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  • Andy Davis

It turns out, western monarchs aren't really that special

Hello everyone,

Sorry western monarch folks, there is more bad news for you. This comes on top of the continuing declines of your winter colonies in California too. A new study was published last month that addressed the long-running question of whether western monarchs are different than eastern, and, you guessed it, it turns out they are not. And, this also means that they don't even qualify as their own population anymore. That's right, you've been downgraded. As of right now, the best way to describe monarchs west of the Rockies is as a "subpopulation". But wait, it gets worse - based on the new evidence in this paper, it looks like western monarchs are essentially redundant with respect to the eastern monarch.

Let me start back at the beginning.

From a scientific perspective, this question is not new. In fact, I can recall at least half a dozen or more other studies that also asked this same question over the last 20 years - are western and eastern monarchs different? Some of these studies were genetic analyses, some were behavioral in nature, and some were analyses of wing morphology. Recall that I blogged about one of the latest genetic studies last year. Anyway, over the years, there has been a lot of information accumulating on this topic. This new paper was in fact a review of all of this evidence. So in the end, this paper could be considered the official consensus on the topic, and (in theory) it should give us the definitive answer on this question. I said in theory there because I think the authors sort of punted on this - more later.

Here is a link to the study, which was published in the journal, Conservation Science and Practice. The paper was titled, "Are eastern and western monarch butterflies distinct populations? A review of evidence for ecological, phenotypic, and genetic differentiation and implications for conservation." As you can see from the weblink, there were a whole slew of scientists who contributed to this paper, and they were led by Micah Freedman, who is currently a postdoc with Markus Kronforst at Univ. of Chicago. I was actually involved in this project at an early stage, but had to back out for a few reasons. But, I do recall that this was a very, very hard paper to write, because it was hard to get broad agreement among the authors on this topic!

Incidentally, I have noticed that the link to the paper has been shared in a few online groups, and websites, though I don't think there has been much discussion over it. I don't know if this means that this is just old news, or maybe that people just haven't bothered to read the paper itself. I do think it is downloadable, so access should not be a problem. I also wonder if maybe folks don't realize the significance of the conclusions from this paper. I'll try to point this out here. This paper means a lot more than most people probably think, especially as it relates to the conservation of western monarchs!

The rationale for conducting this review actually stemmed from all of the events leading up to the recent USFWS decision last year on whether to officially list the monarch on the endangered species list. The USFWS was very interested in this subject last year when they were reviewing the science on monarchs and their populations. In fact there were a number of meetings of all of us scientists (some in person, some on zoom), where this issue was discussed and debated, and really, this paper is the culmination of a lot of those meetings. The reason the USFWS was so interested in this was because they wanted to know how important the western monarch is for the survival of the species in the United States. Apparently, this is something that they research for all of the species that are slated to go on the ESA list - they assess all of the populations and subpopulations of said species, to determine which of them are critical to the long-term survival of the species.

In scientific lingo, they wanted to know if western monarchs provide any "adaptive capacity" to the species. This term is scattered throughout the paper too. This is a fancy way of asking if this population is different enough from the main population so that it provides something unique, and therefore it (the unique population) must be protected. Or another way to think about it is, say there was a major environmental catastrophe and both the western and eastern monarch populations became decimated, would the western population be needed for rebuilding the species? Or still another way to look at this is, say the western monarchs have a special trait like resistance to a bacteria, and this is not seen in other populations. If this hypothetical bacteria eventually spreads and becomes a problem, those populations with innate resistance will help the species adapt to this novel threat. See why the service was so interested in this question? From their prospective, once a species is listed, then they are in charge of conserving it, and in this case, they really needed to know which monarch populations are most critical for the survival of the species.

Back to the paper now. If you haven't read it, I'll provide a bit of a summary of it here. The paper was divided into several sections, where each one discussed the evidence around a specific topic. I'll list those topics here and provide my own summary.

