Study Finds North American Bumble Bee Populations Dangerously Low, Prone to Infection

Healthy bumble bee (Bombus) populations are essential to the pollination of commercial agriculture and wild plant communities in the United States. Unfortunately, initial studies confirmed the widespread fear that American bee numbers were drastically shrinking. Cameron et al. (2011) make important progress in understanding the reasons behind the dwindling number of bumble bees through a three year interdisciplinary research project tracking distribution patterns, genetic diversity and pathogen infection levels in bumble bee populations in each major bee region of the United States. Their research confirms that the presence of four major species has declined by up to 96% while their geographic presence has shrunk 23–87% over the last two decades. Furthermore, declining populations are distinguished by the presence of the pathogen Nosemba bombi and low levels of geographic diversity. While the research provides pathogen levels and genetic diversity as predictors for population collapse, the cause remains unknown.–Michael Landsman

Cameron, S., Lozier, J., Strange, J,. Koch, J., Cordes, N., Solter, L., Griswold, T. 2011. Patterns of decline in North American bumble bees. PNAS 108, 66–-667.

American food production has become dependent upon bumble bees for pollination since recent domestication in major agricultural areas. Their comparatively large anatomy, long tongues and high frequency buzzing, which helps scatter pollen off their bodies, make them uniquely efficient pollinators. Recently North America has joined other global regions in witnessing disturbingly fast-paced bee population decline. Various hypotheses speculating on the causes of bee decline have been made yet Cameron et al. (2011) represents the first major study into the problem in North America. This study sought to accomplish two major tasks; to quantitatively demonstrate a reduction in bumble bee populations and geographic ranges, as well as to test for two of the projected reasons for population decline, pathogens and genetic diversity levels. To quantify current population and geographical ranges, the team chose four varieties of the bumble bee in North America and compared empirical data from the last two decades with a count taken between 2007 and 2009 at over three hundred locations in North America. To determine pathogen infection rates and genetic diversity, the project examined midgut tissue samples of over six thousand bumble bees from representative regions and performed genotypes on both stable and declining populations. Their findings demonstrate both an overall reduction in bumble bee population numbers and geographic ranges and that communities which have suffered the most feature low levels of genetic diversity and a prevalence of the pathogen Nosemba Bombi.

Many followers of bee decline in Europe have blamed the rising temperatures and diminished food sources resulting from climate change for the drastic drop in numbers. Cameron et al. (2011) have observed that in North America, even species which have previously withstood wide climactic variations are facing population collapse, thus they argue for the inclusion of other elements in exploring the root of the bee problem. While the widespread tissue study highlighted the prevalence of N. bombi in declining populations, the authors do not assert their study yielded sufficient evidence to argue the pathogen is a cause of decline; future research is required to determine whether or not N. bombi becomes a hallmark of populations already in decline. While bumble bees can pick up certain infectious pathogens from contact with flowers, no tests have yet explored whether N. bombi is transmitted this way. While the presence of N. bombi is a useful warning sign for threatened bee populations, there remains much more research to do in understanding the infectious agent’s exact role.

An exploration of genetic diversity in bumble bees through genotyping confirms initial suspicions that populations in decline suffered from smaller gene diversity. In a situation similar to that of the N. bombi question, further research needs to be done to determine whether small gene pools is a cause or effect of species dwindling. The genotype study found significant gene flow among populations across a very wide scale. While this is good news in the sense that there is a possibility for dwindling communities to diversify, it also means infectious contagions have the ability to travel long distances as well. The authors expect more research to be done in this area before definitive statements on the cause of this bee crisis. With regards to climate change, it would be useful to determine if limited gene diversity makes bumble bees more vulnerable to temperature variance. Hopefully a major study in PNAS and the dangerous threat to food production dwindling bumble bee numbers present will provoke further research in the area.

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