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  • Andy Davis

Update on eastern monarch breeding population

Hello monarch folks,

I have some exciting (and good) news to report this week. Some new preliminary data has just come in regarding the status of breeding monarchs in eastern North America, and I'll be sharing this, and talking about it, for this week's blog entry on MonarchScience.

A fellow monarch scientist (Leslie Ries, from Georgetown University in Maryland) forwarded me the new (preliminary) data last week - and it's very exciting. Leslie has been working with a variety of citizen science programs to analyze their butterfly monitoring data, including the North American Butterfly Association (NABA), and the Illinois Butterfly Monitoring Network (IBMN). Both of these programs send out people to monitor sites during the summer, and to count all the butterflies they see (not just monarchs). Both programs have been in operation for 20+ years, so they each have a lot of data.

(Credit for the photo above - taken by audreyjm529 on Flickr -

Before I show the new data, let me describe the monitoring programs, to get us all on the same page. The two programs have different monitoring methods, but both programs utilize a small army of volunteers, or citizen scientists, to gather the data. And, for each program, these volunteers need to be adept at identifying most, if not all, of the butterflies (and skippers) in their area (I can't even do this). For NABA, the volunteers go out once each summer (around the 4th of July) and they count all butterflies seen within a circle with a 15mile radius. The circles are usually centered around a town, or county, where the counter(s) live. This is a lot like how the birdwatchers do their annual Christmas Bird Counts across North America. There are not really any specifications on how to conduct the census, just as long as the count is made within the circle in one day. The counters are required to record how many people were observing, and for how many hours. You can actually go to the NABA website ( and view aerial maps of any of the count sites. I did so for one site in Iowa - North Liberty. I just picked this one at random. As you can see, it's a big circle, and the observers are supposed to count as many butterflies as possible in it during one day!

There are NABA monitoring sites throughout North America, but in the graph I'll show you below, Leslie has specifically focused on only those sites within the monarch core breeding range, that is the central United States.

The other program (Illinois) I'm less familiar with, but as I understand it, the observers have more of a standardized 'transect' that they walk to monitor adult butterflies (all species), AND they do this multiple times over the course of the summer (as opposed to just once). They too have a website - - but I'm not sure the individual sites can be viewed here. There are sites throughout the state of Illinois, which conveniently, is also in the core breeding range of the monarch.

So the fact that both of these programs give us data from the monarch core breeding range - in the Midwest- is huge. This is the intense, agricultural, region of the country. The other thing that is great about these programs is that the volunteers are not specifically looking for monarchs, but merely counting any butterfly they see. That means they have no specific bias for monarchs. And finally, Leslie has found that the annual estimates of monarch abundance from both programs are highly correlated, indicating that they both accurately track the abundance of adult monarchs. All told, it is safe to say that these data are simply ideal for estimating the size of the summer monarch breeding population.

So Leslie has been working with the monarch data from these programs for a few years now, and she even contributed a paper that analyzed those data in the special feature last fall in the Annals of the Entomological Society (the collection of 7 scientific studies on monarchs). At that time, she only had data up to the 2014 summer, which was a 'low year' for monarchs, and at the time there was much chatter about how low it was, along with the previous year (2013). Now, Leslie has obtained the summer data from 2015, and has added it to the long-term graph from both programs, which I'll show next. Prepare yourself for a surprise...

This graph, that I got from Leslie, shows the average number of (adult) monarchs per year from both programs, since 1994, and the most recent data points (2015) are circled in red. Both programs estimated nearly identical numbers for 2015 (there are two dots in the red circle). The good news - both programs estimated that the number of breeding monarchs last year was well above average (which is close to 4 on this graph).

Ready for some even-better news? This graph doesn't show any indication of a decline in breeding monarchs in the past 20 years. And, remember these sites are located in the Midwest, in the core monarch breeding range. Back when Leslie wrote the 2015 study (using data up to 2014), there was a small hint of a decline in her graphs of these data, especially in the later years, which is noticeable on the graph above. But now this last data point from 2015 pretty much quashes that argument. In fact, it is safe to say from this graph that the number of adult monarchs seen in 2015 is no different than it was at these same locations back in the mid-1990s.

OK, I know this information goes against everything you've been reading about in the last 5 years and especially in the past few weeks - that monarchs are in trouble, that there is no milkweed anywhere, and 'quasi-extinction' is imminent! I've been reading all of this too, and each time I read these news stories I shake my head, because I've been looking at graphs like the one above for years - graphs that show no declines in breeding, or migrating, monarchs. To be clear, I am well-aware of the clear declines in the size of the winter colonies - those data are statistically significant, and those declines are real. BUT (and this is a big but), how can the entire population of monarchs be declining if there is no corresponding decline in the breeding numbers? This is the question that is currently being debated right now among scientists. I've stated publicly before that the only logical explanation is that the monarchs must be having trouble REACHING Mexico during the migration. In other words, the problem isn't in the breeding population, as the graph above shows.

I know what you're thinking - if these data are so important, why haven't we seen these graphs splashed everywhere like we see with the Mexico data? Good question. My thinking is that the Mexico graph sells more newspapers, garners more clicks, and generates more emotion. The Mexico data also has the advantage of being in the public eye for much longer. The NABA and IBMN programs don't have spokespeople, or press releases, or any PR, really, and Leslie only began compiling their monarch data a few years ago.

Another reason is that some people believe these data are not indicative of the 'population'. The big argument against the NABA and IBMN data is that their surveys are conducted only in towns and other human-dwelling places - in other words, NOT in the middle of cornfields, where the big losses of milkweeds have occurred. According to them this makes these data invalid for assessing the breeding population. However, a counter-argument to that is the majority of survey sites in the Midwest are towns that are completely surrounded by agriculture. If you look closely at the aerial photo above you can see nothing but farm fields all around that one little town. So I would argue that if a town is completely surrounded by cornfields, then the number of monarchs in that town is indeed a good index of the number of monarchs in those nearby cornfields. It's a flying insect, after all.

That's pretty much all I have for this week. So before I go, let me point out something important here - these are not my data. I had nothing to do with their collection. They aren't even Leslie's - she's just the person who is compiling them. These data are taken directly from the two respective citizen science programs. So if there is anyone who doesn't want to believe it, and wants to throw stones, don't direct them at me - I'm just the messenger.

Remember - this is GOOD NEWS!


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