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  • Andy Davis

What's really going on with the western monarch population? A close look at a passed-over study

Hello blog readers,

I guess by now everyone has seen the recent news regarding the western North American monarch population. The press is currently going nuts over a new report from Xerces about an alarming drop in the counts of wintering monarchs. What I read from these stories was that Xerces is reporting that the most recent count of overwintering monarchs was very low, amounting to an 86% drop from the previous year. And these counts had already been declining over time, similar to how the size of the eastern monarch wintering colonies has been declining. These stories seem to paint a very dire picture of the western monarchs - they're using words like "extinction", "precipitous drop", "nearly all gone", etc., etc.

Normally, I don't pay much attention to the news about monarchs these days, because most of the time, the reporting is inaccurate, or carefully worded to sound sensational. However, after my inbox and news feed became inundated with this one story, I decided to look into it more carefully. It's not that I'm questioning the report - I have nothing but respect for Xerces and what they do. And I'm also not questioning the data. I just wanted to see for myself what the science said about the population trends.

I ended up re-reading a very relevant study that had come out a few years ago, and which I don't think had been read by many people. I recall myself only giving it a cursory glance when it was published. But after re-reading and digesting it, I'm now convinced that this is one of the most important studies that has ever been written on the western monarch population! Today I'm going to dive into it and fill you in on what these researchers did and what they found.

The paper was published in the journal, Oecologia in 2016, and was titled, "Understanding a migratory species in a changing world: climatic effects and demographic declines in the western monarch revealed by four decades of intensive monitoring". Here is a link to that study, although unfortunately, it is not free to download. As an aside, this is probably the biggest reason it didn't get noticed. This is a shame! Hey monarch scientists, if you're reading this blog, don't do that! Research on monarchs needs to be publicly available, especially since the public has so much involvement in monarch conservation.

Like I said, this paper is very relevant to the issue of the western monarch population trends. This study was essentially an in-depth statistical investigation of the long-term trends of western (and eastern) monarch populations. From what I can tell, the goal was to determine the patterns of change over time in both (western and eastern) populations, and also to see how the patterns compare between the two populations. To do this the researchers had obtained data on the winter colony size of the western population, based on the Thanksgiving counts that are organized by Xerces. They also used data from a very unique data source to estimate abundance of BREEDING monarchs each year. Here they used a long-term monitoring project that is done by a single person - Art Shapiro. From what I gather, Art is an extraordinary scientist who has been conducting a series of butterfly counts (all by himself) along a number of transects in California, and he has been doing this for at least 4 decades! As I understand it, he walks these transects twice a month, from February to October, and he records all butterflies he sees. These data have become extremely valuable to science, because they show how different species of butterflies are faring over the long-term. They are also of great value because there aren't that many other long-term datasets on butterfly abundance in that region. For this study, the researchers used the counts of monarchs, obviously.

For the eastern monarch population, the researchers used the Mexico overwintering colony data, which dates back 20+ years. Then for a summer comparison, they used the counts of monarchs obtained by the North American Butterfly Association (NABA). I've blogged about these data before - these data track the number of adult butterflies (monarchs, in this case) during the summer at sites throughout the breeding range. It's an extremely powerful dataset - because of its massive scope, consistency of methodology and long history.

I mentioned that this was an in-depth statistical investigation, and as such, the stats were quite heavy. So I'm going to skip over this part, and cut to the chase. Below is my take-aways regarding the results.

First, for the western population:

- The number of wintering monarchs in the west shows a statistically-significant steady decline (that much we know).

- The number of monarchs counted during the summer by Art Shapiro shows a long-term decline early in the season, BUT, not by the end of the season. This last part is very important. I'm going to paste a figure from their study below, and try to walk you through it, because it demonstrates these summer patterns.

The first thing you need to know about this graph is that it is NOT a plot of the abundance of monarchs! The y axis shows the results of their statistical analyses. I actually had to contact the lead author of the study, Anne Espeset, to confirm this too. For this part of their study they had examined if monarch abundance had changed, statistically, over the years, and they had done this at all time points along the x axis. In other words, they ran a different statistical test for each time point of the season. So, each dot in this graph is the RESULT of each of these statistical analyses (they ran a lot of tests). So, where the dots are below zero, that means for that particular date, the test showed there was a negative trend over time in summer monarch abundance (for that date). The lower the dot, the more negative the long-term trend. See how this works? Incidentally, I have added my own marks and text on their graph, to make it more user-friendly. I also added the red circle, which points to something that is very interesting. These circled dots are all above the zero line, which means for those dates (the end of the season), there was a slightly positive trend in monarch abundance over the years, although this wasn't statistically significant. The black dots indicate where these patterns were statistically significant. But I guess it's fair to say that at the end of the season, there was no decline in monarch abundance over the years.

The authors concluded that the western overwintering colonies are declining in size, and the spring and early summer monarch abundance is also declining over time. But, they suggested that the summer monarch population (at least along the routes that Art monitors) is able to rebound by the end of the season. They therefore concluded that the real problem the western monarchs face must have something to do with the spring migration, or alternatively, the fall migration. Hmmm... doesn't this sound familiar? If you've been keeping up with the literature on the eastern monarchs, you know that this is exactly what is happening there!

Their analyses of the eastern population showed that, as expected,

- the size of the overwintering colonies in Mexico has declined over time.

- as we now know from other studies, they found that the counts of adult monarchs in the summer has not declined during this same time frame.

From reading this blog, you know already know that regarding these patterns in the east, a number of monarch scientists (me included) believe that the real problem therefore must lie in the migration south. Either the migration is getting harder over time, or monarchs are going elsewhere besides Mexico, or they are dropping out of the flyway along the way. Something is hindering the migration, leading to smaller and smaller overwintering colonies.

So, their analyses found some very interesting parallels between what is happening in the western monarchs, and what is happening in the east. In both populations, the major declines seem to be happening at the overwintering stage, and in both cases, the summer population is not as affected - at least by the end of the summer season.

Interestingly, despite these parallels, when they compared the western population data with the eastern data, they found that the two don't line up! There was no correlation between the long-term trends in the west and east, at either the winter stage, or the summer stage. They took this to mean the two populations are functionally different, or essentially operating independently. Another way to think about it is that one population can be really low one year, while the other can be high the same year - exactly like last year! Here in the east, the summer monarch population was huge! But in the west, not so much.

So to answer the original question I posed, what's REALLY going on with western monarchs? Based on the most recent science on that population, the western overwintering colony sizes are indeed on a downward trend. But from the data available for tracking summer monarch abundance, the western "population status" depends on when you look. If you only examine how many monarchs there are in the early or middle of the season, these counts show a long-term decline in adult monarch abundance. But, if you look at the counts of adults near the end of the season, it looks like they are able to rebound each summer to some extent.

You know, it's funny. When I emailed the lead author to get her take on this graph, I asked her directly: based on these analyses, are western monarchs declining in the summer? She said yes! But from my interpretation of the summer data as presented in their paper, I think the answer is no. I guess that means that the western summer population status is "debatable".

I guess that about covers this. Thanks for reading.


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