Hello blog readers,
This post is about a paper that came out in 2015 that few people may have seen, but that deserves a lot of our attention in the monarch world. In the journal PLoS One, Nate Lemoine (the sole author) quietly 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. Here is a link to the (open access) paper - link, and a screenshot of the title is below.
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 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. The idea here is that when the climate warms, the more northern regions (mostly in Canada) should become more and more suited for milkweed, and so the range of milkweed should expand northward over time. This same situation is happening with a lot of other species around the world (northward range shift), and this is basically what the researcher was testing in this study too.
Before going further, know that this study was conducted on a computer, using a very fancy program called MaxEnt, which is a program used a lot in the climate change world. To begin, in order to know how far the milkweeds will shift over time, he first had to train the computer program to know where they currently grow, and then the computer could "learn" the rest, like what are the maximum and minimum ranges of temperatures and precipitation (for each milkweed species), and day length, etc. that the milkweeds will grow in. He used current citizen science sightings of milkweeds, and their locations, to train the computer for this. Once that was done, he could then tell the computer program how the temperatures will look across the continent in the future, and then the computer can plot where the NEW milkweed range will be. Keep in mind that I'm purposely oversimplifying all of this here.
I'm going to jump right to the results next. Below are two maps from the paper showing where milkweeds (all species) currently are, plus where these milkweeds WILL be in the future (I think 50 years from now). I made this composite image out of two of the figures from the paper, and placed them side by side to make it easy to see the changes. Remember these are probabilities of occurrence, where the researcher predicted where they should be based on their known climate suitability. Look especially at the new range extending into Canada.
Another thing to note from these maps is how the range of milkweeds will EXPAND in the Pacific Northwest in the future. It looks like the monarchs in the west could even benefit from climate change, because their northern breeding range will expand!
After he figured out the milkweed changes, he then predicted where monarchs will be in the future (because monarchs are where milkweeds are), and the figure below shows the main result of that part. Again, this is a composite image that I made of two figures from the paper.
Note that the researcher did not examine monarch distribution in the west for this part, I think because of there being not enough records to train the program (at the time the paper was done). That's why there are no monarchs "predicted" to live in California or the rest of the west.
By the way, he actually made two predictions for the future milkweed and monarch ranges - one assuming a moderate change in climate and one assuming a more dire climate 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."
In this blog post I'm only showing the results from the "moderate" climate change scenario.
Here's my take - from looking at both the shift in milkweeds, plus the accompanying shift in monarch distributions, it looks like Canada is going to play an increasingly significant role in safeguarding the breeding population. It also means people who live in the southern United States will see fewer monarchs in the summer - not because they are disappearing, but because the monarchs simply moved northward. This is a really important point to keep in mind here - based on these findings, climate change is NOT going to result in a smaller breeding range, but simply a more northern one.
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.
Thanks for reading, and please share this news.
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