Monarch Mysteries Update

“I am sick to death of being told you must use natives, especially if a butterfly has no more interest in it than a fire hydrant.” –Professor Arthur Shapiro, Bay Nature, June 2022

Monarch butterfly populations are studied and quantified during the winter, when they are roosting in the shelter of trees, and during the summer breeding season in warmer climates.  These studies tell different stories.  The breeding population in North America seems to be holding steady since the 1990s in many parts of the country, but the over-wintering population has been steadily dwindling during the same period.  As an academic ecologist recently told the New York Times, “’So it’s not really a production problem,’ said Dr. Davis, an author of the new paper. ‘We don’t have fewer monarchs. We have fewer monarchs reaching the wintering colonies.’”

The most recent study of the breeding population of monarchs is based on a huge data set of 135,000 observations in 403 different sites in North America, partly collected by volunteers of the North American Butterfly Association (NABA) annual summer butterfly count since 1993.  The analysis of current population trends reveals interesting clues about the future of monarchs and probably many other butterfly species:

  • The study “used federal data to estimate how much glyphosate was being used in the area around each survey site. They found that in some regions, especially in parts of the Midwest, glyphosate use was associated with declines in abundance.”
  • “But they also documented a countervailing force: climate change. In the northern part of the United States, increasing temperatures were correlated with increases in monarch abundance. This effect was especially pronounced in the Midwest, suggesting that the warming climate might have partly offset the effects of glyphosate in that region.”

The study of monarch breeding populations in North America found that the Southwest was one of the regions in the US where monarch population declines were greatest.  That finding is consistent with the study of academic entomologists, Matt Forister and Arthur Shapiro, of butterfly populations in the West.  They analyzed data from over 40 years of counting butterfly populations (including NABA data) to learn that 450 butterfly species in western states have declined 1.6% per year in the past 20 years, for a cumulative total of 25% fewer butterflies. Although there are several factors—such as habitat loss and pesticides—their study determined that the strongest factor was climate change, particularly warmer temperatures in the fall.

Professor Shapiro explained during an interview on KALW why extreme heat is harmful to butterflies, although the reasons have not been proven yet. Monarchs are one of the butterfly species that is dormant during winter months. They breed in spring when temperatures begin to warm and days become longer. Warmer winter temperatures are reducing the length of dormancy, which increases their need for year-around food and weakens them if there is inadequate food. Extreme heat and drought have an impact on plants, reducing available food for all butterflies.

Studies of Migrating Monarchs

How do studies of migrating monarchs compare to studies of breeding populations in North America?  There are two major migrations of monarchs in North America.  The migration east of the Rocky Mountains spends the winter in Mexico and the migration west of the Rockys spends the winter on the coast of California.  Both of the overwintering populations have plummeted since the 1990s until the winter of 2022 when the population stabilized in Mexico and increased substantially in California.

The increase in the California monarch migration was described by Jessica Griffiths in an article published by the Sierra Club’s national magazine.  That article is significant for several reasons.  The particular roosting site where the population increase was greatest was a eucalyptus grove in Pismo Beach, California:   “We are standing in a eucalyptus grove on a small patch of undeveloped land bordered by farms near the town of Pismo Beach, on the central California coast. The air smells faintly of brussels sprouts and compost, with an overlay of something like Vicks VapoRub—the distinct scent of eucalyptus. Griffiths gazes up at the branches and smiles. There are so many butterflies.”  The irony is that Jessica Griffiths is the author of a deeply flawed study that claims that monarchs prefer native conifer trees to eucalyptus trees for their winter roost.  One wonders if Jessica Griffiths experienced cognitive dissonance as she counted 17,845 monarch butterflies roosting in a eucalyptus grove where only nine monarchs roosted the previous year.

