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  • Andy Davis

Why I signed the petition to list monarchs, even though the evidence for declines is shaky

Hi blog readers,

That's right, the monarch declines are not a sure thing. I'll say that up front, just to be clear of my stance, in case anyone doesn't know me. This stance is not taken lightly, it's based on careful examination of a lot of data (so I'm not just a crackpot). What IS in decline, is the migration. In this, there is virtually no disagreement among monarch scientists.

The reason I'm spelling this out first is that it has a lot to do with this week's post. As most folks are aware, the US Fish and Wildlife service is currently considering a petition to have monarch butterflies listed as 'threatened' on the endangered species list (link here to press release). This is a special category that is reserved for species at risk of declines, though not considered 'endangered'. This petition was filed last year by the Center for Biological Diversity, the Center for Food Safety, the Xerces Society for Invertebrate Conservation and Dr. Lincoln Brower. It was largely based on the declines happening at the overwintering sites in Mexico in the past 20 years. I do not dispute these declines - they are real and statistically significant. HOWEVER, we also have data on the long-term abundance of monarchs in the breeding range (from multiple sources), and from their fall migration (multiple sources), and these do not show declines in the last 20 years. Most of these data sets were recently published in the special issue on monarchs in the Annals of the Entomological Society of America, which I coordinated (see previous post). These data were not yet available when the petition was first filed. So we now have one graph showing declines and several showing no declines - how do we reconcile these discrepencies?

I can. The way I like to think of it is that the migration is like a grand marathon that is extremely hard. At the start of the race there are all of the participants, but there are a lot fewer participants who cross the finish line. For monarchs, the overwintering sites are the 'finish line' of this marathon, and the breeding range is the 'starting line'. So if the number of 'starting participants' in this marathon has not diminished in 20 years, but the number of 'finishers' has, that tells me the problem lies somewhere within the race itself. In other words, if you wanted to know how many people participated in a marathon each year, would you count the number of people who cross the finish line? No, you would count the starting participants. So if the numbers of finishers is declining, then there is something that is making the marathon (migration) harder to finish in recent years. So this is why I say that monarchs aren't 'declining,' but their migration is. In fact, in my opinion, this is the only logical interpretation of these diverging graphs, and the only explanation that makes sense.

Given this, I would argue that we should be focusing our collective efforts on identifying and reducing anything that is hindering the migration. And guess what, we already know of several things that are preventing monarchs from reaching the finish line. There is one that folks may already know about but I don't think are giving it as much attention as it deserves - Ophryocystis elektroscirrha (OE). This is the protozoan parasite that infects monarchs throughout their range - in all of the known populations around the world. In other words, it is a naturally-ocurring parasite, and not something that we humans introduced. BUT, we humans are repsonsible for increasing its spread, which I'll explain later.

From a scientific perspective, this parasite is pretty much the ONE thing that we know for sure removes monarchs from the migratory pool - monarchs that are heavily infected with it die before they make it to Mexico. We know this from a variety of scientific studies. One of the most significant ones came from work done about 10 years ago, using that super-cool monarch treadmill thingy, that i mentioned last week. A grad student used that gizmo to test if infected monarchs fly less well than uninfected ones, and guess what? They do (see Bradley and Altizer, 2005 in Ecology Letters). There are other sources of data I could mention too, like a paper a few years ago that showed how the number of infected monarchs in the migration declines as they move southward in the flyway - this means that infected monarchs die along the migration trail, leaving fewer and fewer infected ones in the migratory pool (Bartel et al. 2012). There are a couple more sources that I could mention too, but you get the point - from a scientific standpoint we know a lot about this one issue, so we can say with a lot of confidence that OE is a serious threat to migratory monarchs.

So that being said, you might be wondering, if migratory monarchs are struggling to reach Mexico, and if OE is a factor, then is OE increasing? I can tell you that it is, although we do not have published data on this. Sonia Altizer's Monarch Health project tracks these infections, plus she has historical data going back to the 1970s. The infection numbers bounce around from year to year, but in general, it looks like in the past ten years, the rates of infected monarchs have doubled and sometimes tripled over their historical prevalence, which was about 0.5%. The past year (2014) was an exception - see the latest newsletter from MonarchHealth for more info - link here.

Getting back to the petition - if I don't think monarchs are declining, why did I sign it? It was actually because of a small clause that was buried in the fine print - that stated if the listing was enacted, it would become illegal for people to rear more than 100 monarchs. Actually, initially it said only 10 monarchs, but then they changed the amount in response to some intense public outcry (shame on them). I would have preferred the 10, and here's why - if this petition was successful and monarchs were eventually listed, this one clause would essentially eliminate all mass-rearing of monarchs. This is something I've blogged about before, and which has been publicly condemned by other monarch scientists (see link to the position statement). Most scientists, including me, believe that this activity is extremely detrimental to monarch populations, as it fosters the spread of OE. This parasite quickly builds up in captive monarchs, then if those infected butterflies are released, they can spread it in the wild monarchs. And if OE spreads through the local monarch population (around the release site, for example), then none of those infected monarchs has a chance of reaching Mexico.

Unlike other conservation measures people are taking, such as planting milkweed, preserving habitats, etc. (that all take time to do), this one clause in the petition would have pretty much eliminated, overnight, one of the biggest threats to the monarch migration that we know of. That's why I signed the petition. Not because monarchs are declining (which is debatable), but because of this mass-rearing clause, and it's implications for OE. In fact, this one parasite, I believe, is the biggest conservation issue that no one is talking about today.

That's all for this week.

The science of monarch butterflies

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

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