Monarch numbers: trends due to weather and climate

The U.S. Fish and Wildlife Service will be making a decision as to whether to list the monarch as either threatened or endangered under the Endangered Species Act as opposed to the current finding which is “warranted [for ESA listing] but precluded [by other higher-priority listing actions]”. That decision will involve many considerations, most important of which will involve an understanding of monarch biology together with the perceived threats to the monarch population.

One threat we have been addressing is the loss of habitat through many new conservation efforts in public and private settings. These efforts, while impressive, need to increase to offset annual rates of habitat loss as well as past losses due to land management. Counter to these positive developments there are negative trends in weather and climate that must be considered. It is these weather and climate-related stressors which, if they continue, will diminish the monarch migration in the coming decades.

Monarchs are being pinched by changes both at the beginning and end of the breeding season.

Increasing temperatures in March and April in the South Region (Texas and Oklahoma) are allowing monarchs to move too far north too soon and September and October temperatures in the Midwest and South Regions respectively are delaying the migrations in a manner that diminishes the number of monarchs reaching the overwintering sites (Taylor, et al 2020). These relationships are summarized, along with the rate of change in temperature per decade, in Table 1. For reference, rates of change of more than 0.5F per decade are extraordinary and rates of 1.0F or higher are indications of truly worrisome trends that are unlikely to moderate or reverse.

Table 1. Rate of temperature change per decade, 1982-2022. Changes of less than 0.5/decade are considered to be inconsequential due to the impact of other factors on monarch numbers. Temperatures of more than 0.5/decade are likely to have either negative or positive effects on the development of the population but are known to have negative effects on both the spring and fall migrations. Blanks represent months when monarchs are absent or nearly so from the city indicated. The city temperatures are strongly correlated with state and usually regional temperatures. To provide readers with a sense of what the trend lines look like, I have included three images (see Appendix) obtained from the data available via Climate at a Glance. Color code: white – neutral, yellow – either, pink – negative, green – positive*.

March
High temperatures in March in Texas and Oklahoma along with SW winds allow monarchs to advance too far north too soon, i.e., beyond the emergence of milkweeds. Spreading eggs out to the north also has the effect of increasing the mean age at first reproduction. This delay in turn reduces the rate of population growth. Data supporting this interpretation is presented in another blog article: Monarch numbers: dynamics of population establishment each spring.

April-May
The increase in temperatures in the South in April and May could have both negative and positive effects. Positive outcomes could include more rapid development of immatures which would lessen exposure to predators and parasites. Negatives might include shorter lives for egg laying females.

June, July, August
The increase in temperature in June the Midwest stands out relative to the smaller increases in May, July and August. The conditions in June should have a positive impact on the development of the population since it will accelerate the development of larvae and shorten the mean age to first reproduction for the cohort(s) developing that month.

September
Higher temperatures in September in the Midwest and Northeast (Culbertson, et al., 2021, Ethier and Mitchell 2023) are having the effect of delaying the migrations through those regions. Delayed migrations have several causes, but all are associated with lower numbers of monarchs reaching the overwintering sites (Taylor, et al., 2019, 2020).

October
The high temperature effect applies to the South Region in October. Since this region is prone to droughts, high temperatures combined with droughts during October can have a significant impact on the lipid levels (Brower, et al., 2015, Hobson, et al., 2020) and are likely to affect adult survival since monarch numbers are lower at the overwintering sites following these conditions (Taylor, et al., 2020). The elevated temperature in 2019 may be one of the reasons the overwintering numbers declined from 6.05ha in 2018 to 2.83 in 2019. The mean temperatures during October 2019 in San Antonio were the highest in the 128-year record.

Other Threats
Other threats include El Niño and the ability of monarchs to enter diapause. Both constitute existential threats to the continuation of the monarch migration as we know it today. I’ll dive into these topics as time permits.

References

Brower, L. P., Fink, L. S., Kiphart, R. J., Pocius, V., Zubieta, R. R., and Ramírez, M.I. (2015). “Effect of the 2010-2011 drought on the lipid content of monarchs migrating through Texas to overwintering sites in Mexico,” in Monarchs in a Changing World: Biology and Conservation of an Iconic Butterfly, eds K. S. Oberhauser, K. R. Nail, and S. Altizer (Ithaca, NY: Cornell University Press), 117–129.

Culbertson, K. A., Garland, M. S., Walton, R. K., Zemaitis, L., & Pocius, V. M. (2021). Long‐Term Monitoring Indicates Shifting Fall Migration Timing in Monarch Butterflies (Danaus plexippus). Global Change Biology. doi.org/10.1111/gcb.15957

Ethier, D. M. and Mitchell, G.W. (2023). Effects of climate on fall migration phenology of monarch butterflies departing the northeastern breeding grounds in Canada. Global Change Biology. doi.org/10.1111/gcb.16579

Hobson, K.A., O.R. García-Rubio, R. Carrera-Treviño, L. Anparasan, K.J. Kardynal, J. McNeil, E. García-Serrano and B.X. Mora Alvarez. 2020. Isotopic (δ²H) analysis of stored lipids in migratory and overwintering Monarch Butterflies (Danaus plexippus): Evidence for critical late-stage southern nectaring? Frontiers in Ecology and Evolution 8:572140. doi.org/10.3389/fevo.2020.572140

Taylor, O. R., Lovett, J. P., Gibo, D. L., Weiser, E. L., Thogmartin, W. E., Semmens, D. J., Diffendorfer, J. E., Pleasants, J. M., Pecoraro, S. D., & Grundel, R. (2019). Is the timing, pace, and success of the monarch migration associated with sun angle? Frontiers in Ecology and Evolution, 7, 442. doi.org/10.3389/fevo.2019.00442

Taylor, O.R. Jr, Pleasants. J.M., Grundel, R., Pecoraro, S.D., Lovett. J.P. and Ryan, A. (2020) Evaluating the Migration Mortality Hypothesis Using Monarch Tagging Data. Front. Ecol. Evol. 8:264. doi.org/10.3389/fevo.2020.00264

Appendix

Figure 1. Temperature trends in March for selected cities. The horizontal line represents the long-term average. Starting in 2004, most of the yearly means are well above the long-term average.