The UK has 24 bumblebee species left, but we’ve already seen 2 extinctions. In Europe, intensive agriculture is thought to be the greatest cause of bumblebee declines, due to the loss of wildflowers and nesting sites. However, not all species are declining at the same rate and we don’t really know why.
As always, there’s probably no single factor, but some ideas have been put forward. One suggestion is that it’s because bees have different diets depending on how long their tongue is. The flowers favoured by long-tongued bees seem to be more affected by intensive agriculture.
A study of two of the rarer British bumblebees analysed whether one of the problems was dietary breadth – are bees that only gather pollen and nectar from a few species of plants declining faster? This would make sense; there are lots of suggestions that ‘specialist’ species, those which rely on very few species for food, are more vulnerable to change. But this study found no evidence that bumblebee species with narrow diets are declining faster, even though other bumblebee studies have suggested otherwise.
So patterns of dietary specialisation don’t appear to account for national declines, and the study’s results highlight a need for more investigation into why some bumblebees are declining faster than others. This leaves them to conclude that the causes of bumblebee declines are varied and complex; they suggest some possibilities, such as different nesting preferences, or differing sensitivity to pesticides or pathogens.
A greater understanding of pollination ecology and the specific interactions between bee species and the plants they pollinate is needed for effective conservation strategies to be developed.