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

Abstract

Landscape-scale bark beetle outbreaks alter forest structure with direct and 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. Bees were collected across the growing season (early-, middle-, and late-season) for two years using passive trapping methods, and collections were used to analyze patterns in species abundances and diversity. Three important findings emerged: (1) forest stands that were post-outbreak had 62% higher floral density and 68% 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 post-outbreak stands and 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.

Climate Change Alters Temperate Forest Canopies and Indirectly Reshapes Arthropod Communities

“…Likewise, increased canopy openness markedly changes the understory and herbaceous layers (e.g., Dietz et al, 2020). This promotes floral resources and in turn affects some guilds of canopy-dwelling species, relying on below-canopy resources, such as pollinators (Davis et al, 2020;Mathis et al, 2021; but see Urban-Mead et al, 2021).…”

Understory plant community responses to widespread spruce mortality in a subalpine forest

“…The response of the understory plant community to widespread Picea mortality has been understudied despite evidence that the understory is important for carbon cycling (Misson et al, 2007), forest regeneration (Pettit et al, 2019), and wildlife habitat (Davis et al, 2020; Przepióra et al, 2020). To understand how the understory plant community responded to widespread Picea mortality in southeastern Wyoming, we quantified changes in understory cover and richness, tree seedling densities, and understory species responses to disturbance.…”

“…lasiocarpa following the disturbance, suggesting understory cover may influence forest regeneration. Fourth, increases in understory richness may be important to local pollinators (Davis et al, 2020) and other wildlife communities following disturbance by increasing the diversity of plant resources available to them. Finally, priority effects, as measured by species’ pre‐disturbance frequency, as well as the traits plant height at maturity and leaf turgor loss point were predictors of species’ responses to disturbance suggesting that a variety of factors are responsible for variation in species responses.…”

“…Although the effects of spruce beetle outbreaks on subalpine forest overstory structure and composition have been documented across western North America (Veblen et al, 1991; Derderian et al, 2016), the effects on understory plant communities have received little attention (but see Davis et al 2020). The response of understory plants to spruce beetle disturbance in subalpine forests is particularly important for ecosystem functions such as carbon sequestration, wildlife habitat, and forest regeneration.…”

Arthropods and Fire Within the Biologically Diverse Longleaf Pine Ecosystem

“…Restoration thinning in the LLPE increases bee abundance, richness, and diversity ( Breland et al 2018 , Odanaka et al 2020 ). Indeed, it has been well demonstrated in multiple conifer systems that bee abundance and richness increase as basal area decreases ( Taki et al 2010 ; Hanula et al 2015 , 2016 ; Rhoades et al 2018 ; Ulyshen et al 2021a ), even to the point that forest canopy reductions due to a bark beetle outbreak significantly increased bee abundance and diversity ( Davis et al 2020 , Foote et al 2020 ). Upland sites contain higher bee abundance and species richness than flatwood sites ( Bartholomew and Prowell 2006 ).…”