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  • Andy Davis

New study provides more evidence that milkweed is not the answer to the monarch decline

The picture above was taken from the Maryland Biodiversity Project website and has the following caption: Common Milkweed in Garrett Co., Maryland (7/12/2014). Photo by Bill Hubick

Hello everyone,

A hot new study on monarchs just came out last month, and in one of the premier scientific journals in the world - the journal, Science. I'll be delving into this paper in today's post.

Before I begin, let me first make a point of saying that I like planting milkweed and watching it get used by monarchs - I think everyone should have the pleasure of doing so. However, it is becoming more and more apparent by the actual science - like the paper in question here - that planting milkweed will not "save the monarchs". Read on...

The paper in question can be seen here - link - and the last I checked, it was freely available (it's also a short read). It was written by Anurag Agrawal and Hidetoshi Inamine, who are both no strangers to the monarch world - they published a paper two years ago that provided similar conclusions (see here for the previous blog on that). For those non-scientists reading this, let me point out that the fact that this new paper was published in Science is a VERY big deal. This journal is extremely selective when reviewing submitted papers, so the fact that the paper was accepted in this journal speaks to not only the importance of this topic, but to the strength of the study. Moreover, studies that appear in Science also tend to be highly visible to journalists, who often write stories about them, like this one in the Washington Post.

Before I get into this paper, let me set the stage for you a bit, especially for new readers or for those not familiar with the latest goings-on about monarchs. First, most folks are now well-aware that there is a decline in the monarch population in eastern North America, or at least, that is what they have heard, like in news articles and other press releases. However, that's not the whole story. What usually goes unmentioned in these stories is that the monarch "decline" is in only the overwintering phase of the monarch, and this evidence is based on annual estimates of the size of the wintering colonies that are conducted by WWF in Mexico. These data are not in dispute. But, something else that is usually left out of these stories is that there are also a number of other long-term datasets on monarchs from their summer and fall phases, including from the North American Butterfly Association (NABA), the Ohio Lepidopterists, and the Illinois Butterfly Monitoring program. Then there is the long-term counts of migrating monarchs that take place each fall at the Cape May Bird Observatory. Each of these datasets spans the same amount of time that the WWF data does (2 decades plus), but yet they rarely get mentioned in the news. All of these datasets have been thoroughly examined and statistically analyzed by scientists, and they all show the same thing - that there has been no long-term decline in the abundance of breeding monarchs. Again, let me reiterate - they all show the same thing, that the number of breeding monarchs has not changed in 20 years (though they do fluctuate).

Now, the fact that these other datasets do not show declines has been a bit of a dicey issue in the monarch conservation world. It doesn't fit with the story that keeps getting told over and over in the news that "monarchs are declining", plus it flies in the face of another part of the story, that says the declines are due to a loss of milkweed in throughout their breeding range. Think about it - if there has been no loss of monarchs in the summer over the last 20 years, how then can milkweed be declining? I've blogged about the controversy in previous entries, which are summarized here.

The new Science paper provides more evidence for an alternative, and much more parsimonious, explanation for the disparity between the winter vs summer data - which is that there must be something that is preventing monarchs from actually reaching their winter destination - i.e. a problem with the migration itself. In fact, a number of monarch researchers have concluded this too, based on various studies. Basically, the idea goes that since there has been no long-term decline of breeding monarchs in eastern North America, but smaller and smaller numbers of monarchs reaching the overwintering site, the logical inference is that the migration must be getting harder, resulting in lower migration success. The thing I like most about this explanation is that it takes into account ALL datasets on the monarchs, and provides a working explanation for the fact that they don't match up. Some of my colleagues have argued that the lack of a decline in the summer is because the citizen science programs (all of them!) must not be sampling the whole summer population, and therefore we should not even be looking at these data (i.e. the only dataset that counts is the one that shows the decline - Mexico). This argument involves basically disregarding an enormous amount of data, and hard work by volunteers.

