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  • Andy Davis

An open letter to the USFWS regarding the decision on whether to list the monarch

Hello everyone,

Today I have an unusual blog entry. I have written an open letter to the persons at the US Fish and Wildlife Service who are involved in the preparations of the status report of monarch butterflies in the US. As most readers should know by now, the agency recently announced they would be delaying the decision on whether to add monarchs to the Endangered Species list - under the "Threatened" category. As you might expect, I have been following this process closely, and feel the need to voice some concerns, although they are not what you might expect...


To whom it may concern at USFWS:

Your agency is now knee-deep in the process of deliberating on whether to add the monarch butterfly to the Endangered Species list in the United States. I know this must be an extremely difficult task, and I commend your people for their hard work on this thus far. With the recent news about how this process will now be extended until 2020, I can only surmise that this task must be even more onerous than expected. You are tasked with deciding whether an insect species with a continental range, with an undefined habitat, and one that annually crosses three countries, is in need of government protection. Add to this, the public notoriety and emotions that come with this insect, which can only make this job that much harder. I suspect that if you decide to list the monarch, there will be a lot of people upset, and if you decide not to list monarchs, a lot of people will be upset!

I would ask that you hear me out in this letter, and listen to my concerns over this entire process. As a scientist who has been studying monarchs for 20 years, I have a unique perspective on this issue; I am intimately familiar with practically every study of monarch butterflies dating back to the early 1900s. I also have personally written over 35 peer-reviewed studies on the monarch, and I am a peer-reviewer of much of the most recent work. But most importantly, I am an argent believer that all conservation decisions must be made using the best possible science. And to be blunt, I don't believe you are doing this now.

As some of your people know, I was a reviewer of an early draft of the status report - this is the report that attempts to bring together all of the science on the topic of monarch populations in North America. The report provides a summary of what is known about the population status, and it provides predictions about what will happen in the future to the monarch. It is a document that will be presented to the powers that be at the USFWS, and which will be used to guide the eventual decision. Your agency hand-selected the leading monarch scientists to meet to discuss the monarch population status, and to provide feedback on the written draft. While we all spent a great deal of time on this, and we all gave our varied opinions (sometimes too varied!), it was your people who ultimately drafted the report which will form the basis for the decision.

In my opinion, the initial draft of this report placed entirely too much emphasis on the long-term trends that have been occurring at the overwintering sites in Mexico. And, from the most recent announcement regarding the delay, I believe you are still doing this. In the announcement, a key statement reads, "The extension allows the Service to focus additional effort on obtaining the best available science, including data from the latest overwintering surveys." To read between the lines here, given that the monarch had a tremendously successful breeding season last year, which led to a sizable wintering colony, you plan to wait to see if this upward trend continues.

The report, and your current focus seems to lie mostly on the wintering phase of this insect. The overwintering colonies in Mexico have shown a 20+ year trend of decline - of this, no one is arguing. In fact, it was these declines that led to where we are now, where you are trying to decide if this species warrants protection! But, I believe you are misconstruing or misunderstanding what the overwintering colonies in Mexico represent. To be fair, this is a problem that is widespread among those who follow monarchs, and even among some of today's leading monarch scientists! And, to be fair, this is a problem that we scientists even created. But, there has been new research in the past 5 years that dramatically changes (or should change) our perception of these colonies. Mine certainly has.

We used to think these winter colonies represented the sum total of the "entire" eastern population. We had every reason to think this. When the colony location was first discovered 45 years ago by drawing lines on a map from tag recoveries, all of the lines pointed to central Mexico. When we make maps now of the migratory roosts that form in the fall flyway, they also seem to form a direct line going straight to Mexico. People routinely have their tagged monarchs found at these colonies. And when Lincoln Brower tried and failed to find additional mountaintops with colonies (using an ultralight aircraft), it seemed to solidify the view that these colonies were the only ones. Add to this, the fact that these colonies contain millions upon millions of monarchs, it seemed only right to assume that these contained the entire sum of the eastern population.

It was such a useful assumption too - here they all were, all clustered together in one place, and they even appeared to remain still for months so that we can simply record how big the colonies are each year, which would tell us how big "the population" is each year! This then led to the annual indices of the wintering colonies conducted by WWF Mexico (who are doing a fine job), and by plotting these over time we then realized that these colony sizes were diminishing!

