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  • Andy Davis

Fabulous new study by three plant scientists adds to the debate on the importance of milkweed

Hello again everyone,

Normally I'm not this quick in writing new posts but a new study was published just last week that I had to write about! And as my title suggests, it was written not by monarch scientists but by three plant scientists who studied milkweed. And I must say I was very impressed by this paper. So in this blog post I'll be breaking down this paper to sort it all out.

Let's start with a link to the paper itself - here. This will take you to the abstract, but unfortunately the paper itself is not free to download. So unless you want to pay the journal, or if you know a university or government scientist, you won't be able to read it. As an aside, I think this is a tragedy - science shouldn't be kept from the public, especially with projects pertaining to critters with such wide public appeal. Anyway, the abstract will only give you a quick flavor of the paper. It's actually a fairly meaty read and there is a lot left out of the abstract. I'll try to distill it here...

So like I said the researchers here apparently are plant scientists and the goal of this project was to see if there has been any long-term declines in milkweed abundance (in the state of Illinois, where they did the work), and then to see if this "explained" the long-term declines in overwintering monarchs in Mexico. I put the word explained in quotations here because this comes up later. The researchers apparently had access to a long-term data set of milkweed surveys that had been done in the state, and these surveys were done in both farm fields and in natural areas.

Before I go further here, let me note that from what I could tell, these researchers were extremely well-versed in the historical and most recent science around the monarch and the long-term declines, and most importantly, on the current debate of the causes of these declines (see my previous blog posts on this debate), and all of this even though they aren't monarch scientists. And when I say extremely, I mean it - they seemed to know each of the key papers on monarch declines inside and out, and knew exactly how the data from each paper was examined. What's more, they then outlined where there were strengths and weaknesses of each prior study. In essence then, this paper read like an outsider's objective take on the whole debate, and it came across as a well-written summary.

This debate on the cause of the monarch declines was their rationale for their study, so they spent some time delving into it in the first part of the paper. They pointed out that the cause of the monarch declines in Mexico boils down to three non-mutually exclusive hypotheses. First is the one most people know, that is the milkweed limitation hypothesis - that's the narrative that says milkweed has been lost in the American Midwest due to GM crops, and that has led to a decline in the monarch population. This hypothesis is championed by a number of monarch scientists (but not all), and it is based largely on a correlation between the rise in GM crops in the past 2 decades, and the fall of overwintering monarch numbers during the same time. The second hypothesis refers to the idea that monarchs may not be reaching the overwintering sites anymore like they used to, or in other words, the migration survival hypothesis. This idea comes from the fact that most of the long-term surveys of adult monarchs in the summer have not shown a decline over time, meaning that there must be problems during transit to Mexico. This idea has been adopted by an ever-growing number of monarch scientists. Finally, there is an idea that the loss of overwintering habitat is the cause of the declines. I'm not sure if this one is the leading contender - most scientists agree that this is a problem, but the single cause of the decline? Not so sure.

So after laying all of this out, then the authors described their milkweed data, and I'll relay that next. The following text is copied from the paper:

"We used a long-term botanical survey covering the predominant natural and seminatural vegetation types across Illinois to estimate milkweed trends. The survey used randomly selected sites from grassland, wetland, and forest habitats (198, 187, and 160 sites, respectively), which it tracked every 5 years since 1997 using standardized plant sample plots"

The figure here shows a map from the paper with all of these sites in the state of Illinois. This was the figure legend from the paper: Grassland, wetland, and forest survey sites across the state of Illinois. The red triangles represent the survey sites with at least one of the target species included in our analyses. The black circles represent sites where our target species did not occur.

The target species in their case were four types of milkweed: Asclepias syriaca, A. verticillata, A. incarnata, and Cynanchum laeve. This last plant is a close relative of milkweeds and which monarchs do feed on.

