Highs and Lows: How Elevation and Temperature Impact Bee Survival (Ep. 40)

Bees serve as a vital indicator species for the health of our ecosystem. Today, we’re looking at research that not only broadens our understanding of how climate change affects bees but also emphasizes the urgent need to protect these pollinators at all stages of their life. Studies on bees have predominantly centered around adult bees and their foraging behaviors. Research by our guest, Dr. Lindsie McCabe, fills a crucial void by investigating how immature bees weather changing temperatures. Lindsie’s work has shown that the effects of climate change on bees are not limited to the foraging adults but also profoundly impact their immature stages.

Dr. Lindsie McCabe is a research entomologist in the Pollinating Insect Research Unit for the United States Department of Agriculture. Her research focuses on the biology, ecology, and behavior of bees and their community interactions. You can read more about the study we discussed here.

Good to know

Lindsie conducted her study in the San Francsico Peaks in northern Arizona. While most people think of the desert when they hear Arizona, the San Fransciso Peaks feature a variety of landscapes from Ponderosa pine forests at the lowest elevation (6,000 to 8,500 feet/1,800 to 2,600 meters) to an alpine tundra environment above 10,600 feet/3,200 meters. 

Transcript

[00:00:00] Welcome to the Bee’s Knees. I’m your host, Jacy Meyer. The Bee’s Knees is a podcast wild about native bees. Wild and native bees are under threat worldwide. In each episode, we look at actionable things we can do to support these adorable little guys whose pollination work is crucial for maintaining biodiversity.

Thanks for being here.

Most of the existing research on bees has focused on adult populations and their role in pollination. This means there’s a significant gap when it comes to understanding how changing temperatures influence bees in their immature stages. Dr. Lindsie McCabe is a research entomologist with the United States Department of Agriculture.

She and I got together to discuss her study, in which a team of researchers embarked on a three year transplant experiment to uncover the effects of overwintering temperatures [00:01:00] on the survival of cavity nesting bees. What inspired you to look into how winter temperatures affect the survival rates of young bees, especially considering the impacts of climate change?

Yeah, so when I started my PhD work, I was really interested in understanding what drives bees to respond to environmental temperatures and specifically climate change. It was something that was up and coming. I mean, I started my PhD almost 10 years ago now, but at that point, a lot was not known about how the impacts of climate change affected bees.

And so I was really interested in that. What was known was what impacts of climate change affected adult foraging bees, and so I wanted to expand from that and understand, well, actually, they don’t spend most of their life as adult foraging bees. In fact, most of these bees spend very little amount of their life as these adult foraging bees.

So [00:02:00] I wanted to ask the question, okay, during this immature development or non foraging part of their life, what happens? And we know that not just our summers are getting hotter, but our winters are getting warmer. This means less snow melt, earlier snow melt, Less snow pack and all of these are like during a vital life stage of solitary cavity nesting bees.

And so that’s why I wanted to ask this question. So your research involved relocating nest blocks to various elevations. This sounds a bit like a logistical nightmare. Maybe it was just a challenge. But can you explain how you carried out this transplant experiment? Yeah, so, at this time, these experiments weren’t really done.

I had seen them done in plants, but not with bees that can fly. And so, I was like, well, how am I going to accomplish this? This is another reason why it was nice to use [00:03:00] non foraging life stages because I could move them and we know that once they nest in these cells in their nest that they’re not going to move until they become adults.

And so, this was kind of a little bit of a shot in the dark. So, this had not been done before, but we, you know, read some literature, I thought about what we could do, um, and then we put drilled holes in wood blocks, and then put paper straws in the wood blocks, then we hiked to these different life zones, um, which I had been sampling in these life zones for the last four or five years before I started this project.

And so we knew, we knew this area. We knew the composition of the landscape. Um, and so based off of that research, uh, we put blocks where we thought they would be colonized by these solitary cavity nesting bees and it worked. We got them to colonize. We got them to nest and establish in these [00:04:00] nest blocks, but that was only like.

The first part of the experiment, then we actually had to move them, which can be challenging because you don’t know who nested there. And, uh, we can get an idea because these cavity nesting bees will cap the front of their nests with either mud or little sticks. Sometimes they use pebbles, leaf pieces.

And so we kind of had an idea of like what genera these bees were in. Um, and from there we systematically divided up, you know, some stayed at their original location, their natal location, some moved up in elevation and some moved down in elevation. And this took a team, myself and about five other people helped me physically pick up the bees, put them in backpacks, hike up the mountain or hike down the mountain,

it involved a lot of driving. But it was fun and in the end it really worked. I want to talk about the [00:05:00] bees that you move to lower, warmer habitats. Those bees you found, they had a 30 percent decrease in emergence. What do you think caused this drop and what does it mean for bee populations as the planet warms up?

Yeah, these bees are very sensitive to changes in temperature and overall climate, especially during their non foraging adult. So that foraging adult stage is supposed to be pretty robust and they can withstand a lot of climates and temperatures, but non foraging adult, they can’t move. They can’t seek refuge anywhere.

They just are limited to where they’re at. And that impact of microclimate really matters to them. And so this warming temperature, the fact that they moved about four degrees, sometimes in some cases, it was a little more than four degrees Celsius warmer, um, really has a detrimental impact on them. A lot of these bees.

Especially [00:06:00] in the genus Osmia, um, which is what we found to be a lot of them and, and the ones that had the worst survival rate, uh, they overwinter as adults, which means that their adult life, their adult body is fully formed and they burn up fat bodies still while they’re in the overwintering. So. When they overwinter and they’re burning up these fat bodies, higher temperatures make them burn more energy, more storage.

