A new study has shown that the effects of climate change on floral growth may be leading to shorter tongues in bumblebees. Bumblebees and flowers have co-evolved a mutualistic relationship where flowers supply bees with nectar and pollen, important ingredients for hive construction and health, in exchange for transportation of their pollen to other flowers. In this process, some bee species have evolved longer tongues to extract pollen and nectar from deep flower corollas (the part where flowers store their pollen).
Scientists compared the tongue length of bee specimens collected between 1966-1980 and compared their tongue length to specimens collected between 2012-2014. The species they used were Bombus balteatus and B. sylvicola, two alpine bee species that inhabit the Rocky Mountains.
They found that the tongue length in the two species used had decreased by about 24%. The next question they asked was why. The team tested four different hypothesis; decreasing body size of the bumblebees to save energy in warming temperatures, shrinking of flowers with shallower corollas, competition with invaders and a decrease in alpine flowers.
No evidence of a shrinking body size or competition with invading species was found so these weren’t the reason. A comparison of the corolla depth of flowers collected between 1960 and 1982 showed different changes in corolla depth for different species but the ones that did get shorter actually received fewer visits in the past and there was also no increase in flowers with shallow corollas.
The hypothesis that best describes why bumblebee tongues are shorter is a decrease in flower numbers. Bumblebees with long tongues are often referred to as ‘specialists’ because they specialize on a certain species of flower with a deep corolla while shorter tongued bees are generalists because they can visit a wide range of flowers. The flowers with deep corollas have significantly decreased due to global warming so its now more beneficial for bees to be generalists because there is a wider variety of flowers with shorter to mid length corollas.
An assessment of the number of flowers per square metre has revealed a decrease of 73-80% of flowers in some areas. This includes large decreases in flowers with deep corollas.
While being a generalist may always seem like a good idea, being a specialist has it advantages in some conditions. When resources are in good supply, specialists have a food source that they don’t have to compete for with other organisms but as resources decline, generalists have the advantage because they can forage from a variety of food sources, not just one. This is why we are seeing a transition from bumblebees being specialist to generalists. In the population of bumblebees, the short-tongued bees have a better change of surviving and passing on their short-tongued genes than do the long tongued bees.
And as for the flowers with the deep corollas, they have to either evolve shallow corollas or they may go extinct in the area. Flowers rely on bees to transfer their pollen and if no bee can reach it, they have no way of reproducing.