I'd like to point out that one of the strengths of these data is that the survey sites were RANDOMLY-chosen years ago, when the surveys were first started. This is a huge strength from a scientific and statistical standpoint. It means that the site-selection was unbiased. It also means that these surveys are not in people's backyards. The site was chosen by a computer, regardless of what habitat was there.

I'll also add another section of text from their paper that gives another strength of these data -

"Because this study includes randomly selected sites and because of the similarity in vegetation patterns and landcover between Illinois and the rest of the midwestern United States’ “Corn Belt”region, trends here can be extrapolated across this most productive portion of the monarch butterfly’s summer range."

So let's get to their results. I won't get into the stats too much, other than to say the researchers were very thorough with their analyses. Recall that what they were looking for was evidence that the milkweed in Illinois has been declining. Below I've copied four of the main graphs from their paper that shows the long-term patterns of abundance for each of the milkweed types. Note that in each graph you can see two ways that "milkweed abundance" was measured: the percentage of cover at the site (the bars) and the proportion of sites where that milkweed was present (the red lines).

There are some key things to note from these graphs. First, these graphs are only the non-agricultural sites. They separated the data into ag-habitat and non-ag habitat because there were differences in these groups. Second, there was no statistically-significant long-term trend in any of these graphs. From this the authors concluded that milkweeds in natural and seminatural landscapes have not declined in the past 20 years.

But, this is only part of the story. When they looked at the sites in the agricultural areas, they found a strong signal of declining milkweed abundance, and this should come as no surprise to anyone reading this. They next took into consideration which milkweeds and which habitats are most influential to monarchs, based on prior studies and other information. Then they ranked the milkweeds and habitats into a scoring system, that considers which of these is most important for monarchs. Then finally, they looked to see if those milkweeds that ranked highest (for monarch use) have declined over time. Below is a graph that shows, yes, it has. The Y axis shows the milkweed abundance score (they called it Monarch Resource Units).

So when they factored in the farm-field milkweeds, they found a clear decline in overall milkweed abundance in the state of Illinois. I believe they said this was about a 68% decline.

The following is taken from the paper:

"In summary, the best available data indicate that milkweeds have declined precipitously in croplands. In natural areas, declines are more moderate and declines were not statistically significant for any single species."

After describing their results for the milkweed data, the authors then discussed the implications of these numbers for the monarch decline. This led to a lengthy discussion of each of the prior studies, where again, the authors showed a keen command of the literature. I read through this a couple of times because this part was a little loosey-goosey. In the end I'm not sure if they really came to a firm conclusion one way or the other. In other words, they did not state which of the three hypotheses their data supported. I know, the milkweed limitation one seems most obvious, but the authors seemed to go out of their way to emphasize the migration survival one.

I think this is the part of the paper that I'm less enthusiastic about. After all of the analyses of milkweed data, the authors really couldn't draw any conclusions about what this means for monarch populations. Yes, I know that less milkweed means less monarch breeding, but what I mean is that despite the fact that they set out to evaluate the importance of milkweeds to the monarch decline, they really didn't examine any monarch data at all (at least statistically). So all they can really say is that milkweeds in agricultural fields have declined, but those in natural areas have not.

Here's the other puzzling thing to me. There were some news stories that circulated after the paper was published, and where the authors were interviewed. Here's one here, titled "Milkweed losses may not fully explain monarch decline". In the interview, it sounds like the authors have concluded that milkweeds are not as important as people think. I tend to agree with this, but I find it puzzling that the authors came to this conclusion based on these data. Maybe they realized that objectively speaking, their data do not show a cause-and-effect relationship to monarch declines in Mexico? Or maybe after reading all of those monarch papers that describe all of the various hypotheses to explain the decline, perhaps they realized that it is a complicated situation. If so, then kudos to them - I agree.

OK, I think I've covered the main beats of this paper. I hope this paper finds its way onto the internet so more people can read it. I think it will eventually go down as a very influential study in this whole debate.



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