Jessica Griffiths provides an important clue to changes in the monarch migration in the Sierra Club article.  She says monarchs roost in the trees until the temperature rises to about 55⁰ Fahrenheit, when their body temperatures rise enough that they can actively seek the nectar they need to survive.  She says, “They are basically solar powered,” which is another way of saying they are cold-blooded animals that require the heat of the sun to be active.  In the eucalyptus groves that monarchs prefer as their winter roost in California, nectar is close at hand because eucalyptus blooms during winter months, at a time when little else is blooming.

Pismo Beach, November 2021 Source: https://youtu.be/Su2Ma2lUWFY

When the climate changes, entire ecosystems change with it

When days become shorter in the fall, monarchs in California stop breeding and begin their migration to the coast.  Breeding resumes when days become longer in the spring.  But hours of daylight are not the only determinant of the monarch breeding season.  Warmer temperatures at night are triggering the monarch breeding season earlier than in the past.  In fact, some entomologists hypothesize that many monarchs are now breeding year around. The presence or absence of milkweed does not trigger the breeding season, which is determined by hours of daylight and temperature. 

If the warming climate enables monarchs to breed year around, why would be object?  The more monarchs, the merrier, right?  Unfortunately, hobbyist naturalists DO object to altering the timing and location of the breeding of monarchs.  This is a Letter to the Editor of the Yodeler, the newsletter of the San Francisco Bay Chapter of the Sierra Club:

Sierra Club Yodeler, Summer 2022

The author of the letter to the editor of the Yodeler asks us NOT to plant milkweed near the coast or monarch overwintering sites, presumably because she doesn’t want the monarch’s breeding season to begin when and where it has not occurred in the past.  The fact is, the climate has changed and monarchs are responding to those changes.  Who are we to argue with monarchs about what they need to do to survive?

Bay Nature has published an article about monarchs seen in Marin County during their breeding season, where they have over-wintered in the past, but not bred historically.  The warming climate and the availability of perennial tropical milkweed is making Marin County suitable breeding habitat:  “A lot of people have this feeling that without the migration, the monarch is nothing,” says James. “That’s not necessarily true. If we got rid of the migration, the butterflies could still continue. For humans, that would be a pity. But in the ecology of things…it’s not that bad.”  The author of the article welcomes monarchs to Marin County, “A new Bay Area neighbor, adapting to a changing world, making do with what is available, as we all must.”

The monarch migration is not sacrosanct.  Monarch butterflies also live in Central and South America, in the Caribbean, in Australia, and even in parts of Europe and New Guinea. But all of these monarch populations are sedentary, meaning they stay in one place and don’t migrate.  If changes in climate enable monarchs to live and breed year around, why would we want to prevent them from doing so? 

If monarchs can find what they need year around, why should they be forced to migrate? Migration is physically demanding, depleting the physical resources of an animal.  If survival of a species doesn’t require migration, more physical resources are available for other functions, such as increased reproduction or less need for food to fuel the migration.  Images of struggling human migrants come to mind.  Wouldn’t they all be better off if circumstances at home would enable them to stay home? 

A comparable change has occurred in the life cycle of Anise Swallowtail butterflies.  Prior to the introduction of non-native fennel to California, Anise Swallowtails bred only once each year because its native host plant—closely related to non-native fennel–was not available during most of the year.  Non-native fennel is a perennial plant that is available year-around, making it possible for Anise Swallowtails to breed throughout the year.  Thanks to non-native fennel, we enjoy the company of many more Anise Swallowtails.  We should not think of the life cycles of plant and animal species as immutable.  Rather, they are constantly changing to adapt to changes in their environment and adaptation is what will ensure their survival. 

Native vs. Non-native Milkweed?

