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  • Andy Davis

Recently completed analysis of roosts shows the fall migration is down to a trickle

Hi everyone,

I'm afraid I have some very, very, VERY bad news to share today, regarding the famous long-distance migration of the monarchs in eastern North America. I'm going to tell you about a brand new study that I was a part of, and which my team and I just now submitted to a scientific journal for peer-review. That's right, these findings are so new that the ink hasn't even dried on the manuscript. I'm taking this unusual step of announcing this already (before publication), because of the severity of the problem, and, because of the ramifications of these findings to the upcoming decision by the USFWS to put monarchs on the endangered list in the U.S. In fact, I hope that the staff at USFWS is reading this now. Also, let me give you a warning that this new study has left me rather disgusted, and you'll see why later, so it is hard for me to write this in a nice way.

Before going further, I should point out that you yourself can read the entire study from front to back for free right now, as it has been uploaded to a pre-publication "pre-print" website, linked here. This is your chance to hear about the findings before all of the media hoopla starts, because there will surely be a media frenzy when this is published.

This was another collaboration with my UGA entomology colleague, Bill Snyder, and his postdoc, Jordan Croy, who had done most of the heavy lifting with the data analyses. In the end, I think we all added our respective strengths to this project to turn it into a masterpiece (if I do say so myself), although one that tells an absolutely tragic story.

Let's get to that story now.

The title of the paper (and this blog post) give you the gist of the study: "Dramatic recent declines in the size of monarch butterfly (Danaus plexippus) roosts during fall migration" Note the emphasis on "roosts." This paper was an analysis of the thousands of observations by members of the public that have been submitted to the Journey North site over the last 17 years - observations of migratory roosts, like the one shown below. Most monarch aficionados are familiar with these, if they haven't seen them personally. They are aggregations of dozens to thousands of migratory monarchs, that have chosen to stop for the night and rest on a tree or bush. When people see these, they report when and where it was to Journey North, and importantly, they also estimate the NUMBER of monarchs in the roost. This information has been stored in the Journey North archives, and we dug it out for this project (with permission).

(image above from the Journey North archives)

The idea of this project came from the ongoing (and sometimes heated) debates over the "conservation status" of the monarch population, i.e. whether the population is declining or not. This is where the upcoming decision from the USFWS comes in - they are right now deliberating on this very thing. There have been a suite of scientific studies on this in the past decade, and they each came to different conclusions, depending on where and what time points the study was focused on. The studies focused on the Mexico wintering colony numbers have mostly shown how these annual counts have declined in the past 25 years (though not as much in the past 10). Meanwhile, counts of adult monarchs in the summertime have generally shown little to no long-term decline. These two opposing patterns have generated a lot of confusion and controversy. The good news (if any) here, is that this new study may solve this mystery.

Our team used over 2500 records of roosts from up and down the fall flyway, and we specifically looked for evidence that the roost sizes were declining over time. This was done using some very sophisticated statistical analyses. But, we also suspected there would be other factors that influence roost sizes, and we tried to incorporate each of these into our analyses. These included the local weather around each roost (which we obtained from public records), and even the wind speed and direction. We also considered the landscape around the roosts, and especially how "green" it was that year. This is something that can be gleaned from public records too, and it is a good proxy for butterfly food, like flowers, goldenrod, etc. In some years, the fall flyway is less green than in others, because of drought, for example, which means less food for the monarchs, and therefore smaller roosts.

So in the end we (or Jordan, really) had done a very thorough statistical analysis of the roost sizes, to determine if any of these things was affecting how big they were, and, if any of these things has been changing over time too, which is important to know. It turns out that the greenness of the fall flyway has indeed been changing, but not in the way you might have thought - it's been getting slightly more greener over time, on average. This might be due to climate change, and the fact that the summers are becoming extended for longer. And, we found that at the local scale, the roosts tend to be larger when the landscape is greener, perhaps because the monarchs are seeking out places to roost where there are also food resources. We also found that the weather does affect the roost sizes - warmer weather seems to lead to larger roosts. But, none of this seems to matter anyway, as I'll point out next.

