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  • Andy Davis

What would happen if monarchs stopped migrating?

It's migration season for North American monarchs! It's that time of year when their thoughts of breeding subside and the urge to migrate takes over. For people that follow monarchs, it's the time of year when folks start watching for roosts, and begin staking out their favorite tagging sites. For me, this is my favorite part of the monarch life cycle, and one in which I have studied for 15+ years as a scientist. This is the phase that makes the eastern monarch such a celebrety among insects - the incredible, 3000-km journey they make to the overwintering sites in Mexico is a one-of-a-kind thing.

This blog post is about the future of monarchs in the eastern population. While there is a lively and ongoing scientific debate over the long-term population trend of this population (are they declining or not?), there is clear universal agreement amongst scientists that the future of the migration is uncertain. There are a slew of problems going against migrating monarchs - the overwintering sites are being continually degraded and logged (soon monarchs may not even recognize them!), climate change is predicted to extend the migratory journey (see the previous post on this), and last but not least, the rising prevalence of OE is rendering a lot of monarchs incapable of making the long flight (there is solid data on this point). So with all these obstacles in front of it, there may come a day when the migration will cease, or at least monarchs will no longer travel to the Mexico overwintering site. If that happens, what would happen to the eastern population? That's the question I'll be exploring today.

Before you read too much further, first let me point out that this story is not all doom and gloom - if the monarchs stopped migrating to Mexico, it would not mean the collapse of the population. Monarchs in eastern North America will live on, but it would definitely change their biology however.

First, let me point out that some eastern monarchs have already stopped migrating to Mexico in the fall, and this number will surely grow in years to come, thanks to climate change. There are a growing number of sightings of adult monarchs staying in warm coastal locations in the southern and southeastern United States during the winter months. Just look at the Journey North reports for winter sightings in any recent year, and you'll see lots. From viewing the notes on these reports it seems that these adults hang around butterfly gardens in backyards, or can be seen flying across beaches, or whatever. So far, no one has seen anything that resembles overwintering 'clusters' at these sites, i.e. with thousands of monarchs draping evergreen trees. So these sites aren't really the same as the overwintering sites in Mexico, but the point is, the monarchs at these southern sites can survive the winter there. In other words, these sightings prove that adult monarchs can 'overwinter' in the southern US.

If a few monarchs have already figured out that they don't need to go all the way to Mexico, how long will it be before the rest of them do? These places are only going to get warmer in the future. What I'm saying is that these winter sightings of coastal monarchs may represent the early stages of future overwintering sites, that may one day resemble the western overwintering sites in California. If you look at the western population, those monarchs overwinter along the coastline of California at any number of places - some places have one or two clusters of a few hundred monarchs, some have tens of thousands. There are dozens and dozens of sites along the California coastline. This may be what we will see in the future at coastal locations in the southern US. With global warming soon upon us, it's only a matter of time.

So what will this mean for eastern monarchs? Well for one it means not having to migrate as far! And this will surely result in changes to the way they look - they would get a lot smaller. I can say this with confidence because there is really good research showing how monarchs from long-distance migratory populations have larger wings than short-distance or non-migrants (multiple scientific studies report on this). For example, below is a picture of monarchs from the eastern United States (left), compared to monarchs from Puerto Rico (right), which don't migrate. There's a big difference in overall size.

Basically, the long-distance migration acts to keep the entire population big. The way it does this is by natural selection - each fall, only those monarchs with the biggest wings survive the journey, and small-winged monarchs usually don't make it (big wings are key for flying long distances). That means the big butterflies are the only ones that pass on their genes to the next generation in the spring. And this happens EVERY YEAR. So without this long-distance migration to weed out the small ones, the average wing size of the whole population would shrink over time. That's why non-migratory monarch populations have small wings.

The second thing that would happen is that rates of OE would skyrocket. Based on my wife's research (Sonia Altizer), populations of monarchs that don't migrate tend to have very high levels of OE. Basically, long-distance joureys tend to weed out heavily-infected butterflies (just like it weeds out small monarchs), leaving behind only the healthy monarchs, and again, it does this EVERY YEAR. So every year, the entire eastern population gets a 'spring cleaning', and most of the infected monarchs are removed from the population. But without this cleaning, rates of OE increase dramatically. That was the conclusion of a recent paper by her and her graduate Student, Dara Satterfield, that was published in a very good scientific journal (Proceeding of the Royal Society B). They surveyed OE levels in year-round breeding pockets of monarchs along the south coast of the US, and these populations (which were clustered around tropical milkweed) tended to have astronomical levels of OE - in some cases, 100% of monarchs were infected. These pockets can be considered microcosms of the entire eastern population, and they show what happens when monarchs don't migrate - without the migration, we'll likely see high levels of OE in the entire population.

So to wrap things up here, the loss of the migration would certainly be tragic, but it would not mean the end of monarchs in the east. But, without the long-distance migration to weed out the ne'er-do-wells, we would eventually be left with small, highly-infected monarchs. Don't get me wrong, we would still have monarchs - they would just be different.


The science of monarch butterflies

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

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