Hobbyist naturalists also ask that we plant only native milkweed, the host plant for monarch caterpillars.  Such restrictive advice is not beneficial to the survival of monarch populations.  Although a popular opinion among hobbyists, advice against planting non-native milkweed for monarchs is contradicted by scientific sources: 

  • …there is little evidence to support the idea that planting Tropical Milkweeds will weaken Monarch populations and NO evidence to support the idea that Tropical Milkweeds are “trapping” Monarchs and stopping them from migrating…”  American Butterflies, magazine of the North American Butterfly Association
  • A study of lifespan of monarchs breeding on non-native milkweed compared to native milkweed found that monarchs raised on tropical milkweed (A. curassavica) lived as long or longer than monarchs raised on other species of milkweed. They were less likely to be infected, and once infected, tolerated the infection well. (Leiling Tao et.al., “Disease ecology across soil boundaries: effects of below-ground fungi on above-ground host—parasite interactions,” Proceedings of Royal Society of Britain, 282: 2015.1993.)
  • An article from the UC Davis Bug Squad says they plant tropical milkweed and two species of native milkweed. Monarchs have a strong preference for tropical milkweed: “In July, we collected 11 caterpillars from the narrowleaf [native] milkweed; we rear them to adulthood and release them into the neighborhood. But in the numbers game, the tropical milkweed won. From July through today, we have collected a whopping 43 eggs or caterpillars from [non-native] A. curassavica. How many from [native] A. speciosa? Sadly, none.”

Hobbyists theorize that tropical milkweed harbors more parasites than native milkweed because tropical milkweed is a perennial plant, which suggests that parasites could accumulate from one year to the next. If gardeners are concerned about the potential for accumulation of parasites, they are advised to cut tropical milkweed back during winter months. Because tropical milkweed is a perennial, it is available for monarch breeding earlier in the spring than annual native milkweed.  If monarchs breed earlier in the spring, tropical milkweed accommodates earlier breeding.

How to help monarchs

The future of monarchs is uncertain, just as the future of all life in our changing climate is uncertain.  I am betting that monarchs have a future partly because they have survived many changes in the environment for some 50 million years since butterflies evolved from moths.  We can best help monarchs by staying out of their way.  They would also probably benefit if we would stop destroying their habitat, particularly eucalyptus trees and tropical milkweed.

Vegetation changes as the climate changes and animals follow the vegetation they need as they must to survive.  Breeding season of butterflies and other wildlife is also likely to change with the climate. The rebounding monarch population is probably another case of animals moving to find what they need. We should not stand in their way.  They know what they need better than we do.

Birds and butterflies in the eucalyptus forest

The deadline for sending comments to the California Invasive Plant Council (Cal-IPC) about their draft reassessment of blue gum eucalyptus is Thursday, July 31, 2014 (send to info@cal-ipc.org).  We are hoping to inspire you to write your own comment by sharing our personal favorites of some of the goofy statements Cal-IPC uses to justify its classification of blue gum as “moderately invasive.”

Eucalyptus trees do NOT kill birds!

Our regular readers have heard the absurd claim that eucalyptus trees kill birds by “gumming” up their noses or beaks with the nectar of the eucalyptus flowers.  We have published detailed critiques of this claim, so we won’t repeat them because you can visit those posts by clicking HERE and HERE.

Of course, all of this detailed information was provided to Cal-IPC when the original request to reconsider their classification of blue gum was submitted in December 2013.  In their draft reassessment, Cal-IPC now sinks to a new low by claiming Ted Williams as the source of the claim that eucalypts kill birds.  Mr. Williams writes an opinion column published by Audubon magazine, which is appropriately entitled “Incite.”  In 2002, Audubon magazine published Mr. Williams’s opinion of eucalyptus, which he called “America’s Largest Weed.” 

Ted Williams is not a scientist or a journalist.  He is a commentator. His column in Audubon magazine is not entitled “Insight,” because it is intentionally inflammatory.  It engages in rhetoric and hyperbole in support of Mr. Williams’s opinion.  In an article in High Country News, Mr. Williams describes “Incite” as a “muckraking column” and he calls himself an “environmental extremist.”  Citing Mr. Williams as a source of information undermines the credibility of Cal-IPC’s draft reassessment.  Quoting Mr. Williams on the subject of eucalyptus is a bit like quoting Rush Limbaugh on the subject of immigration.

But let’s be more specific with examples of the absurd statements Mr. Williams makes and the evidence that these statements are not factually correct.

