I walk a lot. It hurts less than running and is more fun than swimming but still has some minimal health benefit. Sometimes I walk for miles. I listen to some NPR podcast and think about my route. I choose where to turn and the route I take based on two main criteria: I like to conserve potential energy by picturing the contour map of my neighborhood in my head and try not to go down hill just to needlessly go back up (like I said, minimal health benefits). Second, I don’t like to be in the sun so I think about where there are trees and shade, even if it means going down hill.
Sunlight and temperature vary block by block and sometimes house by house. This is due to the shade that trees provide and the amount of pavement that absorbs heat from the sun and releases it back into the environment. In a matter of blocks we have found that air temperature can change by several degrees C. Imagine if your air conditioner was set at 86 F (30C) instead of 77 F (25C). Lethal? No. Noticeable to a sensitive reader of science blogs? Yes.
So what about the bees already?! Insects experience the environment at much smaller scales than people do. If I’m hot I can look ahead see a shady block and walk there (hopefully down hill). Some insects may not move more than a block in their lives. If they are born in a hot block they may be stuck in a hot block or if not stuck they may have to spend a lot of time and energy to escape. So we figured bees that can’t tolerate heat won’t be found in hot parts of town.
To test this hypothesis we collected 15 species of wild bees around Raleigh, NC and challenged them to see how high a temperature they could stand. We put them in test tubes and heated the tubes slowly until they could not walk or fly. Unpleasant as it was, this procedure tells us, at a physiological level, which bees are more tolerant of heat than others. Some bees fizzled out at 45 C but others lasted until 51 C. That’s a big difference.
Once we knew the range of temperatures bees could tolerate we sampled yards around Raleigh that varied from very urban and hot to less urban and cooler to see where our study bees lived. We found 18 yards and parks that we sampled 11 times over 2 years. We counted and identified a lot of bees then compared the range of yard temperatures to the range of bee heat tolerance.
It turns out that bees with the lowest heat tolerance in the lab were least abundant in the hottest yards. Bees with the highest heat tolerance in laboratory experiments managed to hold their own, even in hot yards.
This may seem obvious. Hot bees can handle hot yards, cool bees need cool yards. However, there are a couple reasons this work is important. First, no bees were more abundant in hot yards. Some bees just manage to hang on in the hot yards. This means that as cities (and the whole world) gets hotter, all the bees we studied will do worse.
Second, to the extent that some bee species avoid hot areas, all the flowers and ‘pollinator gardens’ in the world won’t help the species that get too hot and die. Bee species that get too hot in NC will have to migrate north to find more hospitable temperatures. If they move north it is one less species here. If that species pollinates some important crop or some endangered plant, that plant is out of luck.
When a habitat gets hotter due to urban heat or global warming some species will fizzle out. Our experiments show that we can predict which species will thrive (or at least hang on) in hot habitats and which species will disappear. Wild pollinators are struggling amid multiple stresses. Hopefully work like ours can help predict which pollinator species (and plants that depend on them) will hang on and which may need extra help.
Read the paper:
Hamblin, A.L., Youngsteadt, E., López-Uribe, M.M., Frank, S.D. (2017) Physiological thermal limits predict differential responses of bees to urban heat-island effects. Biology Letters. 13: 20170125.