The last 10 years have been a wild ride for the monarch butterfly population. Between 2010 and 2014 was the all-time population low, as recorded during overwintering in Mexico and reported by the WWF. This terrible low point, something I termed in the “red zone” corresponded to the 100-year drought in Northern Mexico & Texas, and the two are most probably linked. Since then, the population increased, doubled, then tripled, and then stayed more or less comfortably even. Although this bounce back was a relief, there have been questions about whether the monarch butterfly has truly rebounded. I note here that even if there was a true rebound, that does not mean the monarch population is healthy and stable, just that it is above the red zone (even still, the population is more than 60% lower than the highs observed in the mid-1990s, see below).
A new statistical study of the monarch population (not yet peer-reviewed, but available here) authored by Thogmartin et al. stated: “If the population estimate for winter 2019 is ≥4.0 ha, we will then be able to credibly assert the population has been increasing since 2014.” Unfortunately, given that the butterflies have been declining over the past three decades, this year’s annual announcement was not welcome news: the monarch population has not been on a sustained increase over the past 6 years.
The estimates of the monarch butterfly overwintering population were announced today (March. 13th 2020) by WWF Mexico: unfortunately, they are about half the density of last year. The butterflies are so dense at their dozen or so mountain-top clustering sites that overwintering butterflies cannot be counted individually. Instead, the area of forest that is densely coated with butterflies (at about 5,000 butterflies per square meter looking up into the canopy) is estimated as a measure of monarch abundance. Butterflies arrive to Mexico around the day of dead in November and stay until March each year as part of their annual migratory cycle. Indeed, the monarchs of 2020 have already been observed migrating into Texas and laying eggs!
This winter season (2019-2020), there were approximately 2.83 hectares (nearly 7 acres) of forest occupied with dense monarchs in the Mexican highlands (somewhere in the neighborhood of 150,000 million overwintering butterflies). The monarchs end up congregating in a tiny area, with the bulk of the butterflies concentrated among twelve mountain massifs (clusters of peaks) within three hundred square miles (eight hundred square kilometers), an area smaller than New York City. In other words, most of the monarchs from eastern North America, from Maine to Saskatchewan, and south to Texas, probably covering two million square miles, funnel down and overwinter in a location 0.015 percent the area that they occupy in the summer! Unbelievable. This year’s estimate is about half of last year’s, which is NOT great news for monarchs!
Where does this leave us? A year ago, we reported that the population was huge and the overwintering population in Mexico was the highest in 12 years, higher than predicted by many. Last summer (2019) the number of monarchs in the USA was also huge, leading many of us to predict a great migration and population making it to Mexico. Nonetheless, this points to the critical issue of the migration itself, what happens after the caterpillars feed on milkweed. Chip Taylor has noted that “the two biggest factors that appear to account for the lower numbers this winter are the lateness of the migration and the drought in Texas.” As I have previously argued, there is often a disconnect between summer breeding populations of monarchs and the overwintering population. Indeed, this means that limitation of milkweed host plant was not the driver of this year’s halving of the population. Annual fluctuations in spring and summer rains and temperature, coupled with the autumn climate, which dictates the trajectory of the southern migration, have apparently been leading to migratory failure. Drought, in particular, reduces milkweed quality during the breeding season, and perhaps more importantly, reduces availability of flowers and nectar (of other plant species) during the autumn migration.
With 27 years of data, there are various ways to plot and assess the trends. Above I have plotted four year averages for seven periods working backwards (so the first average on the left is only for 3 years). Any way you slice it, the trend has been negative, and the population is not what it was. Nonetheless, the extreme downward trend seems to have bumped up in the last period of four years. Is this the new norm, a winter population hovering between 2 and 4 hectares? How dangerously low are these numbers? And what can be done to continue to reverse the trend and buffer the population? To what extent is limited milkweed the driver of the momarch population’s precipitous decline? I have recently written about these issues in a scientific commentary as a well as my popular science book, which covers much more (read chapter 1 here).
I, for one, needed a distraction from the craziness surrounding Covid19… unfortunately, I wish there was better news for our beloved butterflies. We continue to wait for the Fish and Wildlife Service’s decision, which is evaluating the petition to list monarchs as threatened under the Endangered Species Act (they are compelled to deliver their decision by December 2020). For now, I am looking forward to seeing milkweed, butterflies, and their caterpillars once they arrive to Ithaca, NY in a few weeks this spring.