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  • Andy Davis

New study that examines summer monarch counts from Illinois and Ohio, and the influence of weather

Hello everyone,

In this blog entry I'd like to talk about a research paper that came out last month that is pretty interesting. The paper, titled, "Evaluating confidence in climate-based predictions of population change in a migratory species", in the journal Global Ecology and Biogeography, was written by a postdoc (Sarah Saunders) in the lab of Elise Zipkin at Michigan State University (link to her lab website here). For those not familiar with the work from this research lab, you should be - Although the lab group works on a variety of questions and critters, lately, Elise and her crew have been working on some very important projects that have immense importance for monarch conservation. She and her people are known for doing some very sophisticated statistical (and very technical) analyses of monarch datasets, and this current paper is no exception. I'm going to add a link here to the abstract of the paper, though the paper itself (if you can download it) is a dense read. But have no fear - that's where I and this blog come in!

This research paper was really more of a test of a certain statistical approach, so from my read the focus of the project was more on the stats than on the monarch. However, there were elements of the paper that do have importance for monarch conservation. No, there was nothing in this paper about monarch declines, or even long-term trends either. The researchers on the paper (who also included Karen Oberhauser and Leslie Ries) had set out to examine two sets of data - one from the Illinois Butterfly Monitoring network, and one from the Ohio Lepidopterists Society. Both of these programs solicit volunteers to go out and survey for adult butterflies (all species) in the summer at specific sites in each state. I have a map of all of the sites below, which is taken from the paper. I've talked a bit about these monitoring protocols before - basically, the program volunteers are each in charge of a specific site that they revisit on a regular basis, and over many years, and at each site they record all butterflies they see.

Each of these citizen science programs has a wealth of data on butterfly abundance over the years, including data on monarchs. I'll add a couple of quotes from the paper that speak about the amount of data they had - "The Ohio program was initiated in 1995 by the Ohio Lepidopterist Society, and we included data collected at 116 locations from 1996 to 2011 (513,038 surveys). The Illinois program began collecting data in 1987, though our analysis used data from 1996–2011 to allow direct comparison with Ohio. There were a total of 133 locations for analysis (56,506 surveys)."

So from this information, and from looking at the map above, you can see there were a lot of monitoring sites, and the two programs have a lot of data on monarch abundance. By my math, there were a total of 569,504 lines of data (each line is a single survey of butterflies) in this collective dataset. The other thing to keep in mind here is that both of these states are located well within the historical core breeding zone of the eastern monarch population (i.e. the American Midwest).

From my reading, the goal of this research paper seems to have been to determine how closely the data from the two monitoring programs matched up each year, and to what extent the numbers of monarchs depended on the environmental conditions (like precipitation, soil moisture, growing degree days) during the summer, or the conditions the monarchs faced earlier in the spring in Texas. In other words, can the abundance of monarchs in the summer at these sites be predicted by the weather in Texas early in the spring or the weather at the breeding sites during the summer, and can the counts of monarchs from one program (or state) predict the counts at the other program/state? So there were a couple of interrelated questions here.

There was another factor that the researchers incorporated into their statistical test, which was not highlighted much in the paper, but I think deserves more attention. Within their statistical analysis of the monarch counts, they incorporated an interesting 'habitat' component. Given that these survey sites were scattered across the two states, and each site likely has differing landscapes, the researchers wanted to incorporate this variation in some way. They did so by using a national database of landscape cover, and a GIS program, and looked at the large-scale landscape characteristics within a 1km radius around each monitoring site. This is kind of like using google earth (or using the satellite view in google maps) to look at your house, and then categorizing the landscape features within your neighborhood. The researchers here quantified the degree of 'openness' around the survey sites, which is basically the absence of forest. So one survey site would be 70% open, while another would be 15%, and so on. They figured that monarchs would be more abundant at monitoring sites with more open habitat, since monarchs generally don't go into the woods.

So the statistical analysis of the data revealed a number of things. First, the number of adult monarchs counted each year in Ohio closely matched the numbers counted in Illinois. This finding has big implications. It means that the abundance of adult monarchs in either of these states is a reflection of how the larger population of monarchs is doing. In fact, the researchers concluded that because of their close correlation, counts from either site probably could be extrapolated to the entire region. So for the people reading this at home, it means if you and your neighbors are seeing lots of monarchs one year, for example, you can bet that pretty much everyone else is seeing lots that year too.

Second, they found that certain environmental conditions (not all) during the summer does influence how many monarchs are counted each year; abundances appear to be greatest when the weather is cooler than average at both states in the summer. Precipitation had a minimal influence on the monarch counts. However, the weather conditions during the early spring in Texas were even more important than the weather during the summer. Specifically, cooler, wetter springs in Texas tends to lead to greater monarch abundance in the summer in Ohio and Illinois (and presumably elsewhere in the monarch range). They did not discuss much in the paper about why cool, wet springs lead to more monarchs - perhaps the wetness leads to more milkweed growth, and the coolness delays the spring migration, so that the milkweed in the north has time to grow before they (the monarchs) get there? This is a little unclear.

Finally, remember that habitat thing? While this was not highlighted much in the paper, their statistical test actually found that this was also very important for predicting monarch abundance in both states. It turns out that monarchs tended to be most abundant at monitoring sites with greater openness (i.e. less forest around them). While this makes a lot of sense to people who watch monarchs, believe it or not, this is the first time this has been actually statistically verified in a scientific study. However, this also brings up another important point about 'monarch habitat'. A lot has been made of how much 'monarch habitat' has been lost over the years from human development and landscape modifications, etc., but yet I wonder how much 'monarch habitat' has also been created from these same practices? Based on these data, one could argue that the clearing of forests for human developments (i.e. making landscapes more 'open') may actually be creating habitat for monarchs. This is just a wild and crazy thought (but one that is back up with data).

So I think that covers the basic elements of this paper, or at least, what I think are the important conclusions from this study. I should offer congrats to Sarah Saunders and crew for completing this study, and I hope to see more work like this from the Zipkin lab.



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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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