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Decline in North American bumble bees

While the honeybees were fine there was little interest in pollinators and pollination in general. People just took it for granted. But with the ongoing news about CCD and concerns about declining bee populations worldwide the interest in wild bees as natural pollinators and “backup” for honeybee pollination has risen sharply. It turned out, however, that very little was known about the changes in diversity and abundance of wild bees, despite their importance for natural and agricultural systems.

In North America, there had been fragmentary observations that populations of wild bees were declining, but the evidence for large scale range reductions has been lacking. Now a study has been published in PNAS that for the first time provides nationwide data for eight historically abundant species of bumble bees. And for some of them, the news is not good.

A team of researchers from the University of Illinois and Utah State University compared historical data of the past 100 years from museum collections with current data based on extensive surveys in the US between 2007 and 2009. They focused on eight target species – four expected to have relatively stable populations, and four where preliminary data suggested a decline. Overall, they had 73 759 specimens in the historical data set and collected 16 788 specimens at 382 sites for the current data set.

From this data, they were, for the first time, able to confirm the decline and to quantify its extent for four species: Bombus occidentalis, B. affinis, B. pensylvanicus, and B. terricola. All these species suffered a reduction in their geographic range as well as in their relative abundance. The species most massively affected is B. affinis, with an estimated reduction in range-area of 87% compared to historical data.

It is interesting to see that the declines in relative abundance appear only in the last 20 to 30 years, with, as the authors point out, “values from current surveys lower than in any decade of the last century”.

Concerning possible causes for the decline, they considered pathogens and genetic diversity in their study.
They consistently found higher infection levels of the microsporidium
Nosema bombi and lower genetic diversity in the declining populations, which makes these factors realistic predictors for the trajectory of a population. But they also state that cause and effect remain still uncertain. From the findings in the current study, it is not yet possible to determine, for example, whether the increased prevalence of N. bombi is the result of higher susceptibility to the pathogen or if N. bombi is simply more common in declining species for other reasons. Factors like habitat fragmentation, the loss of floral and nesting resources, or climate change were not considered in this study.

Cameron, S., Lozier, J., Strange, J., Koch, J., Cordes, N., Solter, L., & Griswold, T. (2011). Patterns of widespread decline in North American bumble bees Proceedings of the National Academy of Sciences DOI: 10.1073/pnas.1014743108