Have Monarchs Switched from México to Florida?

Donald Trump would probably be very happy if the conclusion of a new study on the migration of Monarch butterflies is true. According to this study, a large number of these butterflies, which used to return to México each winter have now abandoned that destination to settle down in Florida instead. (At least, that is how the study is being interpreted–for instance, in this article .) It didn’t even take a wall. But, as we should always ask if anything appears good from Trump’s perspective, is any of it true?

Background: Monarchs

Monarch are among the biggest butterflies of the United States (though, by far, not the biggest: that is the Giant Swallowtail). There are several locally resident populations, especially in Florida, but what makes the species interesting are the two migratory populations, one west of the Rockies and one east, with apparently very little exchange between them.  The eastern population undertakes the the longest insect migration in the world. The insects’ journey, from its overwintering habitat in central México to the Midwest and northeast of the United States and southeastern Canada over 3 -4 generations each spring, and back to México in one generation each fall, is up to 3000 miles each way.

For the last twenty-five years Monarch biologists have been arguing that this migratory process is in danger of disappearing. Overwintering monarch populations numbered 400 million individuals in the early 1990s but only a hundred million since 2010 with a historical low of about 35 million in 2013 -14 before getting slightly larger since. (The estimate for this year is slated to be released at the end of January.)

Two hypotheses have been proposed to explain this decline (see my short piece from 2017). The milkweed limitation hypothesis claims that spring Monarch breeding populations before migration are in decline because of the disappearance of milkweed which is the only food source used by Monarch caterpillars. The alternative
migration survival hypothesis states that the decline of wintering populations is due to increased mortality during the fall migration to México. The jury has been out on which hypothesis is correct (and they don’t exclude each other).

But, now, we apparently have a third hypothesis. The study just published (in Animal Migration, with Hannah B. Vander Zanden of the University of Florida, Gainesville as first author) raises the possibility that the decline is due to a fraction of migrant butterflies wintering in Florida rather than in México. Let us call this the alternate migration hypothesis (though I am sorely tempted to call this the America First hypothesis).

The New Study

The technique used was isotope analysis  of hydrogen and carbon. The idea is that the isotope profile of an insect part (the fraction of each isotope for the element) matches the isotope profile in its nutrients (during the development of that part) and that profile varies between geographical regions. The researchers collected 33 Monarch butterflies from three different sites in south Florida (10 from Homestead; eight from Miami; 15 from Naples Botanical Garden). They analyzed the hydrogen and carbon isotope content of small hindwing sections (including both black and orange pigmentation) of each specimen. That is the extent of the experimental work.

They found that only 52 % of the specimens had isotope profiles that matched that of south Florida. The remaining 48 % matched profiles elsewhere, most strongly those from the midwest and from the southwest. The collections from Miami and Homestead were from February and March; that from Naples was from October through December. The published data do not include the geographical provenance of the isotope profiles by month of collection.

Interpreting the isotope profiles not matching Florida as evidence of migration, the thrust of the paper is an evolutionary interpretation of the data as showing that the Florida Monarch populations follow a life-history strategy of alternate migration, that is, some fraction of the populations chooses to breed locally while the rest migrates. (The data suggests that migrants are bigger than non-migrants and that it may be the case that males are more prone to migration than females–though, given that only eight females were collected in total, no firm conclusion can be drawn about that.)

What Does It Show?

Does the study really support the alternate migration hypothesis? Or that an overly sensationalist interpretation of very meager data?

The first point to note is that it has long been known that the Florida resident Monarch population routinely receives migrants from the migratory population. Generally it has been assumed that this influx happens during the northern migration which would mean that the migrants would arrive in early Spring (Agrawal 2017). However, earlier estimates had this immigrant fraction of the Florida Monarch population to be less than 10 per cent. So, the higher number is new and possibly important. Moreover, the data suggest but do not prove that some of the migration into Florida could happen during the Fall southern movement of the migratory population. (Since the midwestern population can span several generations, the data do not permit us to exclude the possibility that some butterflies may still be moving to Florida during later stages of the northward migration.)

Beyond these points, that data don’t establish anything. They do not show that Monarchs from Florida ever migrate northwards. This means that we should not infer that the Florida populations play a role that is analogous to that of Mexican ones. Until it is established that some Monarchs shift to Florida during the southward migration to México each fall, it may well be the case that Florida is replacing the northern United States and Canada for some migrating Monarchs.

What Next?

For the “replacement” interpretations of this paper to be able to withstand scrutiny, two facts must be established. First, it must be shown that Florida is the destination for some southward migrating Monarchs. Now, Monarchs are routinely tagged during this segment of their travels, especially in the north as their spectacular return to México begins. The goal should be to recover some of these tags in Florida. To the best of my knowledge, this has never been done.

Second, it must be shown that Monarch born in Florida migrate northwards. Ideally this should also be done by tag recovery in the north after tagging butterflies in Florida. However, because northwards migrating generations get replaced every few weeks, tag recovery during this process is difficult. Though that must be the ultimate goal, and a determined field campaign would be necessary, we may initially have to settle for more circumstantial evidence: the collection of individuals in the north that have an isotopic profile that matches that of Florida. Again, to the best of my knowledge, that has never happened.




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