Guest post by Kip Hansen — October 17, 2024 — 1,700 words/7 minutes
The magical and mysterious monarch butterfly appears again: leaving lepidopterists (a branch of entomology, the scientific study of two superfamilies of moths and butterflies) perplexed and asking questions they can't answer .
The observational evidence is this:
obviously, [Monarch] Winter populations are declining in Mexico, but some recent studies suggest that summer breeding populations are relatively stable and similar to historical abundance.
What they're talking about here is the monarch butterfly population in the eastern United States—a population that migrates south from as far north as Canada to the mountains of central Mexico every fall. If you are not yet familiar with the mysterious nature of this phenomenon, read my article A Primer on Monarch Butterfly Migration Or any of my previous articles about monarchs.
There's a modern scientific debate about monarch butterflies and their migration—a debate so big that I've dubbed it the “Monarch Wars.” The basis of monarch warfare is:
1) Some government agencies, scientists and activist groups want to declare the monarch butterfly or one of its two northern hemisphere populations an endangered species. In 2022, the U.S. Fish and Wildlife Service “determined the need to list the monarch butterfly under the Endangered Species Act” but excluded at this time Through higher priority listing actions. IUCN unilateral The monarch butterfly was declared endangered and listed on the IUCN Red List of Threatened in 2022. However, the IUCN later delisted monarch butterflies in 2023, declaring them only a “vulnerable species.”
2) Other scientists, groups and interest groups oppose listing this nearly ubiquitous species as endangered – evidence suggests it has not suffered population declines or population dispersal.
3) A third view is that the monarch butterfly migration, as a phenomenon in itself, should be declared endangered.
Unfortunately, the evidence is clear—because all aspects In controversy.
How is this possible?
The latest research is:
Monarch butterflies (Danaus plexippus) have recently experienced dramatic body size declines during their fall migration in habitat [ full pre-print .pdf ]
Authors: Andrew K. Davis, Jordan R. Croy, and William E. Snyder, University of Georgia, Athens, Georgia, USA.
Their summary begins with this line:
“The conservation status of the North American monarch butterfly is a topic of great concern and debate.”
For years I've been calling it monarch war.
Let’s look at the evidence again:
“It's clear that winter habitat is declining in Mexico”
clearly winter colony The amount of habitat in Mexico is declining – in the first 20 years of recording there were some boom years and some trough years of less than 5 hectares, but the entire 20 years showed much greater habitat abundance. Perching populations have been very low in the last decade.
and Summer population?
Pictured above, from Crossley et al. (2022) Posted in Global Change Biology clear indication of summer population danish bush caterpillar Monarch butterflies, seen as vertical orange lines, Not showing a sharp decline the past twenty years, but if anything, it's slightly increased About 0.7% per year. While the increase isn't huge, it's definitely not a sharp, dangerous, or worrisome decline.
[Note: The Mexico overwintering population graph covers three more years than the summer Population study. Those three years were boom years for the overwintering population.]
Recent research by Davis et al. (2024) is approximately the size of the habitats that monarch butterflies form when they stop to spend the night during their southward migration. Davis et al. explain:
“Observations of significant declines in overwintering monarch populations, coupled with an apparent rebound in the summer breeding season, have sparked intense debate about whether monarchs are truly endangered. We used approximately 2,600 citizen scientists to conduct a survey of monarch 'habitat' (fall) Observations of large gatherings of migrating monarch butterflies) to assess whether they are struggling to reach Mexico.
This is an important and interesting question. Are many monarch butterflies unable to reach their wintering grounds in Mexico during the summer?
They summarize it in simple and easy-to-understand language:
“The positive effects of warming and greening along their migratory routes are offset by an inexplicable 80 percent reduction in habitat size, which continues to increase during their difficult southern migration.”
They found “A warm, green flight path” – and feel free to label this CO2-enhanced atmosphere and maybe climate change – already positive impact About the annual migration. Yet there is still “unexplained decline [in during-migration] Habitat size up to 80%”, The decline increases along the southern line.
To the authors' dismay, they concluded:
“This suggests that to save monarchs, we should focus on avoiding well-intentioned efforts such as planting non-native milkweeds, which can harbor parasites, make monarchs sick and disrupt migration.”
I do not think this conclusion is warranted or supported based on the research evidence. This is a valid hypothesis—a possible explanation—but certainly not strongly supported. As with all assumptions, this is guesswork based on some data.
what they said is correct “This suggests that climate change may favor monarch migration overall by creating milder flight conditions and increasing nectar availability along migration routes (but see 24).”