The truth about Anna’s hummingbirds

Hummingbird in eucalyptus flower.  Courtesy Melanie Hofmann
Hummingbird in eucalyptus flower. Courtesy Melanie Hofmann

Cal-IPC’s draft reassessments says, “Williams reported that PRBO found that 50% of the Anna’s hummingbird nests [in eucalyptus] are shaken out by the wind, while only 10% of nests are destroyed by wind in native vegetation.”

Cal-IPC’s quotes from Williams are not found in any publication of the Point Reyes Bird Observatory (PRBO), which Williams claims as the source of the information.  Statements about the nests of Anna’s hummingbird are explicitly contradicted by Cornell Ornithology Laboratory, America’s preeminent research institution of bird biology and behavior:

  • “In the first half of the 20th century, the Anna’s Hummingbird bred only in northern Baja California and southern California.  The planting of exotic flowering trees provided nectar and nesting sites, and allowed the hummingbird to greatly expand its breeding range.”
  • “Anna’s Hummingbird populations increased by almost 2% per year between 1966 and 2010, according to the North American Breeding Bird Survey…The Anna’s Hummingbird is the most common hummingbird on the West Coast, and it has thrived alongside human habitation.  Its range has increased dramatically since the 1930s, when it was found only in California and Baja California.  Thanks to widespread backyard feeders and introduced trees such as eucalyptus, it now occurs in healthy numbers all the way to Vancouver, Canada.”
  • “Females choose the nest site, usually a horizontal branch of trees or shrubs 5-20 feet off the ground (occasionally higher) near a source of nectar.  They often build nests in oak, sycamore, or eucalyptus trees…”
  • “They [Anna’s Hummingbirds] are notably common around eucalyptus trees, even though eucalyptus was only introduced to the West Coast in the mid-nineteenth century.”

In other words, the nation’s most prestigious ornithological research institution tells us that Anna’s Hummingbirds have benefited greatly from eucalyptus trees, which provide both winter sources of nectar not otherwise available in California as well as safe, secure nesting habitat.  Since Anna’s Hummingbirds nest preferentially in eucalyptus, their populations would not be increasing if 50% of their nests were destroyed, as Mr. Williams claims.

Red-tailed hawk nesting in eucalyptus.  Courtesy urbanwildness.org
Red-tailed hawk nesting in eucalyptus. Courtesy urbanwildness.org

Furthermore, the implication that eucalypts provide a less stable nest site than native trees is also explicitly contradicted by a study that Cal-IPC cites elsewhere in its draft reassessment.  Stephen Rottenborn studied the nesting choices and reproductive success of red-shouldered hawks in Santa Clara County.  He found that the hawks prefer eucalypts to native trees and that their nests were more successful when they made that choice.  He attributes that greater success rate to the fact that eucalypts are “large, sturdy trees” that provide “greater stability and protective cover.”

“Fourteen of 27 nests in 1994 and 38 of 58 nests in 1995 were in exotic trees, predominantly eucalyptus.  Nesting and fledging success were higher in exotic trees than in native trees in both years, owing in part to greater stability and protective cover.  Most nest trees in upland areas were exotics, and even in riparian habitats, where tall native cottonwoods and sycamores were available, Red-shouldered Hawks selected eucalyptus more often than expected based on their availability.  Of the habitat and nest-tree variables measured at each nest, only nest-tree height and diameter were significantly associated with reproductive success, suggesting that large, sturdy trees provided the best nest sites.  Red-shouldered Hawk populations in the study area have likely benefited from the planting of exotic eucalyptus and fan palms.” (1)

 Magic!  Turning 2 dead birds into 300

Cal-IPC’s draft reassessment says, “Stallcup reported finding two dead warblers and ‘about 300 moribund warblers with eucalyptus glue all over their faces’ over the years, including ‘a large number of gummed-up Townsend’s warblers, yellow-rumped warblers, ruby-crowned kinglets, Anna’s and Allen’s hummingbirds, and a few Bullock’s orioles. Anyone who birds around eucalyptus trees sees it all the time’ (Williams 2002).”

