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  • Andy Davis

Evidence from monarch DNA shows the population has not declined



Hi folks,


Thanks as always for checking in to hear about the latest science around monarch butterflies. Today's post will be a doozy, as I'll (attempt to) explain the recent study where researchers examined the genetics of monarchs to piece together how their population size has changed in the last 25,000 years! Yes, this is really a thing now in science. With advances in genetic analysis, combined with the latest, high-tech, machine-learning software, researchers can now look at the DNA of any species, and piece together its population history, going back thousands of years. It's super-cool! This is not my area of expertise, but I do know enough to explain how it works. And the especially cool part here is that this new study found exactly what the title of this post states. So, if you are one of those people who thinks that monarchs are in trouble, this will be a game-changer for you.


So, the paper was just published after peer-review in the journal, Current Biology, and here is a link to the abstract - https://www.cell.com/current-biology/pdf/S0960-9822(23)00996-X.pdf. The entire paper is not downloadable without a fee, but, trust me, it is not an easy read. The authors of the study are based at the College of William and Mary, in Virginia, and I see that Anurag Agrawal, from Cornell was also an author. Oddly, I'm not sure how much press this new study received, so you may or may not have heard of it. I did see that their university put out a press release (link here). Then again, given that this new study essentially showed good news, the lack of exposure could also be related to the "inconvenient truth" issue I blogged about before.


From my read of the paper, the goal of this study was to try to look for historical changes in the population size of North American monarchs, and, in the size of their primary host plant, common milkweed. I said primary here, because by all objective measures, this plant is the most widespread of all of the milkweeds, and, it is by far the primary hostplant of those monarchs that reach the Mexican overwintering colonies (>80% of wintering monarchs developed on common milkweed). So, don't nitpick this part. Anyway, the authors here essentially wanted to see if the population size of monarchs tracks the population size of common milkweed.


Before getting too far, let me first attempt to describe how this genetic approach works, like how DNA can tell us the "size" of a population. Essentially, this works because any organism's DNA contains a written record of the level of "genetic diversity" in its population, and this diversity is the key to assessing population size.


Sidebar - what is "genetic diversity?" Well, as you know, the DNA of every animal contains genes that have been passed down through time over each generation. Basically, each time a male mates with a female their genes combine in the offspring. And, the genes from each grandparent, great-grandparent, great-great-grandparent, etc. are also passed down. Over time, with more and more pairings (different matings) from genetically different individuals, the "diversity" of genes in a population increases. But, this only happens if there are a lot of genetically different individuals in the population. In a declining population, where there are fewer individuals and fewer matings, there tends to be a corresponding decline in the "diversity" of genes in the DNA of the individuals left. So think of genetic diversity as a measure of the "genetic relatedness" of animals in a population. In other words, in a declining population, animals tend to be become more "inbred," to use a term people know. However, keep in mind that with the tools we have now, researchers can track even miniscule levels of relatedness within populations - like if two individuals were 17th cousins, or something. So "inbred" is not really the right word here, but it does convey the gist.


Or, try this - imagine if you were to look at the DNA of people that have been living for generations on a small island (like with only a few thousand people on it). There is a good chance that their DNA would show lower levels of genetic diversity than say, any person sampled in Europe (a large land mass). So by this method then, the size of the population can be inferred just by looking at the DNA of a small number of individuals within each population.


This approach is being used more and more in science, and especially for tackling questions about population sizes in animals of conservation concern. For example, a group of researchers recently used this very approach to look at declining honeybee populations in Europe. Here is a link to that paper. The researchers examined archived specimens of honeybees from museums, and compared their "genetic diversity" (of the specimens) to that of living honeybees today. They found there has been a loss of genetic diversity in the population over the last 100 years, which is consistent with overall population declines. In other words, individual bees in that population today are more related to one another than bees were 100 years ago.