But - and listen carefully here - just because monarchs return to this location year after year, does not mean that they all successfully reach it. There is a difference here that is critical. To illustrate this point, consider the Boston marathon - we know the route of travel of the marathoners, and we also know where the finish line is on a map, so in essence then, we know where all of the marathoners are trying to get to. But knowing the route and the destination does not mean that all of the participants make it to the finish line. A lot of them don't make it. This is the same for the monarchs in their fall migration - we know the route, and the eventual destination, and we know there are a heck of a lot of monarchs at the destination. But what we don't know, is how many did not arrive. I think it's a lot.

By my estimate, as many as 70% of the monarchs that begin the fall migration either die along the way or are rerouted elsewhere, so that the numbers that successfully reach these colonies probably only represent a small fraction of the real population - i.e. those at the end of the breeding season. I could cite some of the most recent research on this if need be. Other scientists could argue different numbers, but really, once you start piecing and adding together all of the different threats to monarchs along the fall migration, you eventually realize that there is no possible way that these colonies in central Mexico represent all of the eastern monarchs. In short, these colonies are not the population. They are what's left of the population. Or another way to think of them would be "the remnant survivors of a brutal long-distance migration in which few succeed, and who are destined to re-seed the next generation."

So, if the winter colony sizes are not a real measure of "population size", then why are we (or you) using these as a basis for deciding if this species is imperiled? If anything, the diminishing size of the colonies reflect an increasingly perilous migration, which recent work has shown is the case.

There is another reason to not focus solely on the wintering patterns - because it does not reflect the long-term patterns of the breeding monarch population - not at all.

I'm going to show you some graphs below. Each of these is as up-to-date as possible. First is a graph showing estimates of adult monarch abundance during the breeding season in the Midwestern US - i.e. the core breeding range. I'm sure you are familiar with these data - they are from the North American Butterfly Association, who has volunteers conduct censuses of adult butterflies each summer throughout the country. These data are taken from figures presented in Inamine et al 2016 and Agrawal 2018. Note that I provided the last point in 2018, which was an educated guess, based on reports from last year.

Next is a graph that shows the long-term trend in monarch reproduction in the Midwest, based on censuses from the Monarch Larva Monitoring Project. These data were recently shown in a newsletter sent out to all MLMP volunteers. The annual numbers shown are the peak summer egg density (per milkweed plant), averaged for all Midwestern sites each year.

I hope this is not the first time you are seeing these graphs. If it is, it would confirm my suspicion that you are focusing too much on the overwintering numbers. Regardless, you can see that both datasets show no evidence of a long-term decline, or anything that remotely indicates impending doom for this species!

I don't have time to get into why there is a long-term decline in overwintering numbers, but none in the summer, but suffice it to say that it looks like the monarchs are able to recoup or rebound each summer from the low initial abundance that they start with. In other words, it doesn't seem to matter that their overwintering colony size has gotten smaller - to the population at least.

Now, here is one last graph, and this one I know you haven't seen, because I just made it. As you know, the Journey North citizen science project has had people monitor and report when they see their first monarch of the season each spring and summer for over 20 years, and these data are archived at the Journey North website. From these data, it is possible to generate indices of annual "breeding range size" for the monarch, if you compile all of these records per year, which I have done below. Basically, I counted the number of unique latitude-longitude blocks that have sightings each year, for the entire eastern population (i.e. excluding western states). This is not a great index, but it should suffice for now.

This graph demonstrates that the breeding range of the eastern monarch population has not declined. It has been expanding over the past 2 decades (although this is confounded by the growth in Journey North volunteers). But in any case, this demonstrates the same point as the graphs above did - that despite the declines in overwintering colony size, the monarchs appear to be able to rebound each spring and summer to completely fill in their breeding range. Maybe it has something to do with the fact that each female monarch is capable of laying over 500 eggs...

In sum, I argue that focusing attention on the overwintering colony size as an index of the eastern population size is unwise. There are so many losses of monarchs during migration that the colonies probably only represent a small fraction of the population. Thus is does not seem appropriate to refer to these colonies as "the population". Not only that, the size of the colonies, and the long-term trends in their size, does not reflect the patterns of breeding monarch abundance, reproduction, or range size, seen in the summer. As such, while the diminishing winter colony sizes are indeed cause for concern (over the migration), they do not reflect the state of the monarch population, at least in the non-winter period.

I hope you will take my comments into consideration, and agree with my assertion that sound conservation decisions must be made using the best available science.

With great respect for your work,

Andy Davis, Phd


Direct link to this blog entry:

The science of monarch butterflies

A blog about monarchs, written by a monarch scientist, for people who love monarchs

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