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  • Andy Davis

Is road mortality the real cause of the decline in Mexico?

Greetings everyone,

Get ready for some scary stuff this week. This week, I'm going to rehash the subject of roads and monarchs and pull together all of the evidence that I know of, including in the published and unpublished literature, and I'll also tell you about some unpublished work I did looking into the issue of road-related mortality last fall. As I alluded to in the title, this mini-investigation led to some very interesting findings that may make you question what you think you know about the decline in overwintering monarchs...

OK, here we go - roads and monarchs - I've blogged about this issue before, so you might want to refresh your memory before reading further. If not, here is a nutshell version of the prior blogs. In one case (link here to the blog entry), I delved into an older publication in the Journal of the Lepidopterists Society by McKinna and colleagues. That paper investigated how many monarchs were killed on roadways in Illinois per week during one year. Their estimate - 500,000, was staggering. In my blog I then took this estimate and extrapolated it to the entire fall flyway, for one full southern migration, and came up with 25 million. This is a rough estimate of how many monarchs die from being hit by cars in a typical migration within the United States.

The second paper came out more recently, and was even more scary. This one (link here to go to the blog), by James H. Baxter-Gilbert and colleagues, in the Journal of Insect Conservation, examined how many butterflies and moths (all species) are killed each year in one tiny stretch of road in central Canada. Their estimate was 10 individuals/km/day. And they extrapolated their estimate to all of North America's roads to come up with a staggering number of 9.3 billion - that's how many butterflies and moths (all species) they estimate die on roads each summer and fall.

There was another piece of relevant evidence pertaining to the monarch migration that came out last year, and that was forwarded to me by Elizabeth Howard. This was not a scientific study, per se, but more of a observational report, and it was pretty scary too. It was written by a group of folks in the migration tracking project called Programa Correo Real- Mariposa Monarca, in Mexico. I'm not familiar with this group, but it looks like they have a facebook page (in Spanish) - link. Anyway, these folks wrote a report last fall about some observations on monarch mortality (among other things) on some roadways in Mexico, and included a number of pictures as well, of which I've copied the most pertinent here. In one specific part of the report, the observer, Rocio Trevino, said she was driving near Monterrey, MX at the end of October and witnessed monarchs being killed by cars and trucks (she actually saw them being hit). She also walked along the side of the road and counted dead monarchs. She estimated there were about 10 every 4 meters at one point (or 2.5 monarchs per meter). In one gutter in particular, which was about 20 meters long (pictured below), she picked up 115 dead monarchs (74 males, 41 females). This is about 5.7 dead monarchs per meter. From what I can tell from reading the report, she made similar observations on a number of other roadways.

So this report was pretty scary. To demonstrate just how scary, let's do some quick math on these numbers to extrapolate it to the entire Mexican flyway. And keep in mind this is very hand-wavy, back-of-the-envelope stuff. The high 'rate' of mortality from this report (5.7 monarchs/meter of road) would translate to 5700 dead monarchs per km. There are about 377,000 km of roads in all of Mexico (based on a simple web search I did). From looking at a map of Mexico, I would guess that only a quarter of these roads are within the monarch flyway in Mexico, so that leaves about 94,250 km of roads within the flyway. By extrapolation then from the 5.7 number, the number of monarchs killed on roads in Mexico is about 537,225, or about half a million. Now recall that this observation was based on only a single day, so if we want to know how many monarchs are killed on Mexico roads during an entire fall migration through that region (we'll assume it's about 1 month), then we multiply this number by 30 to get 16,116,750 - about 16 million. NOTE - in a previous version of this blog post I had made an error in this calculation, but I've fixed that now.

Now this estimate for Mexico could be off by a few million, but no matter how you slice it, it seems clear that the amount of road mortality in Mexico is pretty substantial.

So let's recap - we have a project done in Canada that shows millions of butterflies are killed on roadways each year, we have a project from the United States that shows millions are killed each year here, and we have evidence from Mexico that shows millions are killed each year there too. Collectively, it is easily possible that over the span of the entire fall migration, the death toll from roads on the monarch migration is anywhere from 30-50 million. When you consider that the overwintering colonies in Mexico contain about 50-100 million monarchs in recent years, it really hits home just how massive this problem is.

OK, are you scared yet? Wait til you see what's next - So, I got to thinking a while back, if cars are killing such a huge amount of monarchs each year, would the number of cars on the road be related to the number of monarchs that reach Mexico each year? In other words, could the decline of monarchs in Mexico be caused by an increase in vehicle traffic on our roads? To answer this question I tracked down information on the number of registered vehicles in the United States per year (I didn't look at the other countries). I found these data online here at the US dept. of transportation. Unfortunately, this is a count of all vehicles in all states, and I couldn't break it down by flyway region. But we'll assume that the trends in vehicle abundance is the same in all states. Sure enough, it shows that the number of vehicles on our roads has steadily increased in the last 20 years. Meanwhile, as we know, the size of the monarch overwintering colonies has declined in that time frame.

So when you put the two datasets together, you get the graph below:

Crazy, right? From a statistical standpoint, I can tell you that these variables are significantly correlated (p<0.005). In other words, there is a statistical association between these variables, so that as one goes up (vehicles), the other goes down (monarchs).

From looking at this graph, you may also be thinking this looks familiar. It's because this graph strongly resembles the one that supposedly 'linked' the monarch decline with the rise in roundup-ready crops - in that case, the crops have been increasing at the same time as the monarchs were decreasing in the last 20 years, and in that case as well, there was a significant statistical association, and the graph looked just like this one. So which one is right? Well that's a good question. Correlations are frequently found in science, but most scientists don't put much faith in them, because two variables (that sometimes have nothing to do with each other) can be correlated by random chance. For example, did you know that the number of swimming pool deaths in the US each year is correlated with the number of movies Nicolas Cage has been in? I kid you not. Look it up on the internet. The point is, correlation does not always mean causation.

With that said, I think this vehicle graph does have something to say. At the very least, it demonstrates that the number of registered vehicles on the road has grown in the US, and likely in Canada and Mexico as well. And, it is abundantly clear from the available evidence I laid out above, that this can only be bad news for monarchs.

Bottom line - roads and monarchs don't mix. In my opinion we should be doing everything possible to make roads and roadsides as unappealing as possible for monarchs and other insects.


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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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