Sunday, May 15, 2011

Decline by Fall (part II)

As I mentioned, here is the second half of the bumblebee paper (first half here). This section is probably more accessible, as it's a good bit less technical.  A quick summary of the first half:  many species of wild bumblebees are in steep decline around the world, for as yet unknown reasons.  Potential sources of the decline may be parasites spread by commercially reared bees, the impact of pesticides (which are not typically lethal, but which may negatively impact colony health, and on which the research is very conflicted), and problems associated with habitat loss, foraging patterns, and when bees emerge.  The second part of this paper deals with how we might move to protect wild bees.  

Conservation of Bumblebees

The above factors (parasites, pesticides, habitat loss, and bee phenology) may interact in unknown ways to produce the worldwide decline of some species of bumblebees.  Because these bees are important for food production purposes and the maintenance of  current ecosystem structures dependent on their pollination activities, successful conservation efforts will need to mitigate as many potential negative factors acting on their populations as possible, until precise, species-specific causes can be determined, and more detailed conservation plans produced.   Steps such as reducing rates of parasitic infestation in commercial bumblebees are of obvious benefit to the stakeholders, while it may be more difficult to generate interest in others, such as active management of field margins, or a return to crop rotations involving fallow periods.

The knowledge available for creating conservation plans has increased substantially in the last decade, but this appears thus far to have had little impact on the creation or implementation of conservation measures (Goulson et al. 2011).  Dealing with these factors presents a number of problems. (1) Restricting the use of pesticides is controversial, and will likely require litigation, while the research literature does not consistently demonstrate risks to bumblebees from appropriate pesticide use.  (2) Parasites that have become established in new habitats and bumblebee populations are unlikely to be eradicated by any current methods, though long term exposure may force bees to evolve resistance to invading parasites.  (3) Wild bumblebee phenology is more or less impervious to human intervention; queens cannot be roused earlier, excepting perhaps by climate change, and bees that do not forage far from their nests are unlikely to be induced to do so by humans.  Because of these problems, the creation or preservation of habitat appears to be the most practical method for attempting to conserve bees.

Habitat creation for pollinators has already been addressed to some extent by the 2008 Farm Bill in the US, and California has taken significant steps to create habitat for native bees (Spivak et al. 2011).  In the UK, studies have shown that native bumblebees consistently respond well to sewn mixtures of flowers in field margins, but the high level of maintenance necessary to keep these habitats of high quality for bumblebees makes this approach problematical (Lye et al. 2009, Potts et al. 2009).  The inclusion of cultivated land in these plans presents an economic problem, as increases in commodity prices can incentive farmers to convert fallow land (Goulson et al. 2011).  Lack of information about needs of many species of bumblebees makes it unclear how or whether to proceed with habitat conservation efforts.  It is also unclear how best to proceed with habitat conservation efforts outside of agricultural areas.

Fortunately, bumblebees may benefit from habitats in areas not typically considered appropriate. Urban or suburban gardens and brownfields are known to be important resources for several species of bumblebee in the UK (Goulson et al. 2010), and 79 species of bees have been demonstrated to make use of urban green roofs in Canada (Colla et al. 2009).  These findings suggest that relatively small areas of forage scattered throughout the developed landscape may have a positive impact on bumblebee survival.  The rising interest in urban food production (and gardening generally) may provide an opportunity for re-establishing or stabilizing bumblebee populations in areas with large amounts of development, through education of urban farmers and gardeners and the inclusion of species that can be exploited by native bees in flower and food gardens.   Developed areas are unlikely to currently support bee parasites, and the restricted nature of any single food growing operation makes the importation of commercial bees less likely.  Pesticides may still present a problem to bees making use of urban habitat, though this will likely be highly variable across plots visited by bees.  Additionally, the highly heterogeneous nature of developed environments may provide challenges for bees with a limited foraging range or those that specialize in a small number of flowers types.

Articles Cited

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Colla, S. R., E. Willis, and L. Packer. 2009. Can green roofs provide habitat for urban bees (Hymenoptera: Apidae)? Cities and the Environment.
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