So the new paper provides further evidence for this disconnect between breeding and wintering datasets, by shining a light on the 2017 monarch season. If you all will recall, there were monarchs everywhere during that summer - folks were happily reporting them in their backyards in pretty much every place where there was someone looking. To demonstrate this, the authors obtained updated data from NABA on the number of adult monarchs counted during the summer throughout the breeding range, and graphed it, pictured below. To my knowledge, this is the most up-to-date graph using the NABA data that I have seen.

The last two dots on the right are the overall scores from the 2017 summer, and show that the relative abundance of monarchs throughout their breeding range was pretty high that year - not the highest of all time, but definitely above average. Now look at the 2017 overwintering colony size data, graphed below...

The size of the colonies last year was surprisingly low, despite the monarchs having a banner summer. Think about it - if the summer monarch population was above average that year, then why wasn't the overwintering colony size above average too? This was pretty much the gist of this paper - that there is a "disconnect" between the summer and winter abundance, and that this must be getting larger over time.

About last fall, if you recall, there was a lot of online discussion last fall and winter about what happened during the fall migration, and Agrawal and Hidetoshi relayed some of this in the paper. Essentially, the 2017 fall was unusually warm, which some say either delayed the migration onset, or resulted in an extra generation being produced (which themselves were late, etc.). Whether it was an extra (late) generation, or if the warm conditions made the monarchs not want to migrate, either way, this would not have helped the migration. Moreover, from watching the fall migration progress on Journey North, it was pretty clear that the roost observations in Texas (pretty much the end of the migration in the US) were not where they should be, in terms of numbers of observations, and overall roost sizes. So the consensus view was that there was a major problem with the migration that year, which is why the winter colony sizes were so small. Even Chip Taylor admitted this on dplex, the monarch butterfly discussion forum!

The Science paper essentially used this 2017 season as a case in point for why the migration is more important for setting the overwintering size than is the size of the breeding population. And the authors are right - if it happened for sure last year (which most people believe), then why isn't it happening each year?

Now here's where the story goes that no one wants to hear, which is an extrapolation from the conclusions of the Science paper - if the real problem is that the migration is getting harder, then planting milkweed will not help, at least not very much. At best, it will help increase the size of the summer population, which maybe could be considered a good thing. But, this strategy could also end up simply dooming larger numbers of monarchs along the migratory journey! For a human analogy to this scenario, imagine the monarch migration is like one of those adventure/obstacle course things that are becoming more and more popular - like with the ropes, ziplines, waterpits, etc. And imagine that it's really hard to get all the way through this one course, but the course owner really wants to increase the number of people that successfully reach the finish line. One strategy would be to simply allow more people to go through the course, and hope that a few more make it to the end (akin to boosting the breeding monarch numbers). The other is to figure out what part of the course people are having the most trouble with, and fix it so that it no longer acts as such a logjam (akin to addressing the migration issue).

So what exactly IS the logjam when it comes to the monarch migration? Unfortunately, we don't know for sure yet, and this is the thing that the milkweed-strategists usually seize on during the arguments. They see their strategy as more concrete and doable, since it gives people something they can do right now. This is a fair point, for sure. I see that in the Science paper, the authors provided a listing, of sorts, of some possible "threats" to monarchs, including some that occur during migration - this was Table S1 in the supplemental file. Again though, these are just possibilities, and only some of these have been empirically studied. In reality, it is likely that there is a slew of many different "obstacles" that migrants encounter that all play a role, and only some of these can be fixed (i.e. some are completely natural). For example, in multiple previous blog entries, I've covered the issue of road mortality - summarized here - which exacts a crazy toll on the migratory cohort. And it seems there is more research on this topic forthcoming. However, this is just one threat, out of many, and now I'm digressing...

Because of the uncertainties in this area, the Science paper ends with a plea for further research into these migration issues, which I wholeheartedly agree with. Also, kudos to these authors, for getting this paper published in Science.

So as not to end on a low note, let me again say that planting milkweeds and watching monarchs use them is a really, really pleasurable experience, and I love watching my kids experience this. If this is something you enjoy doing too, then by all means, go for it.


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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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