conservation of key ecosystem services across forest landscapes.