海角大神

Scientists say climate change has some insects heading for hills. But bumblebees are lagging 鈥� and that鈥檚 bad news for our most important pollinators.

A new study, which tracked the upper and lower limits of North American and European bumblebee populations, found that most species are losing ground at their southern boundaries. Surprisingly, the displaced bees are also failing to extend their northern range. Lead authors Jeremy Kerr and Leif Richardson propose that this 鈥溾�� is a result of global climate change. Their findings were published Thursday in the journal Science.

By digitizing over 400,000 existing specimen records, researchers were able to map 110 years of bumblebee distribution. They found, both theoretically and observationally, that bumblebees were receding from their southernmost reaches. On both continents, that trend appeared to coincide with shifting climatic conditions.

鈥淭he net effect is that climate change is kind of like a vise, where [bumblebee populations] are being crushed from the southern edges of their range inward,鈥� says Dr. Kerr, a professor of biology at the University of Ottawa.

Other pollinating insects, like butterflies, have also lost habitat at their southern limits. But as once-too-cold regions have warmed, most have managed to extend their northern boundaries. But bumblebees, generally speaking, have not.

鈥淭hat鈥檚 a sign that they鈥檙e adapting to some aspect of climate change,鈥� Kerr says. 鈥淚n general, bumblebee species are not doing this. It鈥檚 like they鈥檝e been locked down in the North.鈥�

It鈥檚 the latest in an ongoing trend of hardship for bumblebees, who have been increasingly facing聽disease and decline in recent years. But what exactly is keeping them from buzzing poleward?

鈥淭he short answer is, we don鈥檛 know,鈥� says Dr. Richardson, a researcher at the University of Vermont鈥檚 Taylor Ricketts Lab. 鈥淏ut there are other factors involved that could be interacting with climate warming and bee ecology to prevent them for expanding northward into novel geographic areas. These could include things like interactions with host plants, interactions with parasites, and other ecological factors. This is an area that probably warrants additional research.鈥�

But this distribution shift wasn鈥檛 limited to just north and south. Richardson and Kerr found that bee populations were also seeking new elevations.

鈥淲e find that bumblebees in the more southern areas of range are moving upslope, thereby tracking their thermal niche,鈥� Richardson says. 鈥淏ut in northern areas, we actually see an overall decline in mean elevation occupied by the bees.鈥�

Richardson suspects this trend may have something to do with changes in vegetation. Many bumblebee species live in shrub-dominated alpine (and subalpine) habitats. These habitats may disappear as the聽tree line聽moves up in elevation, potentially forcing bumblebees to find new ones at different elevations.

The Inheritance of Intolerance

In an attempt to explain the uniquely negative impact climate changes continues to exert on bumblebees, Kerr and Richardson reached even deeper into bee history. They speculate that modern species could inherit their upper thermal tolerances 鈥� that is, how much heat they can handle 鈥� from their distant evolutionary progenitors.

鈥淭he common ancestor of bumblebees comes from a kind of cool, temperate region somewhere in Eurasia,鈥� Kerr says. 鈥淭hey don鈥檛 like it when it gets hot because their evolutionary ancestry comes from a cool place.鈥�

Butterfly species, by contrast, evolved in tropical climates. According to Kerr, that could explain why butterflies are handling climate change more effectively than bumblebees.

鈥淏umblebees have a Palearctic, or temperate, evolutionary history,鈥� Richardson says. 鈥淪o it is possible that they lack the plasticity to adapt to a warming world.鈥�

Richardson is quick to point out that this is merely an idea, not a tested theory. But both authors agree that the proposal warrants future research, which could help identify at-risk organisms.

鈥淚 guess the basic prediction we would make is that for groups that evolved under cool or temperate conditions, climate change is a risk right now,鈥� Kerr says. 鈥淏ut we鈥檝e got to try to identify what those groups are. We haven鈥檛 tried to do that at all. We need to think about the evolutionary origins of species more than we do when we evaluate climate risk.鈥�

From the Farm to the Fridge

As important pollinators of wild plants, bumblebees represent a major ecological concern. They are also essential to human crops, and by extension, economies. Blueberries, pumpkins, tomatoes, peppers, squash, and apples are all pollinated mainly by bumblebees. A decline in bumblebee species could mean lower crop yields, higher food costs, and less variety in our diets.

Luckily, bees are generalists 鈥� most crops are pollinated by several different species. But if you lose even one species, you also lose robustness. One year of bad weather, Kerr says, could result in an entire crop going un-pollinated.

鈥淚f you take that to extreme levels, like we see in some decimated areas of China, you have to have hoards of people out there with paintbrushes hand-pollinating crops,鈥� Kerr says. 鈥淭hat鈥檚 just not something you can do on a continental scale. That鈥檚 a pretty dark sort of world, and I don鈥檛 think we want to live in that place.鈥�

Ethical Questions

In their study, Kerr and Richardson offer up some potential, if not temporary solutions to the problem of bumblebee range compression. By way of assisted migration, researchers could relocate dwindling bee populations further north. They might be better equipped to persist there and stave off the effects of climate changes for a little longer.

鈥淎n advantage of assisted migration is that maybe we can give them a hand to catch up with climate change,鈥� Kerr says. 鈥淭hey鈥檙e just not doing it by themselves. And bumblebee species, in a practical sense, are not really a group you want to try to do without.鈥�

But assisted migration is a controversial measure, especially when it means introducing foreign species to new ecosystems. But in Kerr鈥檚 view, these ethical questions are small when compared to the ethical and practical implications of extinction.

鈥淚s it ethically correct of us to introduce species to places they were never historically present in? In a sense, it鈥檚 kind of like creating an invasive species,鈥� Kerr says. 鈥淏ut these are areas that are adjacent to the places where these species are found normally. So you鈥檙e extending their range. It鈥檚 not like we鈥檙e taking species from Europe and introducing them to Hawaii, which would be crazy.鈥�

Richardson agrees that assisted migration could have some merits, particularly when applied on a case-by-case basis. But it isn鈥檛 the only solution, or even the best one, to the problem at hand.

鈥淭o me, there are several other, more important things we should be concerned with based on these results,鈥� Richardson says. 鈥淥ur study suggests that we need to focus our attention on reducing anthropogenic causes 鈥� the emission of carbon dioxide and other greenhouse gases. This is a large problem, and one not easily solved, but this is just one more reason why we need to tackle this problem.鈥�

鈥淭here are smaller things we can do to help bees in this context,鈥� Richardson adds. 鈥淚f climate warming is putting stress on bee populations, I think we can still do something positive for bees by reducing or eliminating our use of pesticides; by promoting and creating habitats for nesting, foraging, and overwintering.鈥