Rounded Global Status Rank: G2 - Imperiled
Reasons: There was a previous conservation status rank of G4, but that was not based on current information, and actually until the mid 1990s this very common western bumblebee probably would have been best ranked G5. B. occidentalis was among the two most abundant bumblebees in most of western North America. While there were perhaps millions of populations in 1998, and the range and area of occupancy were huge, there is no basis for assessing how many populations still exist, whether any of them are potentially viable, or what the current range is. In less than 15 years this has gone from the second most common bumblebee in the western US to undetectable in substantial areas and rare elsewhere except in the far north and perhaps highest elevations. The decline of this subgenus is on-going and continent-wide. Recently (Cranshaw, 2010) it is reported to have declined even as far east as Colorado. Although Evans et al. (2008) indicate that this species still occurs regularly in the mountains in Colorado and Arizona this information could now be outdated. The species has not yet declined in the Yukon (S. Cannings) or apparently in interior Alaska (Koch, 2011). Although there were a few observations in Oregon and one in British Columbia in 2007-2009, Rao and Stephen ( 2010) indicate that this once common species no longer occurs in coastal and valley regions of Oregon, and it has also disappeared in the San Francisco area (McFrederick and Lebuhn, 2006), not far from where North American declines of subgenus Bombus apparently started from commercial colonies of this species. Rao and Stephen's (2007) reports in the Willamette Valley are from 2006, and with this subgenus records that old should not be assumed to indicate extant populations five years later. This species has declined, and in many places disappeared, from the Pacific Coast as far east as Colorado. It is persisting (at least for now) at the northern end of its range in the Yukon and Alaska, and perhaps still is at high elevations farther south. The decline is probably due mostly to introduced pathogens (e.g. Nosema bombi) and is likely to become range-wide in the 2010s, but there still is some uncertainty, and Koch's 92011) findings need to be explained, although no widely accepted alternative hypothesis has been proposed. Optimistically it is possible that cold climates such as at high elevations or in the far north will provide refugia, but there is little, if any, supporting evidence. The Xerces Society considers this species in steep decline and COSEWIC considers it of conservation concern in Canada. The decline appears to have spread considerably from 2005-2010. Sokolova et al. (2010) suggest there may also be very closely related native North American Nosema, and Koch (2011) finds B. occidentalis to be common, but heavily parasitized by a Nosema in Alaska--which could prove to be native. In a plausible worst case scenario some or all endemic North American species of subgenus Bombus could be extinct within a decade or two, and B. franklini may be already.
Since there is no conservation status rank that addresses a rapidly declining species with so many unknowns, GU would be an easily defensible rank, especially considering uncertainty about scope of threats. NatureServe's Rank Calculator produced a rank of G2G3, which is a reasonable snapshot in time as of about 2010-2011, but under a plausible worst case scenario of rangewide impacts from exotic pathogens in the near future even G2 would understate the degree of imperilment. This rank is very likely to need revision within five years (2016). It is likely that 10-20 years from now one of GH, GX or G4 will apply, in the optimistic scenario Bombus occidentalis would probably be confined to a much more boreal and montane range than it was originally.
Intrinsic Vulnerability: Highly vulnerable
Environmental Specificity: Broad. Generalist or community with all key requirements common.