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  • Andy Davis

What does the size of the Mexican overwintering colonies mean for this summer's population size

Hello everyone,

It's now March and everyone is waiting to hear from the Mexican authorities about their annual estimate of the overwintering colony size. And, every year around this time, folks begin thinking about their butterfly gardens and how many monarchs they will see this summer. Today, I'm going to tell you about some of the recent science that has been done that tries to connect these two dots (winter and summer) for the monarch population.

I'm going to tell you about three different studies in this post, because they all have something to tell us on this issue. Each of them was published in the last 5 years, so this is as up-to-date as you can get on this issue.

The fact that there are a number of studies to talk about here should tell you something about this issue. For a long time, scientists have been questioning how the numbers of monarchs at one stage of their life cycle affect the numbers seen at other stages. It's really only been in the last few years that we have actually been able to address this question statistically, because it requires a lot of data - like 20 years or so. Given that we now have this for most parts of the monarch life cycle in N. America, this question, or variations of it, has now been addressed by a number of research groups.

The first paper I'll discuss is one that I've blogged about before - the conservation "game-changer" study, by Inamine, Ellner, Springer and Agrawal, in 2016 (link to the blog here). This paper was a very comprehensive (statistical) look at the links between monarch life stages in eastern N. America, and it used a bunch of different long-term, citizen science datasets. The authors were specifically looking to see if the long-term declines at the overwintering site were mirrored by declines at the summer breeding grounds. The answer was a resounding no. Using 22 years of monarch abundance data from the NABA monitoring program (with sites across N. America), they found no long-term decline in the abundance of adult monarchs in the summer. They concluded that the monarchs appear to have enough milkweed to sustain their summer breeding population. They also concluded that the declines in Mexico must not be due to a loss of breeding monarchs, but more likely due to problems during migration (i.e. in the journey to Mexico).

There were a number of analyses conducted in that paper, which I won't get into here. But there was one in particular that is important for this discussion - the authors tested to see if the annual size of the overwintering colonies predicted how many monarchs are counted the next summer at NABA monitoring sites, based on the 22 years of data they had (1993-2014). They ran this test twice, once using summer counts from the Midwest, and once with data from the Northeast. Their tests both showed no statistical relationship between the size of the wintering colony and the number of monarchs seen the following summer, in either the Midwest or Northeast. In other words, if the wintering colony was small one year, that did not mean the summer counts would be low, and vice versa, if the winter colony happened to be large one year, it did not necessarily lead to more monarchs later that summer. So based on these results, it is safe to say that the size of the overwintering colony does not matter to the size of the subsequent summer breeding population. Remember, this is based on analyses of 22 years of data, collected by officials at the WWF (Mexico), as well as thousands of citizen scientist volunteers (in the US). These data are not wrong. And keep in mind, no one (especially me) is saying that the overwintering colonies are not important, only that their size is not important for predicting summer monarch abundance.

So you're thinking, if the Mexican colony size doesn't matter, then what does matter to the summer population? I can tell you the answer in a word (or two) - the spring migration, is what really matters. There have been two studies in the past 5 years that have looked into the importance of the spring migration for summer monarchs, and they both came from research conducted out of Elise Zipkin's lab at Michigan State University - link to her lab. Both of these studies have more or less looked at the same thing. A link to one from 2012 is here, and the other from 2017 is here. In both studies, the authors have used long-term, citizen science data to examine (statistically) how the environmental conditions monarchs experience during the spring migration affect how many monarchs are seen during the summer. In both studies, they specifically looked at how annual differences in spring temperatures in Texas, and spring precipitation in Texas, affect how many summer monarchs are then seen in either Ohio (the 2012 study) or Illinois (2017 study). In both studies they used a boatload of citizen science data on summer monarch numbers in either state. So again, these data are strong, and the conclusions are not wrong.

There were a lot of analyses in these papers that are less important to this discussion, so let me cut to the chase with these studies. In both projects the authors found that the environmental conditions experienced during the spring migration was the biggest predictor of how many monarchs are seen in the summer in either state, even more so than the summer environmental conditions in these states. Both studies concluded that the years with the highest numbers of summer monarchs tended to be years when Texas was wetter in the spring (i.e. high precipitation). I guess it doesn't take a rocket scientist to figure out what happens when it rains a lot - you get more plant growth. In other words, wetter spring conditions means more milkweeds for oviposition, plus more nectar sources, for keeping the adults' energy levels high and allowing them to travel farther before they die.

Interestingly, in the 2017 study, the authors also tried to find a link between the size of the overwintering colony and the subsequent summer monarch counts (using a different dataset for summer counts than did Inamine et al 2016). From their analyses of 19 years of data, they found no relationship between the overwintering colony size and the summer monarch numbers in Illinois - just like what Inamine et al found. In other words, they too found that the size of the overwintering colonies does not predict how many monarchs were counted in Illinois in the subsequent summer. So here we have the same conclusion being reached by two completely different groups of researchers, using completely different datasets.

The bottom line is that the size of the Mexican colonies - whether it is large or small this year, does not predict how many monarchs we will see this summer. What really matters is the success of the spring migration.

Speaking of this, if there was ever an event that highlighted what I just said, it is the spring migration from last year! In the spring of 2017, the returning monarchs from Mexico had what I would consider the most successful return migration they have ever had in the past 20 years. I blogged about it last year - link to the blog. Remember, the Mexican overwintering colonies have continued to shrink (no one is debating this), but yet for some reason, last year the returning adults had a stellar recolonization. They were laying eggs like crazy, they traveled much farther than normal, and their spring migration range was huge - all hallmarks of a successful spring recolonization. And then last summer, did anyone else notice how many monarchs there were? And all of this, despite a small overwintering colony size.

Lastly, and just to prove this point, I'm pasting below a couple of spring migration maps from Journey North. One is from a year where there weren't that many monarchs in the summer (2013), and last year, when there were many. Notice all of the early dots in Texas and the south, and note how far north they were in 2017 compared to 2013.

OK, I think that about covers this issue, so I'll leave it here for now. Remember if you have comments on this or other posts at MonarchScience, you can leave them on the Facebook site where each blog article is posted.


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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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