This particular quote from Ted Williams is easily discredited because Rich Stallcup published his theory about birds being harmed by eucalyptus trees (available HERE).  In this publication, Mr. Stallcup reports seeing just two dead birds (one hummingbird and one ruby-crowned kinglet) in the eucalyptus forest during his long, illustrious career as an amateur birder.  He says nothing about seeing “300 moribund warblers” in his publication.  A small measure of common sense enables the reader to evaluate Mr. Williams’s claim:  If Mr. Stallcup had seen 300 dying birds, why would he say he had only seen 2 dead birds in his published article in which he is trying to make the case for removal of eucalyptus?

Overwintering monarch butterflies use predominantly eucalyptus

Monarch butterflies roosting in eucalypus tree.
Monarch butterflies roosting in eucalypus tree.

Cal-IPC’s draft reassessment says, “Natural experimental evidence from mixed stands (native trees mixed with eucalyptus) show that Monarchs do not consistently cluster preferentially on eucalyptus, and at times, appear to prefer native trees in some seasons and locations. (Griffiths & Villablanca 2013)”

This is a misleading statement because it implies that monarchs have the option of overwintering in native trees.  In fact, the reference cited by Cal-IPC is speaking specifically of three species of native trees with small native ranges:  Monterey pine, Monterey cypress, and redwoodsThe study is reporting observations of monarchs within the native ranges of these three tree species.  These tall trees provide a similar microclimate to overwintering monarchs.     However, the native ranges of these tree species are small.

Monterey pines are native in “three disjunct populations in San Mateo and Santa Cruz counties, Monterey County, and San Luis Obispo County.  The native population of Monterey cypress is significantly smaller:  “The native range of the species was confined to two small relict populations, at Cypress Point in Pebble Beach and at Point Lobos near Carmel, California.” Where Monterey pine and cypress have been planted outside their native range, they are being eradicated by the same public land managers who are eradicating eucalyptus.

For example, when UC Berkeley destroyed approximately 18,000 non-native trees over 10 years ago, many were Monterey pines.  Their plans to eradicate 80,000 more trees include all Monterey pines in the project area.  In San Francisco, the plans (SNRAMP) of the Natural Areas Program propose to destroy many Monterey cypresses on Mount Davidson.  The GGNRA has destroyed about 500 Monterey pines on Hawk Hill in Marin County and many Monterey cypresses throughout their properties.

Furthermore, both Monterey pine and Monterey cypress have much shorter lives than eucalyptus.  Monterey pine lives at most 150 years and Monterey cypress about 250 years, compared to E. globulus, which lives in its native range from 200-500 years.  Therefore, even where they are not being eradicated, they will die long before E. globulus and are unlikely to be replanted outside their small native range by public land managers who are committed to a “natives-only” policy.

We are unaware of any attempts to eradicate redwoods outside their native range, in the few locations where they still exist. They seem to have escaped the wrath of nativism. However, the range of redwoods is very narrow:  “The redwoods occupy a narrow strip of land approximately 450 miles (724 km) in length and 5 to 35 miles (8-56 km) in width.  The northern boundary of its range is marked by two groves…within 15 miles (25 km) of the California-Oregon border.  The southern boundary of redwood’s range is marked by a grove…Monterey County, California.”

Although native plant advocates may be willing to plant redwoods outside their native range, they do not have that option because of the horticultural requirements of redwoods.  Redwoods require more water than Monterey pine and cypress and they do not tolerate wind, which prevents them from being successful in many coastal locations, where monarchs overwinter.  Redwoods cannot be successfully grown south of Monterey County where the climate is warmer and drier than its native range.