The benefit of using this approach to study population declines was nicely demonstrated in the example above - all you need is some archived specimens and you can do it (with the right tools and expertise). In other words, you don't even need actual census data on the population size. You can simply infer whether the population has changed over time based on your comparisons of specimens. Another benefit of this approach is that you don't need that many specimens to do it. Unlike with field-based research, where sample sizes usually need to be in the hundreds, it only requires a handful of specimens to determine what the "genetic diversity" of an entire population is. This is because the DNA within each individual tells the story of the entire population. Or think of it this way, your own DNA tells you about the genetic makeup of all of your ancestors - see how this works? One sample of DNA tells you about the genetic makeup of a huge number of individuals.


OK, back to this study of monarchs. From my read, the researchers in this study didn't actually even need to sample any live monarchs themselves. Rather, they extracted previously-published DNA profiles of monarchs from two prior genetic studies. One was by Zhan et al in 2014, and one was by Talla et al in 2020. In each of these studies, monarchs had been collected, and then their DNA was examined. Then, the DNA profiles were placed in a public database. So, for the new paper, the authors simply extracted this information, and then ran it through some really, really fancy computer software. From my understanding, this software looks through the DNA profiles, searching for genes, then maps these, and calculates the levels of genetic diversity.


Next, the authors collected plants (common milkweed) from a number of places throughout the range, in many different states. Here, the authors really did do some field work. In their lab, they extracted the DNA profiles of these plants, and again, used their fancy computer system to piece together the genetic diversity levels of the plant population over time.


Note, that I made this whole lab and computer part sound easy, but it likely took months if not years for this study to come together. I also see that there were many authors listed on the paper, which tells me it was a huge team effort to get this done.


Here is what they found.


First, they found that the monarch population size (as inferred from genetic diversity) has indeed tracked changes in the size of the common milkweed population over time. Every time the milkweed population expanded, there has been a corresponding increase in the monarch population - this makes sense. Importantly, they describe two notable expansions in the size of both over the last 25,000 years (yes, they can do this). It looks like there was an expansion after the last ice age, when the landmass of North America became lush and green. Second, there was a noticeable expansion 200 years ago, which coincides with when most of the eastern portion of the continent was cleared for agriculture. Common milkweed, being a disturbance-tolerant species, took advantage of this, and flourished. So too, did monarchs. In other words, it looks like we humans had already artificially enhanced the monarch population by our landscape changes.


Now, here is the really important part. The authors specifically looked for evidence of a more recent decline in both species, and, they could not find one. That's right, they report finding no evidence of a "population bottleneck" of common milkweed in the last 75 years. This is very important because this milkweed is the one that is found in crop fields, and has supposedly been wiped out by farmers switching to roundup-ready crops in the last 20 years. So, this new genetic evidence does not support this. Note that this does not mean that the milkweed in crop fields is still there, it's not. But, this likely means that there is more than enough milkweed everywhere else. And the other important finding is the authors could not find any evidence of a recent bottleneck in the monarchs either. In other words, the DNA of monarchs shows no recent decline in genetic diversity.


The bottom line from this new study is that the monarch population size is bigger now than it was before humans modified the landscape of North America 200 years ago, and, that there has been no recent decline in the population size of either monarchs, or common milkweed.


The authors point out that this finding is clearly at odds with the 30-year trends in the size of the wintering colonies, and they even suggest that whatever is happening at the winter colonies is misleading, and does not reflect the true size of the population.


Keep in mind that these findings are completely consistent with the study from last year, in which myself and a team of statisticians and entomologists examined counts of breeding monarchs from across the nation and also found no evidence of a decline. That also argued that the winter colonies (or at least their size) are not as important as we all thought. So basically, these authors arrived at the exact same conclusion as my team did.


OK, so now how do we go forth with this new information? This news is basically saying that everything we thought we knew about the monarch population is wrong, and, they seem to be doing ok, at least numerically. There will likely be many different reactions to this news, but lots of resistance for some reason. I always find it odd that this happens, because really, this is good news. For some reason, people don't seem to want to hear this. There are some people out there who will even pretend not to have seen this blog, because it is an "inconvenient" study. I hope you are not one of those people. Others may try to share this blog post, but then have it challenged by some "internet monarch expert." I hope you are not one of those people either!


I hope that you are the kind of person who will share this post because it is the right thing to do. People who love monarchs should know the facts about monarchs, because improving our knowledge of them helps us to tailor our actions, so as to best help the species.


That's all for now.



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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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