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  • Andy Davis

Does tourism negatively impact the monarch winter colonies in Mexico? A study attempted to find out

Hello everyone,

I hope everyone is hanging in there given all that is going on right now. This has certainly been a trying year, in many ways!

As we begin to close out this year, I'm going to take a look back at a largely passed-over study that came out this summer. I had seen and read this paper at the time, and I don't recall ever seeing any discussion about it. Given it's implications, this seems unfortunate. However, this is where this blog comes in! So today I'll give you a breakdown of this new study, plus provide some critical commentary of my own (spoiler - there were some limitations).

This was a study published in a scientific journal called Ecological Indicators, which is a journal that focuses on studies that use wildlife or plant species as indicators of environmental quality. Here is a link to the abstract, but this paper is not public. I see that it was written by a collection of international researchers, coming from France, Mexico and the U.S. I also see that Eduardo Rendon-Salinas is an author; he is the person in charge of the WWF-Mexico's monarch program. That tells me that this study was fully sanctioned by WWF-Mexico.

The goal of this project was to determine if tourism and forest cover impact the physiological health and condition of monarchs at their Mexican overwintering colonies. This is indeed a laudable goal - every year it seems like there are more and more tourists visiting these sites (providing much-needed revenues for the local people), but, I have never seen any research on whether this is harmful to the monarchs themselves. Regarding the forest part, there has been a fair amount of research on this issue in the past, but, not really in this way (i.e. physiological effects). The closest thing I can think of is the prior research from Lincoln Brower and Ernest Williams, which has shown how the degree of forest cover around the colonies is important for proper thermoregulation of monarchs. Lincoln always used to say "the forest is like a blanket for the monarchs, and a blanket shouldn't have any holes in it."

If you think about what happens at these winter colonies, the monarchs form clusters on trees at these mountaintop sites, where they spend most of their days simply clinging to the branches and unmoving. In other words, they spend their days conserving their energy. They don't need to eat while they are there, since the cool temperatures help to keep their metabolism low, and also because they can exist by utilizing their stored fat reserves. On the trip down to Mexico, the monarchs have built up considerable reserves of lipids, which they use as an energy source throughout the winter. If they burn through this lipid supply before the winter is done, then that is bad news.

The authors in the paper describe how both tourism and forest cover loss can cause monarchs to be "perturbed" throughout the winter (their words). For the first issue, I think they are implying that the activity of tourists can cause monarchs to take flight, and thereby burn energy. I don't know how much this happens, but from reading the paper I get an image of a whole bunch of rowdy kids running around the colony and scaring the monarchs into flying away. To be fair, I don't know how much this happens, and, the last time I was there, the tourists were prevented from getting too close to the trees with monarchs. But if you think about it, we actually don't know how close is too close for monarchs! There has been no research on this. For all we know, maybe the clustering monarchs can even see the people from a distance and the very presence of all of those moving people is a stressor to them.

As to the forest cover loss, I think here they are implying that reductions in tree density, or "holes in the blanket" can lead to winds and exposure to storms, etc. That much seems reasonable.

OK, so that was the rationale for conducting this investigation. Now let me talk about what the researchers did. This study involved essentially collecting live monarchs from five different winter colonies in Mexico, and then conducting some interesting laboratory analyses on the physiology of those monarchs. Importantly, two of the five colonies were ones where tourists visit, and the other three have little to no tourists at all. Below is a map from the study which shows the colonies sampled. It sounds like they collected the monarchs in February, so that they had been at these colonies for most of the winter by then.

The butterfly symbols in this map indicate the colonies sampled. They collected monarchs from Sierra Chincua, and El Rosario, which are tourist colonies, and then three others which do not get tourists, or very little visitors: San Pablo Malacatepec and San Juan Xoconusco in Cerro Pelon, and San Antonio Albarranes in the Palomas area. They report that Chincua gets 50-60 thousand visitors per year, and El Rosario gets 160,000 visitors per year - that's insane!

They state that they collected 50 individuals of each sex per colony. This was done by netting a random cluster of monarchs in each colony. It's a little unclear as to what the final sample size was though, because they appeared to run different tests for different subsets of these initial collections.

They brought all of these butterflies back to their lab (I think somewhere in Mexico), where they measured the butterflies and performed a series of different physiological assays. These were a measurement of protein content in the specimens, two measurements of immune status, measures of oxidative stress (which is like an assessment of cellular tissue damage), and an assessment of pathogen resistance. I don't want to get too much into the weeds here by describing all of these, since some are not my area of expertise, and also because a couple of the tests were inconclusive.

Before I get into the assays, let me first point out that they did measure the wing size of all of the monarchs, and there was no statistical difference in overall size across the five different colonies. They stated the averages for each colonies in the text, and for ease of presentation, I made a table below, showing these averages. I thought this would be helpful for people to see. Also, I think it is of interest to see just how big these monarchs all were! Recall that about five years ago, I had written a blog describing the importance of wing size to migration success (link here), and that the forewing length of successful migrants needs to be at least 50mm or bigger. Well, this table validates those statements!