In other words, monarchs do not have the option of roosting in native trees in most of the places in California where they overwinter.  This is a more accurate description of the behavior of overwintering monarchs and the alternatives that are available to them in about 300 locations along the entire coast of California, where they have overwintered in the past:

Three types of trees were used most frequently by roosting monarchs:  eucalyptus (75% of the habitats primarily Eucalyptus globulus), pine (20% of the habitats; primarily Pinus radiata), and cypress (16% of the Cupressus macrocarpa).  Twelve other tree species were identified…with a combined prevalence of only 10%…The negative sign for this indicator means that habitats had smaller populations when the roosting tree type was a species other than eucalyptus, pine, or cypress…our long-term analysis showed that abundance has historically been greater at habitats dominated by eucalyptus, pines, or cypress than at those with ‘other’ species.  Stands of these three signature taxa may be more likely to produce a community structure and associated microclimate that increases the residence time of monarchs.  Furthermore, these taxa may produce a more attractive landscape architecture in terms of sensory cues to migratory monarchs arriving in a certain region.” (2)

For the record, we will add that we would be happy to have more Monterey pines and cypress and if public land managers would quit destroying them, we would consider them attractive alternatives to eucalyptus.  However, for the moment, we must assume that the crusade against all non-native trees will continue unabated.

What is your personal favorite?

We have shared our personal favorites with you, but everyone comes to this issue from a different place.  We have been flabbergasted by the unfounded claims that the eucalyptus forest is devoid of life.  We wonder if the people who say that, really believe it.  Or is it just one of the many strategies used to justify their demands that our non-native landscape be destroyed?

Please choose your own personal favorite and write your own comment by Thursday, July 31, 2014.  Are you primarily concerned about the herbicides that are needed to prevent the trees from resprouting when they are destroyed?  Are you concerned about the loss of your protection from wind or noise?   Or do you value eucalyptus as a sight screen or for shade in an otherwise treeless environment?  Are you concerned about the carbon loss that will contribute to climate change?  Please help us to save our urban forest from being needlessly destroyed by telling Cal-IPC why they should take blue gum eucalyptus off their “hit list.”

Thank you for your help to save our urban forest.

Update:  On March 13, 2015, the California Invasive Plant Council published its final reassessment of Blue Gum Eucalyptus (available HERE).  Cal-IPC has downgraded its rating of invasiveness and ecological impact from “moderate” to “limited.”  Although the detailed assessment is less than perfect, the over all rating itself is an improvement.  Thanks to those who sent comments to Cal-IPC.


(1)    Stephen Rottenborn, “Nest-Site selection and reproductive success of urban red-shouldered hawks in Central California,” J. Raptor Research, 34(1):18-25

(2)    Dennis Frey and Andrew Schaffner, “Spatial and Temporal Pattern of Monarch Overwintering Abundance in Western North America,” in The Monarch Butterfly Biology and Conservation, Cornell University Press, 2004.

Monarch butterflies in California need eucalyptus trees for their winter roost

Monarchs are probably the best-known butterfly in North America, partially because they are distinctively beautiful, but also because of their epic migration.  East of the Rocky Mountains, monarchs migrate from the Canadian border and the Atlantic Coast to spend the winter months in fir trees in Michoacan State in Mexico.  West of the Rockies, monarchs migrate from the Canadian border and the Pacific Coast to overwinter along the coast of California from Mendocino County to San Diego County, near the Mexican border.

Monarch Butterfly.  Creative Commons
Monarch Butterfly. Creative Commons

No single monarch makes the entire journey.  It takes two to three generations of monarchs to make the entire round trip.  How each successive generation knows the route remains largely a mystery, although theories exist.  There are a couple of fascinating books about the migration that we recommend to our readers.  Four Wings and a Prayer is a book about the 38-year effort of Canadian entomologists, Fred and Norma Urquhart, to understand the migration.  It reads more like a suspenseful mystery than the non-fiction book that it is.  Flight Behavior is by Barbara Kingsolver, one of our favorite novelists because nature is often the subject of her work.  Although it is fiction, it has been carefully researched by Kingsolver who studied biology before becoming a writer.  It is engaging both as a cautionary tale for environmentalists and as a personal redemption story.

The western migration of the monarch

Monarch butterflies roosting in eucalyptus tree.
Monarch butterflies roosting in eucalyptus tree.

