Where do monarch butterflies come from?

The plight of monarch butterflies is often in the news and many scientists around the world are working hard to understand their annual migratory cycle.  How do the monarchs produced during summer in the northern reaches of America contribute to the overwintering population in Mexico?  The origins of monarch butterflies that make it to Mexico has been hotly debated and has profound consequences for how we approach monarch conservation.

A new study is remarkable in its use of historical collections over the past 40 years and modern isotopic analysis (download it here).  The scientists address the most important regions in the USA for producing monarch butterflies that actually make it to Mexico.  This sort of data has been very difficult to come by and there has been a lot of speculation.  As outlined in my new book from Princeton, the midwest has dominated discussions as being the most important region in the USA for monarchs. In the study, the authors find that the Midwest contributes a whopping 38% of the butterflies that make it to Mexico.  Figure 1 in the paper blew my mind!

I would add two points for discussion.  The first is that the areas of land that the authors designated as Midwest, Northeast, etc., seemed totally reasonable, but also somewhat arbitrary.  In particular, an issue arises when you consider that, as designated in the paper, the Midwest is about 2.5 times as big as the Northeast. It is therefore not surprising that the Midwest produces about 2.5 times as many butterflies that make it to Mexico (38% vs 15%).  In other words, the butterflies that make it to Mexico have about an equal probability of coming from the Midwest and the Northeast when land area is considered.  Yet another way to think about this is that two states that are about equal sizes in the two regions (for example, Indiana and Maine) will on average produce about the same number of butterflies that make it to Mexico.

The regions studied by Flockhart et al. separated to highlight their relative areas (cut out from Figure 1).

Quite interestingly, the North Central area (including my home in the Finger Lakes region of NY) is slightly more important for butterfly production given its size.  When you factor out the area of the Great Lakes (where there are no monarch caterpillars), the area of North Central is small (36% of the size of the Midwest).  Thus, about 20% more butterflies per square mile come out of the North Central than the Midwest or Northeast.

The annual migratory cycle of the monarch butterfly (a figure borrowed from my new book).  In my past research, we have opted for a three simple regions defined by the butterfly generations. 

Where does this leave us?  The agricultural Midwest is certainly important, but perhaps not as important as previously thought.  The other point worth thinking about is that the Southwest (read: Texas!) comes out as big in terms of area (= to the Midwest) and relatively less important in terms of contributing butterflies (11% of the total).  The critical importance of the Gulf States including Texas, however, is not in the last generation of butterflies produced in fall that migrate south, but rather in the first generation of butterflies that are produced in spring and that migrate north to the Midwest and Northeast.  In other words, the Gulf States are absolutely critical for the annual migratory cycle, even if that is not where fall migrants are produced.  Without a spring generation there, the Midwest and Northeast would be empty!  In chapter 9 of the book, I summarize the critical importance of Gulf States not only for the spring, but also in providing floral resources for fall migrating butterflies.

I hope we see more studies like this in the future, it provides new important information and was inspiring to read.  All for now, and best wishes for the new year! –Anurag

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