Climate change is already wreaking havoc on wildlife in a number of ways, from destroying habitats to throwing off circadian schedules. Mutualism—win-win ecological partnerships honed over evolutionary timescales—is a lesser-known ecological relationship that is also vulnerable to the effects of a rapidly changing planet.
Bees and flowers are prime examples of mutualism. Some bee tongues are perfectly evolved to tap into the nectar and pollen of certain flowers with elongated, tubular petals. By specializing in those plants, the longer-tongued bees reduce competition with generalist insects that can't access those sweet resources, and they ensure that their plant species of choice get in on the pollination action.
Climate change, however, has thrown that mutualistic relationship out of whack in at least one population of bees and flowers. As certain flowers in Colorado have become scarcer due to warming temperatures, the tongues of the alpine bumblebees that historically fed on them have become shorter.
Like many of their pollen-gathering relatives, alpine bumblebees are on the decline. To find out what’s going on, a team of American and Canadian researchers headed to Colorado, where they focused on the plight of two species: Bombus balteatus and Bombus sylvicola.
The researchers examined bumblebee specimens collected on three mountains from 1966 to 1980 and also gathered a fresh set, which they collected in the same places from 2012 to 2014. Suspecting that the relationship between bees and their favorite flowers might be involved, they performed the meticulous task of measuring all the historic and recently caught bees’ tongues.
As the team reports today in Science, both of the species' tongues have declined in length over time, shrinking on average 0.61 percent each year. Cumulatively, the team found a nearly 25-percent decrease in tongue length between the bees collected decades ago and those living in the same region today.
The scientists also found that the bees are visiting more species of flowers—including ones with shorter petal tubes—than they were in the past, and that they are covering greater ground while foraging.
These findings naturally led to a second question: What is causing the tongues to shrink? The bees’ overall body size did not change significantly over the years, the researchers found, which means it's just the tongues that have been affected.
Next they turned to the flowers. Looking at contemporary and historic botanical data, the scientists confirmed that the number of flowers with short petal tubes did not increase in abundance, indicating that the bees weren’t simply ignoring their historically preferred flowers for a more readily available food source.
The team set up sampling plots along different mountain gradients to estimate flower productivity and compare it to past values. They found that in response to warmer temperatures, flowers—particularly ones with deep petal tubes—have been moving up the mountains and becoming scarcer at lower elevations. Because surface area decreases as mountains taper off toward their peaks, this altitude-climbing effect has ultimately resulted in an estimated loss of millions of flowers.
As the authors report, even with some flower gains near the summits, bumblebee food resources on Pennsylvania Mountain, for example, have fallen by 60 percent since the 1970s.
The findings paint a telling picture: hotter summers caused bumblebees’ choice flower species to disappear, forcing them to evolve shorter tongues to tap into the remaining food sources. Then, competition with generalist species, more time and energy needed to collect enough pollen and a forced reliance on suboptimal resources all likely contributed to the bees’ overall decline.
Still, if bumblebees can manage to shift their foraging strategies as rapidly as they did their tongue length, then they might ultimately be able to cope with the ecological shakeup that's now underway. As the authors write, for now, at least, “evolution is helping wild bees keep pace with climate change.”