A recently published study (Flockhart et al. 2017) builds on previous work describing the natal origin of eastern overwintering monarchs, and has new implications for monarch conservation. By analyzing stable isotopes (similar to chemical fingerprints) from the bodies of monarchs, researchers were able to pinpoint more clearly where monarchs overwintering in Mexico were coming from. They did this by detecting the ratios of hydrogen and carbon isotopes in over 1000 monarchs collected from the overwintering sites in Mexico from 1976 to 2014. The tissues of the butterfly samples show different levels of these stable isotopes; by comparing these levels to known geographic variation, the researchers could estimate the natal origin of each butterfly.
Key findings of the study are that: 1) weather patterns influence the proportion of monarchs from different regions, and 2) the Midwest produced the greatest percent of overwintering monarchs, but fewer than previously thought. These results make clear that conservation in all areas of the eastern breeding grounds is essential to safeguard the eastern monarch migration.
Where do eastern migratory monarchs come from?
Monarchs overwintering in Mexico come primarily from the United States and southern Canada, east of the Rocky Mountains. The study area for this research breaks down the eastern population’s territory into six regions east of the Rocky Mountains, as illustrated on the map below. Regions west of the Rocky Mountains were not studied at this time. The map also illustrates the percentage of monarchs that originated in each region over the course of the whole study.
There was a great deal of year to year variation, partially explained by weather. For example, warm years in the northeast, north-central, northwest, and southwest regions resulted in more monarchs originating from these regions, but warm years in the southeast decreased the proportion of monarchs originating in this region. There were also interesting regional effects of precipitation
What’s new about these results?
Previous research using a similar technique with eastern migratory monarch samples from one year showed over 50% of migrating monarchs originated from the Midwest. The earlier study was conducted prior to the broad adoption of herbicide tolerant crops, which over time resulted in the elimination of much of the milkweed from the heavily agricultural Midwest landscape. Based on that knowledge, the authors of this study expected to find a decline in the proportion of monarchs originating from the Midwest compared to other regions, but they did not. The proportional contributions of each region varied from year to year, but no consistent trends in regional representation over time were identified.
What does this mean for monarch conservation?
This study showed that over four decades, the Midwest produced relatively more monarch butterflies that migrate to Mexico than other breeding regions. However, for most of the years for which there were data, more than half of the monarchs came from other regions.
The implication of this result is not that we should focus less on habitat conservation in the Midwest, but that our efforts to conserve habitat for monarchs must be unified across their range. The year-to-year variation in origin, along with other unpredictable events, confirms that a strategy to create, restore, and enhance habitat for migrating and breeding monarchs across North America will help sustain the eastern monarch migration.
In short, this study reveals that creating habitat and getting involved in conservation all over the monarchs’ breeding range is important and necessary. Wherever you fall on this map, you can help monarchs in one of many ways. Find out how to get involved today.
Flockhart DTT, LP Brower, MI Ramirez, KA Hobson, LI Wassenaar, S Altizer, DR Norris. 2017. Regional climate on the breeding grounds predicts variation in the natal origin of monarch butterflies overwintering in Mexico over 38 years. Global Change Biology, doi: 10.1111/gcb.13589
The Monarch Joint Venture is a national partnership of federal and state agencies, non-governmental organizations, and academic programs working together to conserve the monarch butterfly migration. The content in this article does not necessarily reflect the positions of all Monarch Joint Venture partners. Header photo taken by Reba Batalden in 2007.