I heart bumblebees! Part Two: Why the bumblebee is in decline.

Mar 14, 2009 by

So, why have the bumblebees been in decline? Many of the articles I found concerned the U.K., which doesn't mean that the decline isn't happening in the U.S., it's just that the U.K. has been a hotbed of recent bee research. An article titled "Decline of Bumblebees (Bombus) in the North American Midwest" states: "The nature and extent of bumble bee decline in North America is poorly understood due mainly to a lack of baseline and long term data." Clearly, there are reports of bee decline in the U.S. as well, and I think that the two phenomenons are closely related. So let's return to David Goulson's analysis. In the last blog, I wrote about bumblebee's unique characteristics, but let's point out some of the main ones that are contributing to their decline.
  1. Bumblebees tend to specialize more than honeybees, and some only feed off of one plant species, like legumes (Fabaceae) for example.
  2. Bumblebees make their homes in abandoned animal burrows (rodents, typically) and in grassy tussocks, which are usually only available in unmanaged grass and pasture lands.
Let's just think for a moment about the track our agriculture has been on. Farm lands that were once interspersed with wild flowers and grazing meadows for horses are now mono-cropped and farmed for every inch of possible land. In Iowa, for example, my boyfriend has told me how farmers will cut down sections of riparian corridor (the plants that you find along the sides of river beds) to farm corn there. ACK! That's the trouble with subsidized farm products. When the government pays more for a certain crop (think: ethanol), you get all the farmers replacing their more diverse food crops for cash crops that will yield the greatest amount of, well, cash. But I digress. Sort of. Previously, bumblebees and honeybees would feed off of some food crops (like raspberries, strawberries, apples, and legumes) and especially off of the wild flowers that grew in the adjacent meadows and fallow pastures. Bumblebees in particular would also make their homes in the unplowed sections of farms in those abandoned rodent burrows and in hedgerows, the sections between cultivated farm lands where bushes and wild plants grow. One of the interesting things the article about bumblebees in the Midwest pointed out was that bumblebee diversity took a sharp decline in Illinois during 1940 and 1960, which "coincided with large-scale agricultural intensification in Illinois" during that time period. This same process has been occurring in the U.K. as well. Goulson writes that the U.K. has lost 98% of its unimproved grassland since World War II. He then goes on to say that "increased use of herbicides and improved seed cleaning mean that arable crops are now virtual monocultures, whereas once they were rich in flowering seeds." Basically, as a result of this agricultural intensification, there has been a significant loss in wildflower diversity. When bumblebees need certain habitats and those habitats get plowed over, and when bees depend on certain flowers and those flowers aren't cultivated any more...well, you do the math. The bees go extinct. References
Goulson, D. (2006, December). Demise of the bumblebee in Britain. Biologist. Volume 53, Number 6, 294-299. Retrieved March 04, 2009. Grixti, J., Wong, L., Cameron, S., & Favret, C. (2009, January). Decline of bumble bees (Bombus) in the North American Midwest. Biological Conservation, 142(1), 75-84. Retrieved March 14, 2009, doi:10.1016/j.biocon.2008.09.02

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