Here is the big finding - despite these landscape and weather effects, we detected a statistically significant, and dramatic decline in the roost sizes over the last 17 years, that was most pronounced at the southern portions of the flyway (Texas). There was a slight decline in the more northern regions, but the decline was greatest in the south. This means that the number of monarchs in the early (northern) roosts has diminished only a little, but by the time the migration has reached the southern region, the decline was magnified - by a lot. We calculated that the size of roosts in Texas now are about 80% smaller than they were just 17 years ago! Read this last sentence again.

Here is a screenshot of the primary figure in our paper, which shows this pattern.

Here is another way to look at this - if the roosts are getting smaller and smaller as the migration progresses southward, that means the monarchs are dropping out along the way. From what we can guess, they are either getting killed somehow, or simply dropping out of the migration. Either way, they are disappearing as the migration progresses southward. There really is no other explanation, since we ruled out the possibility that the landscape has changed over time, and it seems like there hasn't really been a loss of "greenness" in this timeframe, in fact it is the opposite. I know there have been droughts in some years in some regions, but keep in mind that our analyses looked at the averages across the entire flyway. So in other words, these losses are not weather-related (i.e. climate change), nor is it anything to do with "migration habitat" that we can tell.

This last part deserves more explanation, because this is something that a lot of people have been harping on - that monarchs need more habitat, such as during their migration. Surprisingly, our analyses doesn't show this. And, I should also point out another recent study that has found the same thing. A new paper was just published earlier this year in the journal, Nature Scientific Reports (link here) that was an analysis of migration habitat in northern Mexico. The authors looked for evidence for changes or losses of habitat along the flyway in the last 20 years, and couldn't find any! This is very consistent with our findings. Bottom line here, there is little evidence that the decline in the migration is due to a loss of habitat.

One of our other thoughts on this was that perhaps the monarchs were getting killed by cars and vehicles on roadways. This is certainly a huge problem for monarchs, and especially in Texas and northern Mexico (see blogs about this here). But, this seems like a stretch here, because this isn't something that has increased dramatically over the last 17 years. In other words, it's not like there are suddenly more cars on the road, or, more roads.

Another thought people may have is that perhaps this is from the growing use of pesticides like neonicitinoids, which may weaken the monarchs, making them unable to fly well. This sounds logical, but there have been recent studies on this, where monarch flight was actually tested with and without exposure to neonicitinoids. This has been tested in a lab setting and also in the wild, with radio-tracking migrating monarchs. These studies have not shown any clear negative impact on flight ability.

So what IS causing these migratory losses? This is the million-dollar question for sure. Of course, we can't specifically test these ideas using only the roost sightings database, but I have exactly three guesses, and they are all human-caused: releases of captive-reared monarchs, planting of non-native milkweeds in the flyway, and the rise of the OE parasite (which is being driven by the former two). You've heard me harp on each of these before in this forum and on facebook, for GOOD REASON, because each of these things has a boatload of research that demonstrates the harm they cause to the monarch migration! I mean god-damn, how many times do I have to say it? Captive-reared monarchs are not good at migrating (link here), non-native milkweeds hinder the fall migration (link here), and the OE parasite makes it harder for monarchs to migrate (link here). Each of these things has increased within this exact timeframe (the last 20 years). And now, it looks like the chickens are coming home to roost. For the people who refuse to believe the science and continue with these actions, I consider this to be mostly your fault - you are responsible for the demise of the monarch migration. Is that clear enough?

Do you know who else is at fault here? Everyone who pushed the "save-the-monarchs" campaign. Sorry to be blunt with all of this, but this is an absolute tragedy of epic proportions, because these migration problems are most likely from the actions of people. People who are selling the non-native milkweeds, people who are pushing the rearing hobby, people who are involved in conservation groups who are using the monarchs to generate funding, and even some scientists, who have been campaigning for years to convince people that monarchs are in trouble (which leads to more captive-rearing, more non-native milkweed, and more OE)! All of this, has snowballed into a catastrophe for the monarch migration.

Lincoln Brower once likened the epic monarch migration to the Mona Lisa, which is something special, to be cherished and protected. If he were alive today, I'm not sure what he would think of all of this. He might be as disgusted as I am.

Thanks all for now. Stay tuned for updates as this paper progresses through the peer-review stages. Also, please share this blog, especially to the people who need to hear 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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