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  • Andy Davis

The most important monarch breeding region in North America is...wait, what?

Greetings all,

With a title like that, you know this blog entry is going to be interesting! I can assure you it is, because in this post I'm going to cover a scientific study of monarchs that you probably have never read about, but once you do, it will change everything you thought you knew about the breeding habitat needs of monarchs in North America. Strap in and get ready...

The study I'm referring to is one that was published in the very prestigious journal, Global Change Biology about 5 years ago (2017). It was authored by Tyler Flockhart, Lincoln Brower, Isabel Ramirez, Keith Hobson, Leonard Wassenaar, Sonia Altizer and Ryan Norris - a very prestigious group of scientists! I'll put a link to the study here, as I always do. This will take you to the abstract, though the paper itself is not free to download from the journal. I guess this is why most people haven't seen this, which is a bummer. However, one of the authors has uploaded his personal copy of the paper to his lab website, and here is a link. If you'd like to get the info straight from the horse's mouth, please do read it. Otherwise, read on here.

This paper, titled, "Regional climate on the breeding grounds predicts variation in the natal origin of monarch butterflies overwintering in Mexico over 38 years" involved the use of stable isotopes, which is something I'll explain before going further. Basically, this is a sophisticated way of tracking where a monarch came from in North America by looking at its body (or wing) tissue. If you take a sample of it's wing tissue, grind it up and run some fancy chemical analyses on them to determine their "isotopic signature," you can tell which region of the continent that tissue was produced (i.e. where the monarch grew up, or it's natal origin). This approach was pioneered by two scientists, who are co-authors on this paper - Hobson and Wassenaar. I know Keith Hobson reads this blog, so he can correct me if I get anything wrong here!

The names of those two scientists may be familiar - these are the same two who worked on a very influential early study back in 1998. These researchers had examined a (smallish) collection of overwintering monarchs (collected during the winter of 1997) using this isotope approach, to determine where they had come from. The upshot from that study is that they estimated that 50% of all of the monarchs in Mexico came from the American Midwest. At the time, this was a huge development in monarch science, since we did not know where the "core" breeding range was back then. Based on this one study, it looked like the Midwest was it. After that, a lot of monarch researchers became fixated on this region, seeing it as the key to saving the monarch, because of it's perceived importance within the breeding range of eastern North America. And, once farmers in the Midwest began using roundup-ready crops that allowed them to eliminate all of the weedy milkweeds from their fields, this fixation with the Midwest grew even more. I think a lot of researchers still are fixated, actually. Anyway, I'm not one of them, and I even recall having some important critiques of that study, and the assumptions it led to, and I blogged about these at one point a few years ago. I note that this study is also routinely cited whenever there is talk of the loss of agricultural milkweeds.

So this study that was published in Global Change Biology in 2017 could be thought of as a follow-up to that early one, but with using a lot more monarchs. These researchers looked at samples of overwintering monarchs from many, many years, going back to the 1970s. I understand that they basically raided the late Lincoln Brower's monarch freezer (when he was still with us) to get some of these early samples. They combined these with some more recent samples from other researchers, including from my wife's lab. Collectively, they had samples from across 38 years - quite a time span. Their goal was to examine the samples from each year, using this stable isotope approach, to determine where the monarchs came from in North America. By doing it over a long time frame, they could then determine if there have been any large-scale changes in the natal origins over time. They were specifically looking to see if the relative contribution of monarchs from different regions of North America corresponded with year-to-year variations in climate at these regions.

Importantly, their collection covered the important time frame of the 1990s and 2000s, the years when the farmers in the Midwest started using roundup-ready crops and spraying herbicides on their fields, which dramatically reduced the milkweed in that region. This part is not in dispute at all.

I'm not going to go into the details of the lab procedures, chemical analyses or any of that, since most of it is over my head. The main thing to keep in mind here is that the researchers had a whole bunch of archived (dead) monarchs that were all collected at the Mexico wintering sites, and they used a cool lab assay to find out where in North America they had come from.

So, let's jump right into what the results showed. I'm going to put one of the main figures from the study below, and this figure shows basically everything you need to know from the results. This figure shows a map of North America, with color-coding that indicates the relative contribution of that region to the overwintering sites, based on all of their monarchs. It also shows the year-to-year changes in these proportions (top chart), with the same color-coding. Take some time to look this figure over before reading this blog further. There won't be a test at the end, but you need to understand the figure for the rest of this post to make sense.

OK, now that you've looked this over, I'm sure you'll agree that there are a number of take-aways from this figure, and I'll try to summarize them here.

1. The Midwest typically only produces about 38% of the monarchs that overwinter in Mexico, not 50% as we all thought based on the earlier paper from 1998. From looking at the long-term chart (a), you can see that in the year when Wassenaar and Hobson conducted their seminal study (1997), the Midwest just happened to be a bigger-than-average contributor to the winter cohort. So in other words, we were all wrong about the importance of the American Midwest to the monarchs. In fact, it looks like most monarchs at the Mexico colonies (62%) come from regions other than the American Midwest.

