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  • Andy Davis

Where did all of those monarchs in California come from? A look at some historical and new science

Hello blog readers,

By now, everyone has seen the news about the western monarch subpopulation - I can't open my computer without seeing yet another story about it. The number of wintering monarchs in California has "exploded" this year, going from ~2,000 last winter, to over 100,000 this winter! Perhaps this story is going viral because it is some good news for a change, but regardless, reading these stories got me to wondering where all of these monarchs came from! I suspect that others have thought the same thing, so for this post, I'm going to take a deep dive into some actual research around this topic. That is, I'm going to cover some work that has been done to ascertain where wintering monarchs in California come from! Which geographic regions in particular are we talking about? This question has been addressed by researchers more than once in the past 20 years, and using different approaches.

So below I'm going to take a look at the four different studies that I know of (in order of publication year) where this question has been addressed, and I'll briefly highlight some of the methodology used and of course the results. Then I'll try to make sense of it all at the end. Sound good? Here we go.

To start off with, in 2005 there was a study published in the Biological Journal of the Linnean Society, by Hugh Dingle and colleagues. Hugh is a life-long researcher of insect migration, especially of the bugs that use milkweeds (including milkweed bugs - yes, they migrate too!). Here is a link to this study. Basically, these researchers looked at museum records of monarchs (of about 3000 specimens), in particular the locations of the collection, in order to draw maps of monarch distributions in the west during different seasons. They drew separate maps for each month of the year and then tried to piece together where there was large "movement" of monarchs going into California in the fall, and out of California in the spring. This is an interested approach, and a good use of museum data, though it requires a lot of inference. They were inferring the movement patterns of monarchs by looking at how the maps change from one month to the next.

Here is the main resulting map(s) from the paper. Each dot is a monarch collection. I think the key months to look at here are the fall - August through November. These dots would indicate monarchs that were collected while migrating, so therefore, any region with lots of dots during those months would be where there would be "movement" going toward California.

Based on these maps, the authors concluded that there were large movements of monarchs arriving from the North - Washington and Oregon, going into California. This is actually what has been shown more recently with tagging data. I also note that these maps appear to indicate a cluster of dots in eastern California and Nevada during the summer.

Interestingly, these authors also took note of some "movement" of monarchs through Nevada and Utah in the fall, and appearing to also go through Arizona (note the map for October). They speculated that this indicates that some of these "western" monarchs may be heading to Mexico! This is really speculation here, since they did not know the direction of travel, but it is striking that there are dots that suddenly appear in Arizona in the late fall.

Next, I'll dive into a study published in 2010 in the Journal of Insect Conservation, by Shawna Stevens and the late Dennis Frey. Here is a link to that paper, which I think is open access. This study was also aimed at trying to figure out the "natal" grounds for monarchs that winter in California, but in this case, the authors took a different approach. They examined 10 years of data from the winter colony counts, along with summer environmental data from different regions in the west, to see if they could find correlations. Or in other words, they tried to see if any one western region stood out (i.e. had good breeding conditions) during the years when there were lots of wintering monarchs.

This is quite clever, really. This is another way to ask the same question, but again, it uses a lot of inference and assumptions.

I'll skip most of the paper and just cut to the main results. Here is a map from their results section. The dots show the historical locations of winter colonies. The light-grey shaded regions (CA2, CA5 and CA7) are the regions where the summer environmental conditions best predicted the year-to-year variation in winter colony sizes.

So, these results suggest that most of the wintering monarchs in coastal California actually come from eastern California! They also point out that, based on milkweed record data, that there are more late-season milkweeds present in this region than there is in the Pacific Northwest. This, they argue, means that more monarchs come from this region in the fall, than come from the northwest.

When I look at this map, I find myself wondering if this western "migration" is even a migration at all. If the monarchs simply move from one portion of California to another in the fall, then this is really more of a range shift, than a true migration. Let me come back to this.

Next, I'll take a look at a more recent paper by researchers at UC Davis. In 2016, a study was published in the journal, Ecography, by Louie Yang and colleagues. This was another attempt to determine the natal origins of wintering monarchs in California, using yet another approach - stable isotopes. This is a very fancy and very technical way to estimate where a monarch was born by performing some specialized chemical analyses of its wing tissue. It's a long story, but basically, researchers can perform chemical analyses of the rainfall (H2O) around the country and map out different regions of the country based on the "isotopic signature" of the hydrogen (H) from that rainfall. Then, if the researchers find the same signature of hydrogen in the monarch tissue, they can infer that it came from that region. Keith Hobson (who reads this blog) is the world expert on this - Keith, I hope I got this part right! So in other words, this is a way to "track" where a monarch came from just by examining its wing tissue.

