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  • Andy Davis

Hurricane Patricia - what will it do to the migration?

I had several interesting stories lined up for this week and the next, but it seems that the big news right now is all about this new hurricane that is going to smash into Mexico any day now.

From scanning the dplex emails, and from talking with monarch folks, I know there is a lot of concern over what this storm will do to the monarchs, since it appears to be heading in their direction. As of now, the leading edge of the monarch fall migration is in northern Mexico, based on the Journey North roost maps, so it looks like the migrating monarchs are going to run right into the storm - bad timing all around.

So what is this going to do to the migration? I can't say for certain (not sure who can), but I do have some thoughts on this based on what I know about the science relating to weather and migration in general.

Most importantly, I would urge some calm for monarch folks. Yes, this is going to be nasty, and it will result in the death of a lot of monarchs, BUT, we also know that storms and incliment weather are a normal part of the migratory journey. In other words, monarchs have been dealing with rough weather during their fall migrations for thousands of years (rough guess) - ever since the migration evolved. Storms, even hurricanes, are not new to them!

To be more specific on this train of thought, keep in mind there is a lot of natural mortality associated with the migration already - and this is normal. I've already posted about the number of monarchs that die on roadways (millions), but in addition to that, lots of migratory monarchs die while crossing large water bodies, they die from starvation, from disease, etc. In other words, a heck of a lot of them die each fall, and this is something that has gone on for as long as the migration has been around (though the road mortality is a new thing in the last 50 years). So while it is a little morbid, the bottom line is that it's normal for some monarchs to die in the migration. This actually serves a purpose - it keeps the overall population healthy, by weeding out the weaklings every year. Those that successfully survive the 3000+ mile journey will be the strongest fliers, and they will pass on those strong genes to their offspring. This is what we call a natural selection event.

Speaking of this, I'm reminded of a scientific paper I did a few years ago with Elizabeth Howard, in which we documented a weather-related mortality event that ocurred during fall migration one year at a beach on Lake Michigan. A keen observer noticed this and she took pictures and made detailed notes of the event. She actually declined to be a coauthor on the paper. The paper was in the Journal of Research on the Lepidoptera, and is available to read here - link. Essentially, there was a fierce wind storm that lasted a few days in this region (in 2011), and this lady saw that some monarchs had died and were washing up on the beach. However, the timing of this was very important - this all happened in mid-October, in Michagan. By this time, the bulk of the migration had long since passed, and these monarchs that died were clearly stragglers. So that wind storm essentially cleaned out those stragglers and removed them from the overall population - so no straggler genes ended up being passed on there.

Getting back to the hurricane - the other thing to keep in mind is that those monarchs that do survive this event will be the ones who are smart enough to hunker down and wait it out (this goes for not just the monarchs but the local people too!). During rough weather, monarchs tend to stay put, grabbing onto tree branches, bushes, etc, and they just stay there for the day, or however long it takes before fair weather returns. There actually is research demonstrating this, at least. So in the end what we might see from this hurricane is simply a delayed arrival by monarchs to the overwintering grounds.

Now these are just some thoughts, which may or may not hold water. It's difficult to be more concrete here, since this (hurricanes, anyway) is something that has not been studied before. From my knowledge of the scientific literature around monarchs, I can say this for certain - there is no science regarding the effects of hurricanes on monarchs. So what we see in the next few days or weeks will be new to science!


The science of monarch butterflies

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

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