• Andy Davis

The quietly-published paper on climate change and monarchs

This posting is about a paper that came out earlier this year that few may have seen, but that deserves a lot of our attention. In the journal PLoS One, Nate Lemoine (the sole author) published a study describing how the breeding range of monarchs will shift in the future because of warming climate. From a google search, it looks like Dr. Lemoine is now a post-doctoral researcher at Colorado State University but he did the study as a graduate student at Florida International University.

There was no fanfare around this study. There were no reader comments on the paper at the journal website. The author was not someone known to monarch people, and he apparently did all of his work on his own. By all accounts, this paper slipped into the realm of publication without many people knowing about it. One person did - Elizabeth Howard, since the author used sightings from Journey North (with permission from Elizabeth), but beyond that, this was a fairly low-key study. However, the results of this work are anything but low-key. In fact, they have considerable implications for the future of monarchs east of the Rockies.

In reading the study, I could tell the author was very knowledgeable about the methods used (which I won't get into here, but involved a lot of computer models). I also could tell that he was very thorough in his analyses and in describing the results. So all in all this was a very good study, from my read.

The main question he addressed was how will the spring and summer breeding range of monarchs change in the next 50 years (not sure of the exact time frame), based on the projected distribution of milkweeds. To examine how milkweed distribution will shift, he predicted their likely occurrence in North America based on their known temperature limits, and the predicted temperatures and other climate variables at all latitudes. Basically, he plotted where milkweeds will be based on what temperatures they grow best in. He also double-checked whether this approach works by checking if his models predicted where milkweeds currently are in N. America, and it looked like it worked very well. Incidentally, he examined a bunch of milkweed species, and presented results for each independently, plus all together.

After he figured out the milkweed changes, he then predicted where monarchs will be (monarchs are where milkweeds are), and the figure below shows the main results of that bit. I think this was the most significant figure from the paper (there were many), and the one that is most easily-interpreted. By the way, he made two predictions for the future milkweed and monarch ranges - one assuming a moderate change in climate and one assuming a more dire climte change. From the paper: "The moderate scenario assumes that greenhouse gas emissions rise slowly until 2050 and decline thereafter, resulting in a moderate 1°–3° C increase in mean global atmospheric temperatures." The severe climate scenario "assumes greenhouse gas emissions rise steadily through 2100, resulting in a more severe 2°–6° C increase in mean global atmospheric temperatures."

To understand the maps above you need to look close at the months listed at the top of each. Basically, these maps show climate change is going to shift the adult monarch population more northward than they would have been during each month. And, the shift is even greater with the severe climate change case.

I think the most significant parts of this figure are the maps for June - at the bottom. June is pretty much the end of the recolonization phase of monarchs and the beginning of the summer breeding phase. So these June maps essentially show where the summer range for monarchs will be in the future. Guess what? They'll all be in Canada (my homeland - yay), and they won't be in the American Midwest during the summer months. That means Canada is going to play an increasingly significant role in safeguarding the breeding population. It also means people who live in the United States will see fewer monarchs in the summer - not because they are dissapearing, but because the monarchs simply moved northward.

Here's the really scary thing that is only barely touched on in this paper - When (not if) the breeding range shifts northward that means the late-summer generation (the migratory generation) will need to travel farther each fall to reach the Mexico overwintering sites, and by extension, that means fewer and fewer monarchs will reach these sites. It would be like moving the finish line further back each year in the Boston Marathon and watching how the number of finishers goes down each year. Travelling farther means greater risks of storms, predation, road mortality, etc.

The bottom line from this paper is that climate change is going to change where monarchs breed in the summer (Canada), and this is going to have a huge impact on the overwintering numbers in Mexico. This is definitely a paper that folks should be aware of.

The full citation of the paper is here:

Lemoine NP (2015) Climate Change May Alter Breeding Ground Distributions of Eastern Migratory Monarchs (Danaus plexippus) via Range Expansion of Asclepias Host Plants. PLoS ONE 10(2): e0118614. doi:10.1371/journal.pone.0118614

The science of monarch butterflies

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