In recent decades, many bumble bee species have declined due to changes in habitat, climate, and pressures from pathogens, pesticides, and introduced species. The western bumble bee (Bombus occidentalis), once common throughout western North America, is a species of concern and will be considered for listing by the U.S. Fish and Wildlife Service (USFWS) under the Endangered Species Act (ESA). We attempt to improve alignment of data collection and research with USFWS needs to consider redundancy, resiliency, and representation in the upcoming species status assessment. We reviewed existing data and literature on B. occidentalis, highlighting information gaps and priority topics for research. Priorities include increased knowledge of trends, basic information on several life-history stages, and improved understanding of the relative and interacting effects of stressors on population trends, especially the effects of pathogens, pesticides, climate change, and habitat loss. An understanding of how and where geographic range extent has changed for the two subspecies of B. occidentalis is also needed. We outline data that could be easily collected in other research projects that would increase their utility for understanding range-wide trends of bumble bees. We modeled the overall trend in occupancy from 1998 to 2018 of Bombus occidentalis within the continental United States using existing data. The probability of local occupancy declined by 93% over 21 yr from 0.81 (95% CRI = 0.43, 0.98) in 1998 to 0.06 (95% CRI = 0.02, 0.16) in 2018. The decline in occupancy varied spatially by landcover and other environmental factors. Detection rates vary in both space and time, but peak detection across the continental United States occurs in mid-July. We found considerable spatial gaps in recent sampling, with limited sampling in many regions, including most of ❖ www.esajournals.org 1 June 2020 ❖ Volume 11(6) ❖ Article e03141Alaska, northwestern Canada, and the southwestern United States. We therefore propose a sampling design to address these gaps to best inform the ESA species status assessment through improved assessment of how the spatial distribution of stressors influences occupancy changes. Finally, we request involvement via data sharing, participation in occupancy sampling with repeated visits to distributed survey sites, and complementary research to address priorities outlined in this paper.

Bumble bees (Bombus spp.) are declining across many regions in the Northern Hemisphere, leading to a need for management actions that will protect and enhance their habitats. In the Sierra Nevada of California, USA, montane chaparral is prevalent across the landscape, particularly after forest fires, and may provide important floral resources for pollinators. However post‐fire montane chaparral is often targeted for removal during reforestation efforts, to reduce competition with young trees. In 2015 and 2016, we conducted non‐lethal bumble bee surveys within 2 areas in the Sierra Nevada that burned in forest fires in 2004. Our goals were to describe bumble bee abundance and species richness in a post‐fire landscape, to compare results from chaparral‐dominated upland vegetation with results from interspersed patches of riparian vegetation, and to identify characteristics of individual chaparral stands that might make some stands more valuable to bumble bees than others. We captured 2,494 bumble bees of 12 species, and used Bayesian hierarchical modeling to determine that bumble bee abundance was substantially greater in riparian plots (modeled capture rate truex¯ = 1.10 ± 0.31 [SD] bees/survey in 2015, and 2.96 ± 0.83 bees/survey in 2016) than in upland plots (truex¯ = 0.47 ± 0.07 bees/survey in 2015, and 1.27 ± 0.18 bees/survey in 2016), which comprised a mix of chaparral shrubs and associated herbaceous plants. Modeled species richness was also greater in riparian plots, with an average mean richness of 4.1 ± 1.8 bumble bee species in riparian plots versus 2.3 ± 1.3 species in upland plots across the 2 years of the study. Within upland and riparian areas, plots dominated by herbaceous vegetation had greater abundance and species richness. One chaparral shrub species, bearclover (Chamaebatia foliolosa), was foraged on preferentially over all other shrub species and over all but 1 forb taxon, and was associated with increased occupancy probability in the Vosnesensky bumble bee (Bombus vosnesenskii), the most abundant bumble bee species on our study plots. A complex of closely related herbaceous species in the genus Phacelia, commonly associated with upland chaparral in our study area, was the plant taxon most frequently used by bumble bees, and appeared to be particularly important during mid‐summer after bearclover flowers became scarce. Our findings suggest that post‐fire chaparral communities are generally less intensively used by bumble bees than nearby riparian vegetation but may nevertheless provide important habitat. When chaparral removal is part of post‐fire forest regeneration strategies, bumble bees will likely benefit from retention of a mosaic of upland habitat patches dominated by herbaceous vegetation and, in our study area, bearclover, which may provide foraging resources throughout the life cycle of local bumble bee colonies. Because habitat characteristics affected the occupancy of individual bumble bee species differently, managers should consider foraging preferences of target bumble ...

