As part of their research into the causes of declining bee populations, an international group of researchers, including Jeroen Scheper and David Kleijn from Wageningen University, have made a fascinating discovery. During the past century, the females of large bee species in the Netherlands, such as the bumblebee, have become increasingly smaller. The cause seems to lie in the way bees care for their offspring.
Back in 2014, Jeroen Scheper and David Kleijn jointly published a research paper with colleagues regarding the cause of declining bee populations. They used bee collections from museums as part of this research. Judging by the pollen left on the bodies of 'historical' bees, the researchers were able to determine which host plants they had visited. This research revealed that the population trend of the host plants exerted a considerable influence on the population trend of the associated bees. If the host plants of a bee species declined the bee itself declined. If the host plant increased the associated bee species increased.
In addition, the researchers observed that the large bee species decreased in population size while populations of small bees remained stable. 'This may be because large bees need a large supply of flowers while small species can survive on smaller supplies,' said Scheper in 2014. This pattern suggests that it may be advantageous to shrink in size for large bee species but not small bee species. Current research has now confirmed this pattern. David Kleijn continues: 'Strangely enough, this only applies to the females – the males aren't affected. Over the last 130 years, large female bees have shrunk in size by more than 8%. The males have remained the same.' Scheper and Kleijn draw a comparison with Dutch men, who have grown 10% taller over those same 130 years.
The researchers can only hazard a guess as to the exact reason behind the shrinking pattern of larger bee species, as well as why this only affects females. It is assumed that the large difference in shrinking patterns between both sexes of large bee species is connected to the food supply. David Kleijn comments: 'We know that the females of various large bee species are most reliant on pollen and nectar since they not only forage to maintain their own big bodies but also need to provision the brood cells of their large offspring. Males are not involved in this, so they only need to sustain themselves. The shrinking pattern might therefore be caused by the different ways in which females and males require pollen and nectar.'
The decrease in size can also have an impact on the pollination of our agricultural crops, as larger bees are generally better pollinators. Previous research by David Kleijn and Arjen de Groot demonstrated that the contribution of wild bees to the production value of fruit is considerable. In the case of apples and blueberries, this contribution can amount to thousands of euros per hectare each year. Just for the Elstar variety of apples, this amount adds up to a good sixteen to twenty million euros a year for the whole of the Netherlands.
Size and sex-dependent shrinkage of Dutch bees during one-and-a-half centuries of land-use change. Mikail O. Oliveira, Breno M. Freitas, Jeroen Scheper, David Kleijn, PLOS-One.