Western bumble bee: declines in the continental United States and range‐wide information gaps

Abstract

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.

A conservation conundrum: protecting bumble bees under the California Endangered Species Act

“…1c; Unpublished analysis from Richardson 2019;The Xerces Society et al 2019). These data are corroborated by a recent occupancy modeling analysis, which found that the probability of occupancy by the western bumble bee has declined by 93% over the last 21 years (Graves et al 2020). The western bumble bee is an SGCN in California (CDFW 2015).…”

A contemporary survey of bumble bee diversity across the state of California

“…nevadensis (Cole et al, 2020; Thorp et al, 1983) and B . morrisoni have previously been collected (CDFW, 2019; Graves et al, 2020; Thorp et al, 1983; Table 2). Similarly, the current range of B .…”

“…Bombus occidentalis was historically widespread across the state (Thorp et al, 1983), but is now restricted to high meadows (CDFW, 2019;Graves et al, 2020). This species appears to still be sporadically present in the Sierra Nevada (Cole et al, 2020;Hatfield & LeBuhn, 2007), and the Northern Coast Range (Graves et al, 2020), although we did not sample it in either of these ecoregions, which is consistent with another recent study in the Sierra Nevada (Loffland et al, 2017). The documented decline of B. occidentalis began in the mid-1990s in the most western parts of its range, including in California (Cameron, Jepsen, et al, 2011;Graves et al, 2020).…”

Patch utilization and flower visitations by wild bees in a honey bee‐dominated, grassland landscape

“…Occupancy models use detection-nondetection data collected across multiple surveys of selected sampling units to estimate the probability that a species occupies, or uses, a unit while accounting for imperfect detection (i.e., false absences). While broadly applied in vertebrate systems, occupancy models have only recently been used in studies of bees (Graves et al, 2020;Landsman et al, 2019;McCune et al, 2020). These studies suggest bee detection probability is not perfect and varies with weather, time of year, and sampling effort, thereby highlighting the need to account for imperfect detection.…”