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  • Andy Davis

New study shows there are easily "billions" of native milkweeds in western half of U.S.

Hello everyone,

Yes, you read this right, a brand new study was just published on milkweed densities in the U.S., and it shows exactly what this blog title says! I just read this paper, and I'm going to share some of the key findings here, plus provide some commentary.

Let's start with the basics. The paper was published last month in a scientific journal called, Ecosphere, and a link to it is here. I'm not sure if this full paper is downloadable for free. The paper was authored by four scientists from the United States Department of Agriculture (USDA), plus a researcher from the Xerces Society. I don't know any of these folks personally, although I did reach out to the lead author (Ken Spaeth) by email to get some clarification on a couple points - more on that later.

So the goal of this paper was to come up with some estimates of the amount of native milkweed that currently exists on "non-federal rangeland" in the western half of the U.S. They indicated that their rationale was to provide information that would be useful for conservation efforts of monarchs. The researchers did this by examining a rather obscure source of plant data - something called the "United States Department of Agriculture-Natural Resources Conservation Service (USDA-NRCS) National Resource Inventory rangeland data set." This is a mouthful, but apparently, this is a program run by the USDA whereby its staff visit specific sites each summer and tally the different types of plants at the sites. And apparently, they have been doing this for decades. According to this paper there are over 23,000 records in this database.

Since I was curious about this database, I have been doing some digging to find out more about it. First I'll show a map taken from an online USDA report which shows the locations of all of the "non-federal rangelands" in the western half of the country. I think there are no dots in the eastern half because there really are no grassy rangelands in the east! But, as you can see there are plenty of locations along the middle part of the country, which is some of the heartland of the breeding range for eastern monarchs. So the results from this new study pertain both to the western subpopulation and to the eastern.

The lead author tells me that there are approximately 406 Million acres of non-federal rangeland in the country. Importantly, these rangelands do not include pastureland, cropland, waste areas, roadsides, or urban areas. So basically, these are fairly natural, grassland landscapes - not backyards or anything like that. In other words, it is about as natural as you can get.

To give the readers from the east or elsewhere a better mental image of these landscapes, I'll paste a picture below of researchers from the Dept. of the Interior working on a typical rangeland landscape.

Now back to the paper - from reading the methodology, it seems that the data collection at each site is as follows. Personnel visit each site and go to a specific point at the location, which is flagged or something like that. At each point they document all of the plants surrounding the point, within a circle with a 45 meter radius. They also visually estimate the density of the plants on a coarse scale (low, medium or high). For the purposes of this study, these researchers were obviously interested in the records of Asclepias plants (milkweeds) within this database. It sounds like there were over 2,000 sites where milkweeds were present. And, it sounds like the authors looked at all of the data from 2009-2018, which means that this report is current.

It sounds like the researchers took these records of milkweeds and essentially tallied them to create this report, listing the most commonly-reported species of milkweeds in each state, and by latitude and longitude. They also extrapolated the density estimates from the local sites, to estimate how much milkweed is present across the entire rangeland. This seems fair, and also very useful information. In fact, from my read of this paper, the entire methodology used seems pretty straightforward - in other words, their results are probably not wrong. And, given how consistent the sampling procedures have been over the years, it sounds like this is an invaluable set of data - my only question is, where has it been for so long?

So let's get to what they found.

First, they listed the dominant species of Asclepias, which were A. viridis, A. syriaca, A. verticillata, A. speciosa, A. asperula, A. viridiflora, and A. latifolia. They also provided some graphs showing the rank order of dominance, and I'll paste one here.

Yes, the Y axis of this chart shows the estimated densities of these plants are in the order of BILLIONS. This leads to the next bit, which is their overall estimates of how much native milkweed is present on these rangeland landscapes. It sounds like they provide three overall estimates, and the reader is free to choose which one to use. Recall that the personnel visually estimate the plant densities to be low, medium or high, so the authors here provide similar estimates.

Here is a direct quote - "According to the NRI study, there were 1,303,372,193 plants based on the low-density estimate, 4,100,309,324 plants for the midpoint, and at least 6,919,517,259 plants for the high estimate..." Put another way, there are anywhere from 1 to 7 billion naturally-occurring milkweed plants across the 400 million acres of non-federal rangeland in the western U.S.

Crazy right? And recall that these rangelands do NOT include pastureland, cropland, waste areas, roadsides, or urban areas. So really, these estimates are already at the low end for the entire western landscape. Think of all of the pastures alone in these areas, which also have milkweeds...

The authors also summarized where the highest densities of milkweeds were by latitude and longitude, and I'll paste on of their figures on this below. This map shows the western U.S. with a latitude-longitude grid overlaid (in black). They tallied how many milkweeds were present in each black square, and then ranked these in order of density, with 1 being the block with the highest milkweed density.

So, this map makes it clear that the region with the highest milkweed density is along the center of the country, which is more or less the heartland of the eastern subpopulation. The lowest densities of native milkweeds (on rangeland) were actually in California!

OK, so there was a lot in this paper that I didn't cover here, including some interesting analyses done to quantify the health of the various plant communities, the soil profiles, and a lot of other stuff. To me though, the most important bits are what I covered - i.e. how many milkweeds are available for monarchs.

So after reading this, I was blown away by these numbers, as I'm sure those reading this are too. Given how much emphasis is placed on "restoring milkweed" for the monarchs, this seems to argue that there is plenty already. So, I reached out to the lead author, Spaeth, by email and asked him this very question. I asked, given the estimates you provided in this paper, do you believe that there is enough milkweed for monarchs right now?

He did politely reply to this question, although I will point out that his reply was not really an answer. Ken's reply - "The question as to how many milkweed plants are adequate for monarchs is an ongoing debate, and some suggestions have been made for the Midwest population (I cited the papers) in the literature. That is an answer that butterfly biologists need to formulate. To make some kind of statement whether certain numbers of milkweed are adequate is subject to controversy."

I also note that nowhere in this paper did the authors make any claims about how much milkweed is necessary for monarchs. I can get why he and the other authors did not want to make any actual judgement or proclamation on this controversial topic either way. In the politically charged and divided world we live in now, it seems like the best way to not get yelled at is to simply not make a decision, and not make any statements.

But as my readers know, I don't care, so I'll say it. To me this evidence shows that there is plenty of milkweed for monarchs. Period. Mic drop.

That's all for now.


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The science of monarch butterflies

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

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