We will focus on the western migration of the monarch because that’s our neck of the woods, but also because this migration is one of the reasons why many people who care about nature and wildlife object to the destruction of eucalyptus trees.  Eucalypts are the preferred trees for over-wintering monarchs“Three types of trees were used most frequently by roosting monarchs:  eucalyptus (75% of the habitats primarily Eucalyptus globulus), pine (20% of the habitats primarily Pinus radiata), and cypress (16% of the habitats Cupressus macrocarpa).  Twelve other tree species were identified…with a combined prevalence of only 10%.” (1)

For those who may not know the botanical names, that’s Monterey pine and Monterey cypress that are the runners-up to eucalypts as the most popular trees for over-wintering monarchs.  Although monarchs roost in those trees in their native range on the Monterey peninsula, they also use those species outside their native range.  Unfortunately, just as the eucalyptus is a target of native plant advocates who demand their destruction because they are not native, both Monterey pine and Monterey cypress are targeted for destruction outside their native range.  For example, both Monterey pine and Monterey cypress will be eradicated from hundreds of acres of public land if the FEMA grants are funded in the East Bay.  This is just 150 miles away from where those trees are native and there is fossil evidence that they existed in the East Bay in the distant past.  In other words, most of the trees used by monarchs for their winter homes are in jeopardy of being destroyed by the native plant movement.

Another nativist myth BUSTED!

One of the reasons why we are telling this story is that it is a tidy little example of the justifications fabricated by native plant advocates to support their destructive agenda.  In the case of the monarch, native plant advocates claim that prior to the arrival of Europeans, before eucalypts were planted and Monterey pines and cypresses were planted outside their native range, the monarch used native trees for their over-wintering habitat.  The “assessment form” used by the California Invasive Plant Council to classify Blue Gum eucalyptus as invasive says,  “[The Blue Gum] provides roost sites for migratory monarch butterflies…ecological niches for butterflies and raptors probably formerly filled by native plant species.”  No evidence is provided in support of that statement.  We have also read that claim in comments of native plant advocates on internet articles in response to those who defend eucalypts because they are needed by monarchs.

Like many of the “cover stories” of native plant advocates, this is just not true.  A search of the scientific literature about monarchs enables us to bust this particular myth to smithereens.  It would be simple enough for native plant advocates to look at the evidence before spinning their tales, but it is apparently easier to make it up, especially when they are rarely questioned.  Million Trees exists to fill this informational void.

The historical record of the western migration of monarchs

The earliest record of over-wintering monarchs in California is from 1864, when monarchs were observed over-wintering in Monterey pines in their native range.  Richard Vane-Wright, the scientist who reports this record, explains why he believes it is probably the first incidence of over-wintering monarchs in California:

“’Previous to that, no mention has been found of this interesting phenomenon…The early Spanish chronicles and traditions make no mention of it, although Monterey, a scant three miles distant, was gay with life when the last century came in…even David Douglas, the world famed botanist, and the keenest-eyes of all the strangers who came [to California] is silent regarding it.’…Douglas, the indefatigable fir tree collector, appears to have made no mention of the phenomenon in 1830-1832, despite spending two winters at Monterey.” (2)

Vane-Wright believes the eastern monarch migration to Mexico also began around the same time.  His theory is that the agricultural practices of early settlers, which cleared trees, created a population explosion of the milkweed that is the host plant of monarchs.  More milkweeds resulted in more monarchs and monarchs began to migrate in response to population pressure, he believes.  He calls this the “Columbus Hypothesis.”  (2)

Biological facts explain why monarchs choose these species of trees

Aside from the historical record, the biology of monarchs and the physical characteristics of the trees in which they over-winter explain why these species of trees are required by the over-wintering monarch.  During the late fall and winter, monarchs enter a dormant phase called diapause.  They continue to need nectar and moisture during that period, but they are not very active, so these resources must be close by.  Although they migrate to the coast from Mendocino to Mexico, they are most abundant around the mid-point of that range, where temperatures and rainfall are moderate.  Most of the approximately 250 roosting sites are within 2.4 kilometers of the ocean, so wind protection is important for them while they are roosting.  All of these factors predict the ideal conditions provided by eucalyptus trees:

  • Monarchs need tall trees (of at least 60 feet) because they roost in the intermediate level of the canopy where wind protection is greatest (3)
  • The forest must be dense enough to provide wind protection,
  • The tree canopy must be open so that the roosting monarchs receive filtered sunlight to keep their bodies warm enough.
  • The monarchs need enough moisture for hydration, but not so much that they are soaked and lose their body heat.  So, dew and/or fog provide the ideal amount of moisture.   (1 & 4)

All of these requirements for the monarch’s winter roost point to their dependence on eucalyptus, pines and cypress.  The trees that are native to the narrow strip of the coast of California do not meet these criteria.  They are not tall enough and they do not grow that close to the ocean because they do not tolerate wind.  The native vegetation of that narrow strip of California coast is predominately dune scrub and coastal grassland prairie.  And these are the vegetation types that the ecological “restorations” in the Bay Area are trying to re-create.  These vegetation types will not be suitable habitat for over-wintering monarchs.  Furthermore, plans to drastically thin eucalyptus forests on hundreds of acres of the East Bay Regional Park District will render those habitats useless for over-wintering monarchs.

In addition to the physical properties of eucalyptus, the monarch benefits from the fact that it is flowering from about December to May, while the monarch is roosting in the tree.  The flowers of eucalyptus contain a copious amount of nectar which is also important to the honeybee because it is flowering at a time when there are few other sources of nectar.  One study reported observing monarchs feeding on the flowers of Eucalyptus globulus. (5)

Risky Business

We have mixed feelings about reporting this research about monarchs to our readers because there is some risk to the monarchs in doing so.  The evidence suggests that monarchs did not over-winter in California prior to 1864, after the magical date that nativists have selected to freeze-frame California’s landscape to their nativist ideal.  Here in the San Francisco Bay Area, this magical date is 1769, when the expedition of Portola laid eyes on the San Francisco Bay.  Will nativists declare the monarch an alien invader to be eradicated along with the hundreds of plants and animals they claim “don’t belong here?”  This may seem a far-fetched conjecture, but keep in mind that the European honeybee is being eradicated in some “restorations” because it is not native.  The honeybee is essential to the survival of American agriculture, yet its existence is threatened by the radical agenda of the native plant movement.

That’s the risk we take in reporting this evidence because we hope that it helps our readers to understand the absurdity of the nativist agenda.

Update:  Monarchs have returned to Natural Arches State Beach in Santa Cruz in big numbers.  Here is a link to a report that includes a lovely video of the roosting Monarchs.  

***********************************

(1)    Dennis Frey and Andrew Schaffner, “Spatial and Temporal Pattern of Monarch Overwintering Abundance in Western North America,” in The Monarch Butterfly Biology and Conservation, Cornell University Press, 2004.

(2)    Richard Vane-Wright, “The Columbus Hypothesis:  An Explanation for the Dramatic 19th Century Range Expansion of the Monarch Butterfly,” in Biology and Conservation of the Monarch Butterfly, Natural History Museum of Los Angeles County, 1993.

(3)    Andres Kleiman and Miguel Franco, “Don’t See the Forest for the Butterflies:  The Need for Understanding Forest Dynamics at Monarch Overwintering Sites,” in The Monarch Butterfly Biology and Conservation, Cornell University Press, 2004.

(4)    Kingston Leong, et. al., “Analysis of the Pattern of Distribution and Abundance of Monarch Overwintering Sites along the California Coastline.” in The Monarch Butterfly Biology and Conservation, Cornell University Press, 2004.

(5)    Susan Chaplin and Patrick Wells, “Energy reserves and metabolic expenditures of monarch butterflies overwintering in southern California,” Ecological Entomology, 7:249-256, 1982