Ah yes, there is one but. You can read it here however. This is a complex calculation of “energy expenditure” that may apply to monarchs migrating at higher temperatures, but certainly not to butterflies that actually migrate.
The reasoning of Davis et al. It goes like this:
“That is, while habitat size declines throughout the migratory route, this decline becomes increasingly severe along the migratory route. The latitudinal gradient of decreasing habitat size is consistent with increased mortality during migration and/or the gradual abandonment of migration by monarchs as they move southward. This apparent migratory disruption may be a missing piece of the puzzle to explain the relative stability of summer populations in the Midwest and the decline of overwintering populations in Mexico.”.
Unfortunately, Davis et al., despite conducting an excellent survey, provide no clear or actionable findings. “So what is causing the apparent, dramatic decline in habitat size seen in these data? We can only speculate,…”
And presumably they did. While none of these speculations are new, they include:
1) Epidemic of monarch butterfly parasites Electra algae (“OE”) has increased over the past decade, some say “tenfold.”
2) Extensive cultivation of exotic milkweed, e.g. milkweed and Horn melon Homeowners and even land managers. Some believe this will lead to more parasitism. Some studies suggest that emperor caterpillars that feed on these species may be stunted or weak. Or the longer growing seasons of these species may attract monarchs to stay north later in the fall.
3) Thousands of captive monarch butterflies are raised and released each year. Some studies suggest that monarchs in captivity are weaker or less capable of navigation. There are concerns that hybridization between captive and wild monarchs could harm the entire species.
4) Finally, year-round resident populations of monarchs appear to be increasingly common and growing along the western and southern edges of the species' range. Vague concerns include interbreeding between non-migratory and migratory monarchs, leading to genetic dilution. This is the hypothesis that “monarchs gradually gave up immigration as they migrated south.” Another possibility is that more and more monarch butterflies are migrating south, finding it very suitable, and settling there for the winter.
Like every butterfly, all monarchs found in northern regions are the descendants of monarchs that migrated north from overwintering in warmer locations in the south, where winters are cold and cold. Therefore, the stability and even growth of northern summer populations seems inconsistent with the extremely low numbers of monarchs overwintering in Mexico. Yet it is what it is.
West Coast Monarch Hotel
A similar but more profound situation was found in the West Coast monarch population during the winter of 2020-2021 and the following year (2021-2022):
You see, recovery is biologically impossible. Therefore, a false version of Sherlock Holmes' famous quote can be applied: “When you have eliminated the impossible, what remains, no matter how improbable, must be the truth”. In order to achieve recovery (which is a physical fact) we must eliminate the impossible, and to do this requires: Monarch butterflies spend the winter somewhere – just not where expected and therefore not counted.
This is one of the speculative explanations for the mystery of the eastern population—monarch butterflies may be overwintering in Mexico after all, just not in the usual or known locations, and thus cannot be counted.
Bottom line:
1) We simply don’t know how or why this happens: There are far fewer monarch butterflies overwintering in Mexico, but northern summer populations are stable or even growing.
2) There are four or five good speculative answers, but none stand out or have stronger evidence. This situation is more likely to be combination one or more suggested explanations; or dire something else.
3) One thing is now fairly certain: the slight warming (about 1.5 degrees Fahrenheit) that has occurred on the North American continent over the past 20 years (often called climate change) is not the cause, but more likely a mitigating factor that favors monarch butterflies.
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Author comments:
Last summer, where I live in the United States, I saw very few monarch butterflies.
I'm fascinated by the evidence about monarchs and increasingly realize that much of it is driven by a “save the monarch” bias. As in almost all fields of science, researchers' desires directly lead to their discoveries. Those who look for disaster will find it, as was the case with monarchs. But these researchers were stymied by other evidence – stable summer populations.
In science, evidence should lead to an appropriate framing of a situation, and if the situation is considered to be an undesirable condition, then the evidence should suggest solutions to correct or mitigate the discovered causes of the undesirable condition.
If the cause of the problem is only suspected, rushing to come up with a solution sets us on a merry-go-round and often makes the original problem worse.
As for the monarchs, I still favor planting native milkweed in your gardens (private and public), discouraging mowing of highway edges and the edges and spaces between cornfields, and of course, acknowledging that we still don't understand what's going on The monarch should bring more and better research.
Thank you for reading.
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