OK, so for the assays, the measurement of protein content was interesting. Protein content in animals is a universal sign of health - too little protein is usually an indication of anemia or sickness. I won't get into the nitty-gritty, but this assay was done using a sophisticated procedure involving grinding up the specimens (after euthanasia of course) into powder, then combining this into a mixture of different reagents, adding saline, and there even is a neat benchtop machine involved. In the end, the machine reads out how much protein was in the original specimen (i.e. the whole monarch). Anyway, they performed this assay on the monarchs in the tourist colonies, and on those from the non-tourist colonies. Below is a graph from the paper showing the average protein content of monarchs in each colony. Remember, more is better.

Note that in this graph, the two tourist colonies are in blue, and the non-tourist colonies are in grey. Also, the degree of forest cover around each colony is given too. From this graph I think the take-away is that the monarchs with the best protein content are in the Albarranes colony, which has the highest forest cover (78%) and has no tourists!

Now, let me talk about the pathogen assay, since this was a fairly easily-explained test. One way to measure health in animals is to assess how well they can fight off a rudimentary or benign pathogen. In insects, this is routinely done by injecting a known dose of a pathogen into the animal, then waiting to see if it survives. Sometimes this is done using a fungal pathogen, or maybe a virus, or bacteria. I know this sounds gruesome but it is a very powerful and easily interpretable test. In this study, it looks like the researchers injected a standardized dose of staph bacteria into a subset of the monarchs, then waited 24 hours to determine if they survived. I'll paste a graph below from their paper, which shows the % of monarchs that died from the bacteria.

The two tourist colonies are again on the left. It looks like the monarchs from Chincua (labelled State Property on this graph) had vary high mortality (40% died), and this is the colony with the lowest forest cover (43.7%), and, it is visited by tourists. I note too that hardly any of the monarchs from the three non-tourist colonies died from this test. In two of those there were zero deaths!

I'm purposely not going to get into the other assays here, because frankly, I don't think they were conclusive. Either there was no difference across any of the colonies, or, I'm not sure if the authors interpreted the numbers appropriately. So in the end, I don't think these other assays amounted to much. The ones I did point out here were the most decisive, in my opinion.

Speaking of my opinion, I'd like to offer my impressions of this study next. Let me start by pointing out that I still believe this study is a great step in the right direction. We (scientists) really need to be getting into this issue - how tourism and other stressors affect monarchs. If our goal is to protect the monarchs from harm, then we really should be working to identify what harms monarchs in the first place! And, I really like the approach here of assessing the physiological condition of monarchs. Physiological measurements can be a useful way of gauging the overall health of animal populations, and in ways that are unseen! Consider that the simplistic measurement of wing size here revealed that the monarchs were all pretty much the same size across all colonies. But that doesn't mean they were all equally healthy! In fact, the monarchs from Chincua appear to be the worse off - their forest cover is very low, and they appeared to be incapable of surviving a very ordinary pathogen. Meanwhile, those in the Albarranes colony survived that same pathogen very well, and they had a very high protein content; that colony has a high forest cover and no tourists.

I think the take-away message from these findings is that high-tourism and low forest cover combine to negatively impact monarchs at winter colonies. This is a pretty big message!

But hold, on, this study did have some limitations for sure. The biggest one is the sample size. While the researchers measured a bunch of monarchs from each colony, they really only measured 5 colonies. So the real sample size (in statistical terms) was n=5. I would have liked to see them measure monarchs from multiple tourist colonies, and multiple non-tourist colonies, and then do the same for forest cover. If all of the findings remain the same for each of the tourist or non-tourist colonies, then that would strengthen their argument here - that the tourism is negatively affecting the monarchs.

The other problem here is that we can't be sure if the findings from any one assay are because of tourism, or from the forest cover loss, or is it a little of both. Both are "confounded" to use statistical terms again. But, I have to admit that the monarchs from Chincua don't look good, while those from Albarranes do look good.

OK, so this all brings me to something else, which is kind of like the elephant in the room - the COVID-19 pandemic has dramatically curtailed the tourism activities at these colonies this winter! Might this be a good thing (for monarchs, anyway, not for the locals)? Ellen Sharp, who helps run JM's Butterfly B&B in Macheros, near one of the colonies (, tells me that Chincua and El Rosario are open for business this year, while Cerro Pelon will remain closed all winter to visitors. But, all open sanctuaries will operate at 50% capacity. With this, combined with the reductions in all travel this year, especially in international travel, I suspect the overall number of visitor to this region will be low this winter.

Incidentally, I hope the authors of this paper are reading this, because I would tell them, if you really want to study the effects of tourism on monarch condition, you should study the monarchs this winter (without tourists), and see if their condition improves. If it does, then we would know for sure if it is causing harm. In fact, I'm beginning to see other research projects being conducted this year (on other critters) that are along the same lines. It seems that this pandemic has provided an unexpected opportunity for science.

So as not to end on a low note here, I should put in a plug for Ellen's virtual colony tour program ( This seems like a very good idea, in the face of all that is going on right now with the pandemic, and it seems like it would benefit the monarchs too, by providing some funding that could replace a little of the lost tourism revenue. I may join this myself.

That's all for now.


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