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  • Andy Davis

Two independent insect scientists weigh in on the debate over monarch population status

Greetings readers,

The debate over the eastern monarch population status rages on - at least in the pages of the Annals of the Entomological Society of America. Right on the heels of the recent editorial letter writen by Pleasants, Williams, Brower, Oberhauser and Taylor, another letter was just published this week in this journal, this one written by two (non-monarch) insect scientists, Lee Dyer and Mathew Forister. These two guys are very reputable scientists who study insect populations, though not monarchs, so they are somewhat independent of the goings on within the monarch world. Dr. Dyer was the guest editor of the recent special issue on monarchs, and Dr. Forister was one of the reviewers of some of the papers.

Their letter (you should read it before going further - link here) was a counter-rebuttal to the critique written by Pleasants et al last week. Recall from my last post that their (Pleasants and colleagues) letter to the editor was about the eastern monarch population status and how they did not believe the conclusions of several papers in the special collection that showed monarchs were not declining. They listed a number of criticisms of those studies that they believe make them not reliable for guaging the population status.

Dyer and Forister made a number of well-crafted arguments in their letter, and I'll try to summarize the major points here. They attempted to remain somehat neutral on the subject of monarch population status, although they did have some strong rebukes of the arguments made by Pleasants, I think because they felt the arguments were lacking in evidence and/or scientific rigor.

Some major points of the letter:

- It seems they had great respect for the work that has been done to advance knowledge about monarch biology, and they praised all of the monarch scientists involved in the special collection for making great strides.

- Their main argument running throughout their letter was that the only way to solve this debate (i.e. are monarchs declining or aren't they?) is by using sophistocated statistical techniques and by incorporating a lot of citizen science data from all stages of the life cycle. They said so far this has not happened in the monarch world. To be fair, I think there is some of this in the works right now (and that these guys are unaware of), but sure enough, there is nothing yet in the published scientific literature.

- They responded to the specific arguments made in the Pleasants et al letter. One was about milkweed and its importance to monarch conservation. Pleasants et al were adament that milkweed is 'key' to the monarch population, and they contend that milkweed loss is why monarchs are declining. Dyer and Forister countered by saying that no studies have been done to provide legitimate scientific evidence that milkweed is limiting to monarch populations. In other words, we need studies looking at all of the various factors that are important to monarchs, including milkweed, and compare them all to find out which one is the most important, before we can say milkweed is 'key'. So in essence Dyer and Forister are saying, yes, milkweed is important, but are there other things that have a greater influence on the monarch population numbers? We don't yet have this answer from a scientific standpoint. A side note here for blog readers - if you are thinking about all the fuss that has been made over the genetically-modified, roundup-ready crops and how they are removing milkweeds, keep in mind that this is not what Dyer and Forister are talking about. They want to see rigorous scientific research (preferably using sophistocated statistics) on all stages of monarch life cycles, and for someone to tease apart which stage, and what resources are the MOST critical to monarchs. Simply verbally saying that milkweed is the most important is not evidence enough. Plus, a lot of the evidence for GM crops as the culprit for monarch losses is based on a simple corellation (GM crops have increased at the same time monarchs in Mexico have declined) which was done in the Brower et al. 2012 study. However corelational evidence is pretty weak in science.

- Pleasants et al had been critical especially of the paper by Leslie Ries and colleagues, who analysed data from both NABA and the Illionois Butterfly Monitoring Network, which are programs that count adult butterflies in the summer months, and in the core breeding range. Both of those data sets showed no evidence of declines in monarch numbers in the past 20 years. Pleasants et al made the argument that the places where people survey for butterflies in those programs are not in agricultural settings, where major milkweed losses have ocurred. They then used this argument to dismiss the long-term patterns observed from those data sets. While Dyer and Forister acknowledge the issue of placement of monitoring sites, they provided some example data of their own that they statistically examined and which showed that the placement of the survey sites would not matter in this case - in fact monarch declines would be seen anywhere, even in non-agricultural sites. This means that the patterns observed from the NABA and Illionois data are valid.

- Dyer and Forister were critical of the overall conservation response so far to the declines in Mexico - not that they thought it was lackluster (it's been anything but), more like they thought it was one-sided. They called the focus on only planting milkweeds 'myopic', and they argued that there is no reason not to focus on protecting migratory resources too, at least until we know more about what's really taking monarchs out of the migratory race.

OK, I think that's all the take-aways from their letter. Now, what happens next? Well, this letter and the previous one by Pleasants et al. have surely stirred the pot, which will make for some lively conversations at the next monarch scientist meeting. I wonder if this will also make is way onto dplex? It probably should, though I usually don't get around to posting anything there. I think people at least need to be aware of this debate. Also, I mentioned before that there are some efforts underway to actually do some of the sophistocated statistics that will help to answer these questions being raised, and maybe this debate will get those efforts moving faster.

That's all for now.

The science of monarch butterflies

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

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