And so by the time summer, spring comes, they can’t get out of their nest. Um, they are functionally dead. Um, and that’s what actually what we were finding. Some were dead and some were just functionally dead. So you noticed that bees that are habitat generalists were more likely to emerge when moved to different elevations compared to habitat specialists.

What does this say about the adaptability of different bee species to climate change? [00:07:00] And kind of a two-part question here, how could this information help you with conservation efforts? Yeah, so I think this is a really critical piece found from this research. Mostly because we know that, well, from this, we know that species with wide ranges, and this is in a small scale, so this was on a mountain that we looked at this, probably true throughout United States and the rest of the world, is when you have the bees that have evolved to be plastic across the wide range are going to be more responsive to changes in fluctuation in temperature, humidity, other environmental stressors.

But when you have bees that are very specialized to one range, these are the bees that are most susceptible to climate change, pesticides, some other stressors in our environments. And so we particularly need to pay attention to these species that may [00:08:00] contribute a great deal to biodiversity in their environment.

But overall, they are very restricted. And this is why you think this is why we conserve rare plants. Um, it’s the same type of thing. When you have rare plants that are restricted to one area, you were like, Oh, we need to conserve this. It’s the same thing for bees. Bees that are restricted to small areas are less likely to adapt.

So your study found that bee emergence increased at the highest elevations when the nest blocks were moved higher. What does this suggest about the potential for these bee species to survive in colder environments? And do you have any concerns or limitations with this survival strategy? Mainly I’m thinking about the availability of resources above this tree line that these bees were relocated to.

Yeah, this was a cool unintended consequence result of this work. We weren’t there testing nesting resource [00:09:00] limitation or distribution. It was kind of just like an opposite control of moving them down. And what we did find is those bees that were at our highest elevation naturally, that occurred naturally at our highest elevation, when we moved them up to the very top of Humphrey’s Peak where there weren’t any trees, they did

well, and better than when in their natal habitat. But what’s interesting is that they couldn’t exist there normally. They could forage there, but they couldn’t like produce there because there’s not nesting resources. These are cavity nesting bees. They have to have those nesting resources. If they don’t have nesting resources, they may like forage there, but their foraging range is ultimately very small.

And so we need to keep that in mind also, you know, if we’re thinking from conservation terms, we can’t just think of, oh, well, they have flower resources, especially for cavity nesting bees. It’s so [00:10:00] important that we look at other biotic resources, like where they nest in cavities. Are there enough dead and down wood?

Is there pithy stems? It’s stuff to keep in mind if you want to plant a garden, that you also have to have, uh, nesting resources. On top of that, some of these bees use mud to make their nests, and some of these bees use leaf pieces. And so having availability of leaf pieces is really important. Drought can cause leaf toughness, and that can cause bees to

not have the available resources. And bees and actually all animals have this thing called thermal breath. It’s like, what’s their maximum that they can live in and what’s their minimum and in between that is their thermal breath and most bee species don’t live anywhere near a maximum or minimum, they live somewhere in the middle and this allows them to be adaptable to be plastic and how narrow or wide that thermal breath [00:11:00] is dictated or can tell us a lot about risk.

And the fact that a lot of these bees were able to move up and survive at colder temperatures just means that they were well above their critical minimum prior to where they’ve just evolved to live. And probably because of the nesting resources. That is so interesting. You mentioned the term bee plastic.

Could you define that for me? Yeah. So, It’s sort of what you imagine, um, when, so that they’re more flexible, they’re able to acclimate or adapt to more areas or more regions. If they were very rigid in where they had to be, you couldn’t move it up and down, they would all die. Thank you. So based on that and kind of what you found through this study, bees might be a bit [00:12:00] more resilient than we give them credit for.

Could we say that? I would think that. Some bees are going to be very at risk for climate change and warming temperatures and their non foraging adult stage. So egg to overwintering adult is going to be the most critical life stages that we look at to understand, are these bees susceptible to climate change?

And that’s just not done. Most people are looking at adult foraging temperatures and stop. Um, and so really if we want to focus on conservation of bees, we need to move away from just looking at what’s going on in their adult stage and really understand what’s going on during their development when they are the most susceptible.

So why do you, what’d you do? Why bees? Uh, so there’s a great quote by EO Wilson that [00:13:00] says every kid has a bug phase. I just never grew out of mine. This is something that I’ve. always felt deep in my heart. I have loved insects from a very, very young age. And I said, this is what I’m going to do when I grow up.

I’m going to be an entomologist. And halfway between when I got where I am now, I was like, Oh, maybe I want to be a botanist instead of an entomologist. And then I found out about. It’s about pollination and bees and it really just took a hold of my heart and I love working with bees. I think they’re fascinating.

They really are a good indicator species for how our world is doing, how the environment is doing, and if you’ve ever seen a bee up close, they’re so cute. Bees are cute, aren’t they? This was such a great conversation on a really overlooked aspect of bees and climate change. [00:14:00] When bees were moved to lower, warmer elevations, their emergence rates plummeted by 30 percent.

However, moving them to higher, cooler elevations showed a different story, with some species actually thriving. This suggests that while some bees can adapt to colder climates, a lack of habitat resources would harm those bees who may be seeking refuge from rising temperatures. And we can’t forget our specialist bees who just aren’t as resilient as the generalists.

My thanks to Lindsie for sharing her important research and thanks as always to you for listening and sharing the show. Please visit the website, thebeesknees.website, to learn more about Lindsie’s study and see some photos of her bee boxes in situ. Until next time, stay special.