Monarch Population Status

The WWF-Telmex Telcel Foundation Alliance, in collaboration with the National Commission of Natural Protected Areas (CONANP), the National Autonomous University of Mexico (UNAM), and the Monarch Butterfly Biosphere Reserve (MBBR), announced the total forest area occupied by overwintering monarch colonies today. Eight (8) colonies were located this winter season with a total area of 1.79 hectares, a 99% increase from the previous season (0.90 ha).

Figure 1. Total Area Occupied by Monarch Colonies at Overwintering Sites in Mexico.

A link to the full report will be provided here once it is made available. Please see the article below for more information.

Note: The WWF-TELMEX Telcel Foundation Alliance collaborates with CONANP to systematically monitor the hibernation of the Monarch since 2004, and they join the Institute of Biology of the National Autonomous University of Mexico (UNAM) to analyze changes in forest cover in the area core of the Monarch Butterfly Biosphere Reserve in order to have scientific bases that support the implementation of conservation strategies for the benefit of the species, ecosystems and human beings.

Monarch overwintering numbers in Mexico for the 2024 season

by Chip Taylor, Founding Director of Monarch Watch

The monarch numbers are in, and they are of interest again, but this time not for the fact that the population declined but for the rate of increase. Last year the overwintering population was extremely low, occupying only 0.90 hectares when all colonies were counted. That number was second only to 2013 when the total was 0.67 hectares. The number this year is 1.79* hectares and that is remarkable. It’s a doubling of the population. How many vertebrate species can double their numbers in one year? Surely few, except for some fishes and a few birds that brood large clutches.

Out of curiosity, I went through all of the records for increases to see whether there are other years with remarkable recoveries and there are 7, including this year. The rate of increase data, represented by the number for the following year divided by the previous year, are as follows:

As you can see, the 2.0 increase from 2023 to 2024 is in line with increases (1.7-2.7) that have occurred 5 times in the past.

Following that, I checked the spreadsheet I maintain of all the weather and other conditions that might moderate population growth. That revealed there were negative growth indicators for all the low years (droughts, low summer temperatures, etc.) and various degrees of positive growth conditions (near average temperatures and precipitation) in all the following years. In fact, the temperatures and precipitation were close to the long-term averages for the entire growing season for 5 of the years in which the populations increased. Negative years do follow negative years and that has been more common since 2010 than earlier in the record (none prior to 2010 and 5 since 2010). That may say something about greater instability in weather patterns during the growing season since 2010. However, for now, we should put that idea on the watch list.

A takeaway here is that monarchs demonstrate resilience over and over again. Weather knocks them down, but spectacular recoveries are the rule if negative conditions during one year are followed by favorable conditions for population growth. As some of you may recall, I wrote a text for the Monarch Watch Blog that summarized the data that speaks to the three r’s (resilience, redundancy and representation) that are at the core of the Species Status Assessment (SSA). That document is prepared and used as the basis for determining whether a species should be regarded as threatened or endangered. My comments were in reference to the SSA prepared in 2020 but seem relevant now that we are dealing with a new SSA.

*There is one more thing to tell you about the 1.79hectare measurement. It is close to the three- and five-year running averages, which are 1.98hectares and 2.18hectares respectively.

Reference

Taylor, O. R., 2023. The species status assessment (SSA) and the three r’s. Monarch Watch Blog.
monarchwatch.org/blog/2023/10/13/species-status-assessment-and-the-three-rs

Addendum

If you go through the updates I wrote for the Monarch Watch Blog from March through June, you will see that I was really high on the possibility that the recovery this year could be the best ever. It wasn’t. Something happened during late May and early June that limited the reproductive success of the first generation in the Upper Midwest and therefore the size of the second generation that reached maturity in July. That, in turn, limited the size of the third/migratory generation and ultimately the size of the overwintering population.

So, what happened? Probably too much rain and too many days with unfavorable conditions for egg laying in the Upper Midwest, the region that produces the majority of the monarchs that arrive at the overwintering sites. The foundation for this interpretation was outlined by Myron Zalucki and Wayne Rochester in a chapter in The Monarch Butterfly: Biology and Conservation (2004). These authors pointed out that weather which limits egg laying results in a reduction of the total number of eggs laid during a lifetime. In other words, female monarchs don’t fulfill their full reproductive potential if egg laying is significantly delayed and that limits the number of offspring and the size of the next generation. This interpretation deserves a follow up, but the outcome is similar to a reduction in population growth that occurred following a long rainy period I tracked in Ontario a number of years ago. Demography isn’t easy and it becomes really difficult when events that occur over a few days, or a week or more, have a big impact on mortality or reproductive success.