The first section examined the evidence for ecological differences, such as host plant usage, thermal tolerance, natural enemies and migration traits. Of the literature around host plants (two studies), there was no evidence that western monarchs prefer (or require) different species of milkweeds than eastern monarchs use. Both populations appear equally capable of using whatever milkweeds are present. For thermal preferences, there wasn't a lot of evidence to look at, in fact it was really just one study that Sonia and I did back in 2005, in which we grew eastern and western monarchs in different temperatures. In that study, there was some minor evidence that western monarchs were lighter in color, but in the end, both populations were capable of developing in a range of temperatures. Next, they looked at the evidence for natural enemies, and here they focused mostly on the research around the parasite, OE, since there has been a lot of studies. Here, the evidence indicates that even though OE rates are higher in the west, there is not much difference in overall susceptibility between western and eastern monarchs. That means if you give an eastern monarch caterpillar and a western monarch caterpillar an OE spore from the east, or from the west (researchers have), then they both become similarly infected. Or in other words, neither population is more or less resistant to OE infections.

Lastly for this section, they looked at migration-associated traits. Now, we all know that the western migration is much shorter than the eastern one, but that is not what the researchers were looking at -they wanted to know if the western monarchs have different traits, like wing shape, or size. There is a fair amount of research on this one actually, but the collective evidence seemed to indicate that while eastern monarchs are statistically larger than western monarchs, this difference is trivial compared to the range of sizes one would find in North America. There was also some evidence that western monarchs have a reduced flight capacity, based on recent laboratory trials from the lab at Emory University. I blogged about that study last year. But, that same study also examined the genetics of monarchs from both populations, and concluded they were not genetically different. So, the conclusion from the Emory study, which was also stated in this review, was that the flight differences of western monarchs must be from environmental effects, and it is not something they are born with.

The next section was devoted to the evidence for genetic differences, or lack of differences. As I mentioned, this question has been asked a number of times over the years, using the available technology of the times. As the tools of this trade become more sophisticated, so too does this particular analysis of monarch DNA. In this review, the authors compiled all of the literature on this topic, which all pretty much shows the same thing - eastern and western monarchs are genetically the same. The authors then go on to speculate how this could be true, given that the two population are divided by this massive mountain range! They point out that from a genetic perspective, if even one monarch passes between the two populations per year, then that is enough to keep the gene pool mixed enough so that they all share the same genes!

Also in this section, the authors pointed out that there is some evidence that the monarchs in south Florida do seem to be genetically distinguishable from all others, but they did not make any claims that this helps the monarch species. Also, see my prior post on the extremely high rates of OE in south Florida!

The last section of the paper was devoted to discussion of the concept of adaptive capacity in monarch populations around the world. While this seemed a little tangential, it was interesting to hear the authors' thoughts on this topic. They pointed out that there are many global populations of monarchs, including in New Zealand, Australia, the Carribean, Hawaii, etc., and most of these are non-migratory. A recent study, by Nail and colleagues, took a close look at these non-migratory populations and suggested that their very presence provides the monarch species with a lot of adaptive capacity. However, the authors of this new review indicated that these non-migratory populations may not be a good stand-in for the truly migratory monarchs in North America, since they may have lost the ability to become migrants! The jury may be out on this issue.

Finally, I mentioned earlier that I thought the authors may have punted a bit in their conclusions. After showing all of the evidence, they conclude by saying that it is up to policy-makers to decide if western monarchs NEED to be saved. They suggested their are two approaches one could take, an approach based on the evidence (which shows western monarchs are not unique), and therefore not deserving of special protection, or one that is more precautionary, in which we should save the western monarchs anyway, just in case there is indeed something special about them, that we don't yet know. Reading between the lines here, I suspect that there was some arguing among the authors over this conclusion, which may explain the wishy-washiness here.

To be absolutely thorough on this issue, I actually reached out to the lead author, Micah Freedman, and asked him to weigh in on the topic of the eastern and western North American monarchs. I specifically asked if he himself thought the western monarch provides any "adaptive capacity" for the monarch species. Below is his reply.

"To answer your question simply: No, I don't think that western monarchs provide non-redundant adaptive capacity. The evidence pretty strongly suggests that they are genetically indistinguishable from eastern monarchs, and the (relatively minor) differences that do exist between wild eastern and western monarchs are likely environmentally determined. If western monarchs are going to be given special consideration during the annual review of the ESA listing decision, I think that there would need to be new data presented to justify doing so."

Andy here again - to put this answer in even simpler language, this new evidence shows that western monarchs are not special, and therefore their decline (while sad) will not doom the monarch as a species.

That's all for now folks. Kudos to my colleagues for putting this thorough and timely paper together!


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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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