Evidence for the decline of the western bumble bee (Bombus occidentalis Greene) in British Columbia

Abstract

The crucial ecosystem service provided by native pollinators has been recentlyregarded as threatened globally (e.g., Biesmeijer et al. 2006, Klein et al. 2007, NRC2007). Bees (Hymenoptera: Apoidea) are a speciose taxon containing a large portionof native pollinating insects. As in many invertebrate groups, baseline data andnatural history knowledge for most bee species is highly deficient. As a result,important pollinators may undergo drastic declines unnoticed (Buchmann &Nabhan 1996).Bumble bees (Bombus spp.) are among the best studied bee taxa as they are large,colorful and relatively easy to identify (Kearns & Thomson 2001). The declines ofsome species have been documented globally (Williams & Osborne 2009) but thedeclines in western Canada have been largely anecdotal. In particular, NorthAmerican members of the subgenus Bombus sensu strictu have declined rapidlythroughout their native ranges (reviewed in Evans et al. 2008). While the decline ofB. franklini in southwestern U.S.A. and B. affinis in eastern North America havebeen well documented and quantified (Thorp & Shepherd 2005, Colla & Packer2008, Grixti et al. 2009), there are few baseline data for comparing previous andcurrent abundances of B. occidentalis, the Western Bumblebee. Here we provide acomparison of the relative abundance of B. occidentalis after 20 years in the FraserValley of British Columbia, Canada.In 2003 and 2004, a study on the abundance and diversity of wild bees incommercial highbush blueberry and cranberry fields was performed (Ratti et al.2008, Ratti 2006). The study was located in the Fraser Valley of British Columbiawhere a total twelve sites were surveyed using sweep nets and pan traps (Ratti et al.2008, Ratti 2006). Collected specimens were identified by C. Ratti with vouchersbeing confirmed by T. Griswold and deposited in the Packer Collection at YorkUniversity, Toronto, Ontario, Canada and the USDA-ARS Bee Biology andSystematics Laboratory, Logan, Utah. Similar surveys were performed in variousberry fields and natural vegetation in the same region in the early 1980s (Winston &Graf 1982, MacKenzie & Winston 1984). While the exact same sampling methodswere not used for both time periods, the recent study (Ratti 2006) sampled more sitesand used an additional sampling method (pan trapping in addition to sweep netting).This indicates that differences noted in B. occidentalis abundance are not the result oflower sampling effort during the more recent time period.Bombus occidentalis was the second most abundant bumble bee in blueberry fieldsin 1981 (27% of collected bumble bees) (Winston & Graf 1982), and it was the secondmost common Bombus species, overall, collected in berry fields and naturalvegetation (approx 22%) in 1982 (MacKenzie & Winston 1984). In 2003–2004, thisspecies represented less than 1% of the Bombus collected (26 individuals of the 2738total). Likewise B. occidentalis was the most abundant bee (55% of bumble bees, 312individuals) in cranberry fields in 1982 and second most abundant bee in 1981 (41%of bumble bees, 104 individuals) (MacKenzie & Winston 1984, Winston & Graf