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  • Andy Davis

Drought, schmought, new study shows migrating monarchs can deal with it

Hello blog readers,

Happy new year 2024! I'm glad to have you along for the ride through another year, as we journey through the fascinating world of monarch science. Today's blog post will be a summary of a paper that was published in late 2023, that hardly anyone noticed, but that has some pretty big implications for homeowners and conservation efforts. I'll use this blog to bring it to people's attention. And the title of this post is accurate too! Really!

The paper in question was published in one of my favorite scientific journals (and which is fully online), called Conservation Physiology. Here is a link. This is a journal that specializes in research where physiological applications and approaches are used to tackle real-world conservation issues (my kind of research). And even better, the paper was authored by my friend Keith Hobson (professor at Western Ontario), and a number of his colleagues. I have a lot of respect for Keith, because he (like me) is one of the few remaining objective monarch researchers out there, as his only agenda is just to simply collect good data, and tell people what the data show. As of late, a lot of Keith's work has been focused on the monarch fall migration, since he (also like me) realizes that this is where we need to be focusing our conservation efforts.

So, full disclosure, I had nothing to do with this paper, nor did I even review it. I am reading it now just like everyone else. I did give it a good read, and from what I gather, this new study was mostly focused on understanding how and when migrating monarchs acquire their lipid stores during the fall, which they use to sustain themselves throughout the winter. This is a huge conservation issue, because (in theory anyway) if monarchs can't get enough lipids to sustain themselves, they won't last the wintering months, and then we'd see fewer monarchs making the spring migration north.

This was also on the mind of Keith and his colleagues, and it looks like during their three year study (2019, 2020 and 2021), there was one year in which there was a natural experiment about this very thing. In 2019, the American south experienced a major drought, and especially in Texas. This dried up a lot of the flowers that monarchs use to get nectar during their journey (which they convert to lipids). The drought was lessened or gone by fall 2020 and 2021, and this made for a nice comparison of monarch lipid levels between years, which would allow the authors to draw conclusions about how droughts affect monarch migration.

During these three seasons, Keith and a whole slew of collaborators made efforts to collect migrating monarchs at various places along the fall migration (see map), and for each of these collections, they took a number of measurements of the monarchs, including their overall wing size, their mass, and then later (back at the lab) they determined how much fat each monarch was carrying. Unfortunately, this is a destructive assay, but, keep in mind this was for science. There were some other measurements of these monarchs too, like the amount of water they were carrying, and then some calculated measurements of their wing loading, though these data weren't as important. You can check out the paper itself to see these analyses.

The primary statistical analysis was a comparison of how much fat (lipids) the monarchs had at these different locations along the flyway, and importantly, across the three different years (one of which was a drought). And also keep in mind that two of the collection sites were at the winter colonies themselves, which allowed the authors to see if the drought in 2019 had affected how much fat reserves the monarchs arrived with.

Here is what they found:

In 2019, the monarchs collected in Texas did indeed have lower than normal lipid levels, and their lipids were lower than in the other two years too. However, in all three years, the monarchs collected in northern Mexico (sites 6-8) had more lipids than those in sites 1-5 in the map above, and this was especially pronounced in 2019. And, in all three years, the monarchs collected in Mexico had even more lipids than that. In other words, the monarchs were able to procure their necessary winter lipid reserves in each season, even during the 2019 drought. From the comparisons across collection sites, it is clear that the majority of lipid accumulation seems to happen at the northern Mexico areas.

The authors speculated that the monarchs seem to be able to "compensate" for any droughts that affect their flyway resources in the northern regions, by loading up on nectaring areas in Mexico. This is a fair conclusion to reach. It sort of implies that the monarchs maybe just kept flying over Texas altogether that year. However, I might have another to put forth regarding the pattern from 2019. This was briefly mentioned in the paper too, but I think it deserves more consideration. My thought is that the 2019 drought may have acted as a natural selection event, so that maybe the drought in Texas resulted in lots of monarchs with low lipid levels that perhaps that did not survive the journey beyond that region, but, maybe there were still enough monarchs that avoided that region altogether, and those were the ones that made it to Mexico. Thus, Keith's team was only measuring the survivors that did make it to Mexico with enough fat reserves. If this is indeed the case, it actually has an upside - it means that the survivors (the ones who knew how to deal with drought) would be the ones to pass on their genes to the next generation.

The interesting thing about this paper is that this is the second study to come to the same conclusion regarding the effect of drought (or lack of effect) on migrating monarchs. Lincoln Brower actually led a similar study which was focused on a drought that happened in 2011 across Texas. That paper was published as a chapter in the 2015 book about monarchs. Lincoln and his team went to Texas to collect monarchs during that drought, and also went to the wintering colonies in Mexico, they measured the amount of lipids in those monarchs, and they came to the same conclusion - the drought in Texas did lead to low lipid levels there, but it did not reduce the amount of lipids in monarchs at the wintering colonies. Lincoln also concluded that the monarchs must have made up the difference after they left Texas. Keep in mind that my idea would still pertain to that scenario too (drought kills some, but not all monarchs).

Sidebar - I kind of see these findings as yet more evidence for how adaptable, and robust, monarchs are. The doom and gloom that permeates the news about them on a weekly basis sort of gives people the impression that they are somehow these fragile critters that are on the brink of extinction. But in reality, they are a massively successful and extremely adaptable insect that is capable of overcoming tremendous obstacles year after year. Sure, some of the population succumbs to these stressors and risks, but then some don't. That's simply the way nature works.

To be clear, there is good evidence from other research that drought has some effects on the size of the winter colonies, so it's not all hunky dory. But, from these two studies it seems that whenever there is a drought, there are enough monarchs that do arrive at the wintering colonies with the necessary fat reserves.

So I mentioned at the outset that these findings have implications for conservation, and for homeowners, and they do. Clearly the one thing that migrating monarchs really need during the fall is nectaring resources. Also, it is clear that the need for nectar resources is most apparent in the southern U.S., and also northern Mexico. So the logical takeaway is that if people really want to help the monarch fall migration, they should plant fall-blooming native flowers. These flowers are literally fueling the fall migration, and helping to ensure that the migration is successful. But the other thing that this research demonstrates is that people really shouldn't freak out whenever there is a drought. The monarchs will find a way to deal with it.

That's all for now.


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