2. The region that contributes the most monarchs to the Mexican overwintering sites tends to vary from year-to-year. This is really big news. Yes, on average, the Midwest tends to have a large contribution (graph C), but there are years when the northeast actually produces the most (1983), and in the most recent years, a lot of monarchs come from the Great Lakes region. In fact, this point is rather important for another reason - the authors found that the relative contribution of each region from year-to-year depended a lot on the climate in each region. If the conditions were good for milkweed and monarch reproduction in any given region, then that region had a large proportion of monarchs make it to Mexico. Sometimes, this wasn't the Midwest.

3. The southern states never contribute much to the Mexican overwintering colonies. This is a little surprising - you would think that being closer to the Mexican overwintering sites would mean that you would have a greater chance of reaching the colonies. I guess this could imply that monarchs produced in this region are not good migrators, or maybe they don't even migrate...hmm. Or, perhaps they tend to migrate elsewhere... Lots of questions here that may take more study to find answers.

4. Here's the other big take-away - if you focus only on the Midwest region on the long-term chart above, you see that there is no overall trend in the size of this dark-green bar over the 40-year time period. There is an initial rise and then a fall, but this could be random. But if you focus on the latest year, it looks like there is just as many monarchs coming from the Midwest now as there was back in the 1970s. Wait, what? Yes, that's what the graph says. I'm not making this up. In the mid 1970s, before roundup-ready crops were even invented, the Midwest produced about 30% of all monarchs that reach the winter colonies. And 40 years later, long after most agricultural milkweed has been eliminated from roundup-ready crop fields, the Midwest still produces about 30% of all monarchs.

Crazy stuff, right?

I've had conversations about this last point with some people more knowledgeable about this than me and there have been a couple of thoughts kicked around. One is that maybe the lack of a decline in the Midwest region on this chart is misleading because this chart shows proportions only, not actual abundance. Similarly, maybe the declines in overwintering numbers stem from a large-scale decline in breeding numbers across the entire range, which is not reflected when you look at relative proportions. If you read through to the end of the paper, it seems that this explanation was also suggested by the authors - that perhaps the breeding population is declining across the entire range.

Another explanation for the lack of a decline in the proportion of monarchs coming from the Midwest - and I know this sounds crazy - is that there really hasn't been a decline of adult monarchs in the Midwest, despite the losses of milkweeds there, or at least this decline is not as pronounced as we all once thought. In other words, the monarchs were never as dependent on those milkweeds within the farmers' fields as we once thought. I know this sounds crazy, but this is actually consistent with other research where monarch abundance has been tracked in various regions around the continent. I've presented many blogs on this, and shown you study after study that draw similar conclusions. In the latest paper that myself and colleagues completed (also published in Global Change biology), we specifically examined the long-term changes in adult monarch abundance in every region of their breeding range, including the Midwest. We found that in local areas were lots of herbicide had been applied, there was a noticeable drop in adult monarch abundance, but these drops were offset by increases elsewhere. In other words, the losses of agricultural milkweeds do not spell doom for monarchs.

Here is my take on this - even though there really has been a decline in milkweeds in agricultural fields in the Midwest (this is documented), the monarchs just don't care. Or at least, the monarch population doesn't care. Think about it - with the breeding range of North American monarchs being so incredibly huge (continental), the breeding population can and does occupy vastly more landscapes and regions than just those in the Midwest, and, based on these data, it looks like the population simply pumps out monarchs wherever the conditions are good in any given year. This is why some regions produce more monarchs in some years than others. This was also the conclusion of the most recent paper in Global Change Biology (see the blog here). If you think about it, this is a really great ability to have for an insect, to have an enormous breeding distribution that covers vastly different regions, and to be able to capitalize on the variable climates within your own range in any given year. This is something that other butterfly species with smaller ranges WISH they had.

Now, let's get back to the original question here: which is the most important region for monarch breeding activity? Clearly, the Midwest still is the largest one contributor (38%) to the monarch cohort in Mexico, but, this is not always true. Sometimes it is the Atlantic coast region, and sometimes it is the upper Midwest/Canada region. In other words, it depends on where the best climate is for breeding that year. This is probably how monarchs have been doing their thing for hundreds, maybe thousands of years in North America - simply breeding everywhere, in all regions, and some regions do better than others in any given year.

So does this new study change what you know about the Midwest? About monarch breeding needs? I know that ever since I read it back in 2017, it changed my views. To me, this study reinforces the sentiment that I've been conveying here for a while now, which is that the monarch breeding population in North America seems to be highly resilient and adaptable, and able to overcome any changes we make to their landscapes.

Lastly, let me take a step away from this paper and briefly mention something else that is on my mind these days. If you've read up to this point, let me end here by saying thanks - thanks for reading this blog, and for educating yourself by reading the science around monarchs. This is a very important thing to be doing in this crazy world we live in now, which is rife with misinformation and science-denial. And, there is a lot of denial going on right now with information around monarchs - to the point where some people will refuse to share this blog post because they don't want others to know what the science actually shows about monarchs. Don't be one of those people.



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The science of monarch butterflies

A blog about monarchs, written by a monarch scientist, for people who love monarchs

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