Many bumble bee species (Bombus Latreille) have declined dramatically across North America and the globe, highlighting the need for a greater understanding of the habitat required to sustain or recover populations. Determining bumble bee species’ plant selection is important for retaining and promoting high-quality plant resources that will help populations persist. We used nonlethal methods to sample 413 plots within riparian corridors and meadows in the Sierra Nevada of California for bumble bees during two summers following extremely low and normal precipitation years, respectively. We assessed the five most abundant bumble bee species’ plant selection by comparing their floral use to availability. Additionally, we described the shift in plant selection between years for the most abundant species, Bombus vosnesenskii Radoszkowski. Bumble bee species richness was constant between years (13 species) but abundance nearly tripled from 2015 to 2016 (from 1243 to 3612 captures), driven largely by a dramatic increase in B. vosnesenskii. We captured bumble bees on 104 plant species or complexes, but only 14 were significantly selected by at least one bumble bee species. Each of the five most frequently captured bumble bee species selected at least one unique plant species. Plant blooming phenology, relative availability of flowers of individual plant species, and plant selection by B. vosnesenkii remained fairly constant between the two study years, suggesting that maintaining, seeding, or planting with these ‘bumble bee plants’ may benefit these five bumble bee species.

For many avian species, predation is the leading cause of nest failure. However, relationships between predator abundance and nest predation often differ across spatial scales. We examined the relationship between environmental characteristics in meadows and mammalian predator activity, the relationship between predator activity at 2 spatial scales, and the probability of nest predation of willow flycatchers (Empidonax traillii), dusky flycatchers (Empidonax oberholseri), and yellow warblers (Dendroica petechia) in the central Sierra Nevada Mountains, California, USA. Environmental characteristics associated with the detection of nest predators varied depending on species. Douglas's squirrel (Tamiasciurus douglasii) and chipmunks (Tamias spp.) were associated with characteristics common along edges of meadows; short‐tailed weasels (Mustela erminea) were associated with willows, whereas mice (Peromyscus maniculatus, Reithrodontomys megalotis, and Microtus spp.) and long‐tailed weasels (Mustela frenata) were distributed throughout the meadows. The probability of predation of willow and dusky flycatcher nests increased with increasing short‐tailed weasel activity, and the probability of predation of yellow warbler nests increased with increasing activity of chipmunks and short‐tailed weasels. Variation in the occurrence of predator species in different areas of the meadows likely influences the probability of nest predation by each species and the nesting success of birds. Identifying factors that influence the distribution and abundance of common nest predators will likely be integral to the development of conservation efforts to increase the reproductive success of some bird species.

Riparian corridors can be highly biodiverse but are often degraded by human activities, and are therefore frequent targets of restoration actions. Yet managers often lack clear guidance on how to conserve or restore riparian vegetation structure and composition to promote wildlife biodiversity, due to the difficulty of balancing the needs of multiple species and taxonomic groups. We used independent multi‐species occupancy models to assess the response of riparian bird and bumble bee assemblages, respectively, to variation in vegetation structure and composition in montane riparian corridors. We sought to identify vegetation characteristics associated with relatively high richness across each taxonomic group to define target conditions for habitat restoration. Riparian bird occupancy increased with more willow (Salix spp.) cover and less overstory cover, while bumble bee occupancy increased with greater flowering plant richness, more forb cover, and less shrub cover. Relatively distinct habitat preferences of bumble bees and riparian birds emphasize the value of managing for habitat heterogeneity to promote biodiversity across multiple taxonomic groups. Multi‐species modeling distills the responses of numerous species down to a single estimate of a covariate effect for an assemblage of species and can provide land managers with empirically derived targets for habitat restoration that will benefit many species.