Changes in the Summer Wild Bee Community Following a Bark Beetle Outbreak in a Douglas-fir Forest

Abstract

The status of wild bees has received increased interest following recent estimates of large-scale declines in their abundances across the United States. However, basic information is limited regarding the factors affecting wild bee communities in temperate coniferous forest ecosystems. To assess the early responses of bees to bark beetle disturbance, we sampled the bee community of a Douglas-fir, Pseudotsuga menziesii (Mirb.), forest in western Idaho, United States during a Douglas-fir beetle, Dendroctonus pseudotsugae Hopkins (Coleoptera: Curculionidae), outbreak beginning in summer 2016. We resampled the area in summer 2018 following reductions in forest canopy cover resulting from mortality of dominant and codominant Douglas-fir. Overall, results from rarefaction analyses indicated significant increases in bee diversity (Shannon’s H) in 2018 compared to 2016. Results from ANOVA also showed significant increases in bee abundance and diversity in 2018 compared to 2016. Poisson regression analyses revealed percent tree mortality from Douglas-fir beetle was positively correlated with increases in total bee abundance and species richness, where community response variables displayed a cubic trend with percent tree mortality. Percent reduction in canopy cover from 2016 to 2018 was also correlated with bee species richness and diversity. These findings suggest that wild bee communities may benefit from changes in forest structure following bark beetle outbreaks.

Canopy cover and seasonality are associated with variation in native bee assemblages across a mixed pine‐juniper woodland

“…These collective results provide new data on the importance of variation in T A B L E 2 Summary of a generalized linear model analysing effects of canopy openness (%), collection period, and their interaction on bee abundance in a pine-juniper woodland In the fire-prone conifer ecosystems of North America, thinning of overstory trees to reduce canopy density and increase canopy base height is often applied as a tactic for enhancing ecological restoration efforts and reducing wildfire risk (Brudvig et al, 2011;Harrod et al, 2009). A growing body of evidence demonstrates that the canopy plays a key role in regulating localized pollinator community αand β-diversity, with consistent effects across a wide variety of forest ecosystems (e.g., Burdine & McCluney, 2019;Foote et al, 2020;Hanula et al, 2016;McCabe et al, 2019, Odanaka et al, 2020and others); our data match these earlier studies. This pattern implies most forest management activities (as well as natural disturbances) that result in reduction of canopy density or cover are likely to have beneficial effects on pollinators, and this relationship can be exploited by managers to improve pollinator habitat across a variety of ecosystem cover types widespread in temperate regions.…”

“…Although this is convenient for vegetation management, our sampling design in the present study does not allow for conclusions about the causal mechanisms linking canopy cover to bee abundance. Some recent studies of forest bee communities show that reduced canopy cover is associated with recruitment of forbs and therefore drives higher bee abundances by improving foraging habitat (e.g., Davis et al, 2020;Foote et al, 2020;Rhoades et al, 2018), but is also possible that (1) greater visibility of capture devices (blue vane traps), (2) changes in thermal conditions, or…”

“…Although this is convenient for vegetation management, our sampling design in the present study does not allow for conclusions about the causal mechanisms linking canopy cover to bee abundance. Some recent studies of forest bee communities show that reduced canopy cover is associated with recruitment of forbs and therefore drives higher bee abundances by improving foraging habitat (e.g., Davis et al, 2020; Foote et al, 2020; Rhoades et al, 2018), but is also possible that (1) greater visibility of capture devices (blue vane traps), (2) changes in thermal conditions, or (3) availability of nesting sites underlie the reported pattern of greater bee abundance and diversity in areas with low canopy cover. We only use a single sampling method for surveying bee populations (blue vane traps); greater sample richness and diversity can be obtained by incorporating other collection methods including netting and use of coloured pan traps (Prendergast et al, 2020).…”

“…Throughout most temperate regions of the northern hemisphere coniferous forests are a predominant cover type, and a variety of recent studies show that forests are an important reservoir for native bee biodiversity (reviewed by Hanula et al, 2016). Bee assemblages in temperate conifer forests are responsive to disturbances including fire (Burkle et al, 2019), insect outbreaks (Davis et al, 2020; Foote et al, 2020), and forest management (Rivers et al, 2018). One apparent link among abiotic, biotic, and anthropic disturbances is that they each typically involve either destruction or removal of live canopy, which has cascading effects on physical (Prévost & Raymond, 2012) and ecological (Kneeshaw & Bergeron, 1998) properties of forest stands and can impact plants and animals inhabiting the understory, including bees.…”

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 ).…”