Understanding the eastern monarch population’s extinction risk is an important benchmark to gauge current threats and effective conservation. Previous research from 1993 – 2014 predicted a 10-year extinction risk of 42% and a 20-year risk of 62% (Seemens et al. 2016). Wanting a more up-to-date evaluation, Dr. Wayne Thogmartin recalculated the eastern monarch’s extinction risk using data from 2006 – 2022. By comparing these two datasets, he hoped to inform and guide future conservation efforts.
Overwintering surveys collected by Dr. Rendón-Salinas, in collaboration with the World Wildlife Fund-Mexico and the Monarch Butterfly Biosphere Reserve, were combined with annual butterfly summer counts from the Illinois Butterfly Monitoring Network. Using the updated timeframe, Dr. Thogmartin found that the mean population size from 2006 – 2022 was 7.81 acres of overwintering habitat with an annual growth rate of 0.93. Stable populations typically have an annual growth rate of 1, while increasing populations experience growth rates greater than 1. The monarch’s growth rate of 0.93 indicates that the eastern monarch population is continuing to decline.
Pulling all these findings together, Dr. Thogmartin was able to calculate an updated 10-year extinction risk of 28.7% and a 20-year extinction risk of 52%. While these risks might be slightly lower than those calculated based on the 1993 – 2014 data, it is important to note that these predictions have wide uncertainty. The 95% confidence interval for the 10-year prediction includes an extinction risk ranging from 1.9-81% while the 20-year prediction ranges from 3.2-97.9%.
The reason for this wide uncertainty is due to stochastic, or random, events that affect the monarch’s migration and overwintering habitats. These stochastic occurrences often come in the form of extreme weather events such as sudden winter freezes at overwintering sites, severe droughts during the monarch’s fall migration, or intense wildfires at overwintering or breeding grounds. These stochastic events can have devastating impacts on monarch populations: killing off large sections of the overwintering colonies, drastically reducing migration success, or wiping out large areas of overwintering and reproductive habitat.
The additional problem with these stochastic events is that they are becoming more frequent. Due to the increase in extreme weather events and increasing temperatures, it is more likely that a stochastic event will occur and have devastating impacts on monarch populations. If these events become too frequent, it will be difficult for monarch populations to recover.
So how do we help protect monarchs from extinction? The two main ways to decrease the risk of extinction are to increase the monarch’s population size or to decrease the frequency of stochastic events. As many of the stochastic events impacting monarchs are related to changing weather patterns, the most feasible means of mitigating monarch extinction risk is by increasing their population size.
“Species are more likely to face extinction when their populations are small,” said Thogmartin. “Monarch numbers dropped sharply in the 1990s and early 2000s” and afterwards “stabilized somewhat, but at a much smaller population size”. This, combined with the tendency for the population size to “change dramatically from one year to the next … makes them more vulnerable to extinction.”
We can help increase the monarch population size through voluntary conservation efforts that establish native milkweed and nectar plants in areas where habitat has been lost. These actions are especially important in Iowa and the Midwest as 40% of the annual overwintering numbers originate in Iowa and surrounding states. Habitat can be established in all landscapes from agriculture and public lands to people’s yards and even urban environments. Increasing the monarch’s population size decreases the pressures they face from stochastic events as they are more able to withstand sudden population declines. This can help decrease their risk of extinction.
The necessity of increasing monarch habitat was also highlighted in Dr. Thogmartin’s assessment of the monarch’s carrying capacity. A species’ carrying capacity reflects the number of individuals that can be supported based on current habitat availability. For monarchs, this number is based on milkweed habitat availability, which, due to losses over the past few decades, Dr. Thogmartin expected to be significantly reduced. His findings showed that the monarch’s current carrying capacity is estimated to be around 7.91 acres, aligning with their average population size.
The problem with this carrying capacity is that to achieve a sustainable monarch population, scientists recommend an average population size of approximately 15 acres, much higher than the current carrying capacity. This disparity demonstrates that the current available monarch habitat is unable to support a sustainable monarch population, and unless voluntary conservation efforts increase its availability, the limited habitat will continue to restrict monarch population growth.
Luckily, the solution to this problem can be quite simple if we collaboratively work to increase monarch habitat establishment, planting native milkweed and nectar plants in our yards, landscapes, and wherever people are willing. Through voluntary conservation efforts, we can continue to increase monarch habitat availability and in turn help decrease the extinction pressures that monarchs face.
Full article:
Thogmartin, W.E., 2024, Non-negligible near-term risk of extinction to the eastern migratory population of monarch butterflies—An updated assessment (2006–22): U.S. Geological Survey Open-File Report 2023–1097, 10 p., https://doi.org/10.3133/ofr20231097.
Additional citation:
Rendón-Salinas, E., Fernández-Islas, A., Mendoza-Pérez, M.A., Cruz-Piña, M., Mondragón-Contreras, G., and Martínez-Pacheco, A., 2023, Area of forest occupied by the colonies of monarch butterflies in Mexico during the 2022–2023 overwintering period: Monarch Butterfly Fund, 4 p., https://www.worldwildlife.org/publications/area-of-forest-occupied-by-the-colonies-of-monarch-butterflies-in-mexico-during-the-2022-2023-overwintering-period