Large bee species have become smaller over the past 150 years, as ecologists discovered when they measured bees from Naturalis’s historical collection. The shrinkage occurred only in the females, not the males. While the reason is not known, the Brazilian and Dutch researchers see falling biodiversity in Dutch fields as the main culprit. Their results have been published in PLoS ONE.
Photo: 8 of the 18 studied bees / Wikipedia
The finding that large bee species are currently declining faster than smaller species had already been published. The researchers suspected that this is putting those large bees under evolutionary pressure to become smaller. The team of ecologists investigated this by taking 4510 preserved bees and measuring the distance between the points where the wings are attached to the body, which is a measure of the body size. They studied bees belonging to 18 species found in the Netherlands. In large bee species such as the buff-tailed bumblebee, the females turned out to have become 6.5 percent smaller. The researchers were able to exclude most environmental factors so this does indeed seem to be an evolutionary change. This is a surprising result even for a biologist, says David Kleijn. ‘You do tend to think that bees are fixed. But they have to adapt too.’
Kleijn sees the reason for the shrinkage in the changes in Dutch agriculture. Farming has become more intensive and on a larger scale over the past 150 years. Pastureland is now just grass with few wild flowers or plants, so bees have access to fewer flowers – and therefore less nectar. In addition, there is more variation than in the past in what is on offer. Larger bees need more nectar and are therefore affected more by the changes.
A surprising aspect of the results is the fact that the males have not shrunk. ‘There must have been an opposing pressure,’ says Kleijn. ‘It might be important for them to be large, for example to mate with as many females as possible.’