Wildfires Influence Abundance, Diversity, and Intraspecific and Interspecific Trait Variation of Native Bees and Flowering Plants Across Burned and Unburned Landscapes

Abstract

Wildfire regimes are changing in the western United States, yet the ways in which wildfires influence native bees, the resources they depend on for food and nesting, or the traits that influence their interactions with plants are poorly understood. In burned and unburned areas in Montana, USA, we investigated the abundance and diversity of native bees, floral and nesting resources, nesting success, and traits of flowers and bees. In two of the three localities studied, burned areas, including areas that burned with high-severity wildfires, supported higher density and diversity of native bees and the flowers they depend on for food and larval provisioning. Burned areas also had more bare ground for ground-nesting bees and more available coarse woody debris for cavity-nesting bees than unburned areas. Moreover, cavity-nesting bees were completely unsuccessful at nesting in artificial nesting boxes in unburned areas, while nesting success was 40% in burned areas. Mean bee intertegular distance (a trait strongly correlated with tongue length, foraging distance, and body size) was similar between burned and unburned areas. However, wildfires influenced both interspecific and intraspecific trait variation of bees and plants. Intraspecific variation in bee intertegular distance was higher in unburned than burned areas. Both interspecific and intraspecific variation in floral traits important for interactions with pollinators were generally higher in burned than unburned areas. Thus, wildfires generally increased the density and species diversity of bees and flowers as well as trait variation at both trophic levels. We conclude that wildfires-even large, high-severity wildfires-create conditions that support native bees and the resources they need to flourish, but that unburned areas maintain trait variation in landscape mosaics with heterogeneous fire conditions.

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

“…In addition to Osmia spp., bumblebees (Bombus spp.) were abundant in our study and are often among the most common taxa found at high-elevation sites 31,32 , due in part to their relatively high thermal mass in comparison to other bees 24,25,33 . Our collections provide evidence of shifts in Bombus assemblages following bark beetle outbreak: B. flavifrons was substantially more abundant in post-outbreak stands, whereas several high-elevation specialists were more frequent in non-affected stands including B. balteatus and B. melanopygus.…”

“…A comparison of several recent studies and the current report reveals a growing body of evidence that both natural and managed ecosystem disturbances are important for maintaining biodiversity of native bee populations across coniferous forest landscapes. For example, complementary findings from Burkle et al 32 and Galbraith et al 20 indicate that wildfire disturbances are associated with increased diversity of multiple bee communities and across multiple conifer forest cover types. In addition to effects on taxonomic diversity, bee functional trait variation both within and between species are also responsive to these disturbances 32 .…”

“…For example, complementary findings from Burkle et al 32 and Galbraith et al 20 indicate that wildfire disturbances are associated with increased diversity of multiple bee communities and across multiple conifer forest cover types. In addition to effects on taxonomic diversity, bee functional trait variation both within and between species are also responsive to these disturbances 32 . Similarly, studies from boreal forests in Europe report that prescribed fires in Scots pine (Pinus sylvestris L.) forests result in distinct bee assemblages with improved pollination services 34 , and most solitary bees are not predicted to be directly negatively affected (e.g., by mortality from soil heating) during fire disturbances 35 .…”

“…Across many ecosystems, bark beetle outbreaks are drivers of landscape-scale spatial heterogeneity across large areas and contribute to the structural complexity of forest systems 42 , 43 . Structural heterogeneity and complexity are strong predictors of gene flow and biodiversity in bee communities 43 , 44 and various authors have concluded that a landscape mosaic of disturbances (and varying disturbance severity) that results in a range of structural classes enhances collective pollinator richness, ecosystem function, and ecosystem services 32 , 45 – 48 . Among ecosystem managers, as well as the general public, bark beetle outbreaks are typically considered as undesirable disturbances for which prevention strategies should be sought 5 , but our study suggests that outbreaks may help to create habitat for pollinator communities in wilderness settings.…”

Biotic and abiotic drivers of plant–pollinator community assembly across wildfire gradients

“…Plots were visited in random order during these hours. We considered pollinators to be any Hymenoptera (Burkle et al, 2019;Reese et al, 2018), and results were similar if we conducted all analyses using only Hymenoptera.…”