Fewer and fewer monarch butterflies are reaching their overwintering grounds in Mexico every year, and new research might shed light on why.

A 2016 study found that the monarch population in Mexican overwintering colonies has declined by approximately 80 percent over the past two decades. Pinpointing the causes of this decline has proven difficult, however.

A study published in the journal Animal Migration last month suggests a possible cause: The monarchs are simply finding places other than Mexico to spend the winter months, and possibly even giving up their migratory ways altogether, in order to survive.

Fewer and fewer monarch butterflies are reaching their overwintering grounds in Mexico every year, and new research might shed light on why.

The mountains of central Mexico represent such critical overwintering habitat for monarch butterflies (Danaus plexippus) that a number of reserves have been created specifically to preserve the famous winged insects. Though they still face threats from mining, deforestation, and other human activities once they arrive, ecotourism centered entirely around visiting the monarchs in their winter retreat is so prevalent that it’s a significant factor in the local economy.

A 2016 study found that the monarch population in Mexican overwintering colonies has declined by approximately 80 percent over the past two decades. Given its implications for the welfare of the North American monarch butterfly population as well as human livelihoods, the fact that the number of monarchs reaching their Mexican overwintering grounds is declining is a cause for substantial concern.

Pinpointing the causes of this decline has proven difficult, however. Some scientists argue that it’s due to an overall drop in the eastern North American monarch population driven by the destruction of their breeding habitat in the Midwestern United States — monarch adults deposit their eggs on milkweed, which has been greatly reduced thanks to the use of herbicides and genetically modified crops in agricultural fields. (There are two populations of monarchs in North America, one east of the Rocky Mountains and another to the west; the western population overwinters primarily in southern California, though they have been discovered in Mexico as well). Another theory is that the fault lies with the increasingly perilous migration the butterflies must make every year to reach Mexico.

A study published in the journal Animal Migration last month suggests another possible cause: The monarchs are simply finding places other than Mexico to spend the winter months, and possibly even giving up their migratory ways altogether, in order to survive.

Dr. Hannah Vander Zanden of the University of Florida in the United States led a team of researchers that captured monarchs wintering in southern Florida and used a technique called stable isotope analysis to determine the geographic origins of the butterflies based on a tissue sample from their wings or body. The researchers found that nearly half (48 percent) of the specimens they caught had come from the American Midwest, which is believed to be the core breeding grounds of the eastern North American monarch population.

Scientists have typically assumed that monarchs from that region travel exclusively towards the mountains of central Mexico in the fall, but Vander Zanden and team have shown that at least some monarchs from the Midwest U.S. are choosing to fly to southern Florida instead.

“Previous research had suggested that some migrating monarchs may wind up in southern Florida if they become waylaid by strong westerly winds, but this evidence makes it seem like they purposely traveled to this location,” Andy Davis, a monarch migration expert at the University of Georgia and editor of Animal Migration, said in a statement. If more of these kinds of alternative overwintering locations are found, the discovery made by Vander Zanden and colleagues could help explain the shrinking overwintering population in Mexico.

In the Animal Migration paper detailing their findings, the researchers theorize that monarchs may be employing an “alternative life history strategy” in choosing to overwinter in south Florida instead of central Mexico. “Alternative life history strategies are mechanisms by which organisms are able to maximize fitness across a range of environmental conditions. Fitness is maximized by different strategies depending on context, resulting in trade-offs between life history strategies,” the researchers write.

Monarchs are known to use two different life history strategies: migratory monarchs pause breeding during their time in their winter grounds, while resident monarchs live year-round in one region and breed throughout the year. Vander Zanden and team discovered that the migrant butterflies in south Florida had larger wingspans than the residents, meaning that “switching to a resident strategy could alter their probability of reproductive success.”

In order to develop conservation measures for the butterflies, the researchers note that more research is needed to figure out exactly what factors determine the life history strategy monarchs choose, which in turn could influence migratory trends and geographic distribution in monarch populations. “Further work is needed to investigate the mechanism underlying this pattern, but these findings show that alternate life history strategies and sex-specific behaviors are underexplored factors influencing monarch migration and evolution,” the researchers write.

CITATIONS

• Semmens, B. X., Semmens, D. J., Thogmartin, W. E., Wiederholt, R., López-Hoffman, L., Diffendorfer, J. E., … & Taylor, O. R. (2016). Quasi-extinction risk and population targets for the Eastern, migratory population of monarch butterflies (Danaus plexippus). Scientific Reports, 6, 23265. doi: 10.1038/srep23265

• Vander Zanden, H. B., Chaffee, C. L., González-Rodríguez, A., Flockhart, D. T., Norris, D. R., & Wayne, M. L. (2018). Alternate migration strategies of eastern monarch butterflies revealed by stable isotopes. Animal Migration, 5(1), 74-83. doi:10.1515/ami-2018-0006