So, Yang's team set about to track where California-wintering monarchs came from using this stable isotope approach. They collected about 100 monarchs from a number of wintering sites during one season and then performed this analysis to determine where they were "born". Below is the primary map they derived.

This map is color-coded with different geographic regions (based on hydrogen isotopes) of the west. They then show pie charts for the 4 different winter colonies they sampled from, and each pie shows the relative number of monarchs in that colony that came from these different regions. Don't worry about the units in the legend, but just look at the colors in the pie charts.

Unfortunately, the limitation of this approach is that the chemical analysis does not narrow down the region very well, or as well as we'd like it to. That's why these colored regions are so big. But, if you study this map closely, this analysis seems to indicate that a very large proportion of wintering monarchs comes from Idaho, Montana and Wyoming! This is quite a different conclusion than the other two studies. To be fair, this tan region does include eastern Washington, which is similar to the paper I'll describe next. Also of interest, this paper seems to disagree with Stevens and Frey, in that eastern California is NOT the largest contributor of monarchs, though it does produce some (if you combine the green and blue-green pie slices). Interestingly, this paper indicates that a non-trivial number of wintering monarchs are local monarchs, from southern coastal California!

Even though the conclusions in this paper seem a little different, I would actually put more stock into this one simply because it is the most direct approach - it directly examined winter monarchs to ask where they came from. The other two were mostly drawing inferences based on correlations or assumptions of movement.

The last paper I'll talk about here is the most recent, and it used a still-different approach. A study was published in 2019, in the online journal Frontiers in Ecology and Evolution, by a group of researchers led by Thomas Dilts, and Matt Forister, from the University of Nevada. Here is a link to the paper (free to read). The goal of this study was to map out the important breeding regions of western monarchs by using a specialized computer program called MaxEnt. This software allows the user to input information on the environmental limits of whatever critter or plant is in question, and it can generate estimated range maps, on where that organism can theoretically live. These maps are typically verified by also inputting direct observations of the plant or animal in question. So, by knowing where and in what conditions the organism lives, it can extrapolate this to where it should also be.

As you can tell, these researchers wanted to know where the important breeding areas are for monarchs in the west, and so they used this program to generate maps of milkweed ranges, and of important monarch breeding regions. They incorporated many thousands of direct observations of monarchs and milkweed. Then, they generated large maps of the entire western range, showing where "high quality" breeding habitat should be. Note, they their goal here was not specifically to ask which regions contribute to the winter colonies, but only to find out where the important breeding habitat is - see the difference?

Anyway, below is a map from their paper. Actually, this is just a portion of their map, which shows where breeding regions should be. They had generated a separate map for each of the native milkweed species, which I won't get into here. These two show the "relative habitat suitability" for breeding monarchs using all milkweed. Interestingly, they generated two maps for this, one where tropical milkweed is included (C) and one where it is not included (D). The difference in these two maps, especially for the California region, is striking.

Based on these maps, the researchers concluded that a sizeable area for monarch breeding is in eastern Washington state, and southern Idaho. This is consistent with what Yang et al found using stable isotopes. But, the researchers concluded that the largest region with high-quality habitat is actually in California. This seems to agree with the 2010 study by Stevens and Frey. Again, this study did not try to determine where these monarchs were heading in the fall, but it seems reasonable to assume that these are "natal" regions for the monarchs that end up in the coastal California wintering colonies.

So, if you're following along, and taking notes, you may have already started to see some common patterns in these studies, which I'll summarize here. If you recall, the original question we were trying to answer here was where do monarchs from wintering colonies in California come from?

- It seems clear from multiple studies that a large number come from the Pacific Northwest, especially eastern Washington, eastern Oregon and Idaho.

- It is also clear from multiple lines of evidence that a large number come from within the state of California! There is evidence that eastern portions of the state are important natal grounds for monarchs, and there is now evidence that some come from the coastal region of southern California! So, a lot of California-wintering monarchs are California-breeding monarchs!

Regarding this last statement, this implies that a large number of the California wintering monarchs did not travel very far at all to get to these sites. They basically just had to meander across the state until they reached the coast! So I guess this means that the monarchs we are now seeing in California are essentially a mix of migrants from the Pacific Northwest, as well as semi-migrants (?) that came from within the state. I find this last bit fascinating - until now, I always assumed that these monarchs had all travelled some great distance to reach these sites, but really, only some of them had.

Ok, I hope everyone learned something from reading this. I know I certainly did when researching it!



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