Climate change is shortening bumblebee tongues

Climate change is predicted to cause plants and animals to migrate to new locations, and reshuffle biotic communities in various ways. Now, scientists have documented not just predictions but concrete evidence of climate change driving the evolution of bumblebees that forage in the mountains of Colorado.

Researchers collected specimens of two bumblebee species, Bombus balteatus and B. sylvicola, from three locations in the Rocky Mountains, and measured the animals’ tongues.

A bee’s tongue says a lot about what a bee eats. Bees that specialize in sipping nectar from flowers with deep tubes have long tongues. Those that are generalist foragers, visiting many different types of flowers with varying anatomy, have shorter tongues. B. balteatus and B. sylvicola are long-tongued, specialist bumblebees.

The researchers also measured tongue length in museum specimens of the two bee species collected from the same locations between 1966 and 1980. The bumblebees’ tongues have gotten about 25% shorter over the past 40 years, they reported last week in Science, a striking example of evolution occurring rapidly enough for humans to witness and record.

The researchers watched the bees foraging and confirmed that the two species have broadened their palates. They now regularly visit a wider variety of flowers, including those with shorter and more variable tube depth, than they did in the past.

That observation is particularly surprising because warming temperatures have enabled other species of short-tongued, generalist bees to expand their range up the mountainsides. This increased competition would normally cause resident bees to specialize even more narrowly.

The explanation for this pattern is found among the petals, the researchers say. They examined herbarium specimens of flowers collected from the bees’ habitat from 1960-1982 and 2012-2013. The bees’ preferred long-tubed flowers haven’t changed shape. And short-tubed flowers haven’t suddenly become more prolific.

Instead, the bees’ favorite flowers have become scarcer because flowers in Rocky Mountain alpine habitats have become less abundant overall, a consequence of warming temperatures and drying soils. These factors are causing decreased flowering of plants in arctic and alpine ecosystems throughout the world.

The researchers used data on the density of flowers at one of their three study sites to calculate that total food resources available to the bees have declined by 60% since the 1970s. Being a specialist forager only makes sense when resources are plentiful, they say; times of scarcity favor generalists, so the bees are becoming more generalist.

“We are not saying climate change isn’t a problem for bumblebees – it is a major problem,” says University of Missouri – Columbia biologist Candace Galen, a member of the study team. “However, these findings indicate that some bumblebees may be able to adapt if provided adequate habitat, and are largely shielded from environmental pollutants, such as pesticides.”

Bumblebee populations worldwide have been declining in recent years, so the finding that these two species have been able to evolve in order to adapt to climate change provides welcome bit of good news.

Still, all this leaves open another worrisome question, which the paper addresses only obliquely: What happens to long-tubed flowers when they lose their long-tongued bees? – Sarah DeWeerdt | September 29, 2015

Source: Miller-Struttmann N.E. et al. “Functional mismatch in a bumble bee pollination mutualism under climate change.” Science DOI: 10.1126/science.aab0868

Header image: Bombus sylvicola, a Rocky Mountain bumblebee species that has evolved a shorter tongue in response to a climate change-induced decline of flowers in its alpine habitat. Credit: Nicole Miller-Struttman.

Recommended

white-bar
CLOSE
CLOSE