Monarch myths revisited

Debunking the myths of nativism—especially those that justify the eradication of non-native trees—is the task we have assigned ourselves, which requires us to revisit a few of the misconceptions about monarch butterflies in California.

Monarch Butterfly. Creative Commons
Monarch Butterfly. Creative Commons

When application for endangered status for monarchs was filed in August 2014, a few new monarch myths emerged and have since been faithfully repeated by native plant advocates who are demanding the eradication of our urban forests.  The monarch migration in California is using predominantly non-native trees, which should afford those trees some protection.  Unfortunately, it has only produced more convoluted theories that deny the value of non-native plants and trees to monarchs.

Myth #1:  The California migration of monarch butterflies prefers native trees for their winter roost. 

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

A study of the trees used by monarch butterflies for their winter roost in over 300 different sites in California reported that the vast majority of monarchs are using eucalyptus:

“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%.” (1)

Unfortunately, this fact has been obscured by a small study of a few selected sites used by monarchs during their migration.  Griffiths and Villanova (2) observed the monarch migration in a few sites in Monterey and San Luis Obispo counties.  They report that the monarchs moved around among three tree species including eucalyptus, suggesting to them that Monterey pine and cypress are equally important to the monarchs.  While we don’t doubt that this may be true, we don’t think we can generalize from this study because it was conducted in the small native range of Monterey pine and cypress.

The California migration of monarchs spends the winter months roosting in tall trees in 17 counties along the coast of California, from Mendocino County in the north to San Diego County in the south. (1) Most of that expanse is outside the native range of Monterey pine and cypress.  Griffiths and Villanova do not acknowledge that both Monterey pine and cypress are being eradicated outside their native range for the same reason that eucalyptus is being eradicated, i.e., they are considered “alien invaders” where they have been planted outside their native range.  Here in the San Francisco Bay area, for example, 500 Monterey pines were destroyed on the Marin headlands a few years ago and an untold number of Monterey pines will be destroyed by the FEMA projects in addition to those that have already been destroyed here.

If, in fact, monarchs do have a preference for Monterey pines and cypress for their winter roost, they do not have that option outside of the small native range of those trees in Monterey County. 

For the record, we should tell you that we are just as opposed to the pointless destruction of Monterey pine and cypress outside their small native range as we are opposed to the destruction of eucalyptus.

There is paleontological evidence (fossil cones) that Monterey pines lived in the San Francisco Bay Area several times in the distant past.  That finding was reported (4) in Fremontia, the journal of the California Native Plant Society.  The author asked that Monterey pines be allowed to remain where they lived in the past because the species is threatened in its small native range.  Unfortunately, her advice has been ignored by native plant advocates, who continue to demand that all Monterey pines be destroyed where they have been planted outside their present native range.  This extreme viewpoint is one of the reasons why native plant advocates have earned their reputation as fanatics.

Myth #2:  The California migration of monarch butterflies used exclusively native trees before eucalyptus was planted in California.

Those who wish to discount the value of eucalyptus to overwintering monarchs often assume the California monarch migration predates the planting of eucalyptus in California in mid-19th century.  That assumption supports their claim that all of our eucalyptus can be destroyed without having a negative impact on monarch butterflies.   In fact, there is no historical record of the monarch migration until the mid-19th century.  The historical record of the monarch migration was reported by Vane-Wright (3), who tells us the California monarch migration is probably a 19th century expansion of the range of the eastern monarch migration, from east of the Rocky Mountains to Mexico.  Recent molecular analysis of the monarch migration confirms that the eastern and western migration of monarchs in North America are genetically identical, suggesting that the populations might be dispersing east and west from their Mexican winter roost. (5)

Monarch caterpillar on milkweed. Tilden Botanical Garden
Monarch caterpillar on milkweed. Tilden Botanical Garden

Monarch butterflies are not usually eaten by birds because their host plant contains a toxin that makes birds sick and the monarch’s warning colors broadcast that fact.  The warning colors of butterflies that are toxic to birds are often mimicked by other species of butterflies to confer that protection.  There is a monarch mimic, the Viceroy, in the eastern US, which occurs in California only in a tiny bit of riparian habitat in southeast California.  “The lack of mimics suggests the [monarch] may not have been here long enough for any to evolve.” (6)

Myth #3: Non-native species of milkweed is harmful to monarch butterflies.

Monarch butterflies lay their eggs on milkweed and their larvae, the caterpillar, feeds exclusively on milkweed.  Many native plant advocates believe that the monarch requires a native species of milkweed.  They are mistaken in that assumption.  Wikipedia lists over 35 species of milkweed (genus Asclepias) all over the world and many are known to be used by the widely dispersed populations of monarchs.

The dispersal of monarchs from their original range in North America is approximately 200 years old, according to molecular analysis of populations across the Pacific Ocean (Hawaii, Samoa, Fiji, New Caledonia, New Zealand, Australia) and across the Atlantic (Spain, Portugal, Morocco).  These dispersals are assumed to have been aided by human transportation of both milkweeds and monarchs and extreme weather events.  “For example, monarchs were recorded in Australia in 1870 and were most probably carried there on cyclonic winds from a source population in New Caledonia.” (7)  These populations do not migrate and are therefore genetically distinct from the ancestral population of North American monarchs as a result of genetic drift.

In many of the homes of new populations of monarch butterflies there was no native species of milkweed before being introduced simultaneously with the monarch populations.  Although there are numerous members of the milkweed family native to Australia, monarchs do not appear to utilize the native species, preferring the introduced species of milkweed.

Tropical milkweed (Asclepias curassavica) Creative Commons
Tropical milkweed (Asclepias curassavica) Creative Commons

In California, a tropical species of milkweed is popular with gardeners (Asclepias curassavica).  Unlike the native species of milkweed, tropical milkweed does not die back in winter.  Gardeners therefore tend to prefer the tropical milkweed because it makes a colorful contribution to their gardens year around.

Of course, native plant advocates prefer native species of milkweed and they justify their preference by claiming that tropical milkweed is harmful to monarchs.  They claim that the monarch parasite (Ophryocystis elektroscirrha) can accumulate on tropical milkweed because it doesn’t die back during the winter.  Tropical milkweed is the only milkweed available in winter.  The parasite disrupts some winter breeding of monarchs, but that breeding would not occur in the absence of tropical milkweed.  If more monarchs are the goal, tropical milkweed is making a contribution to the monarch population.

New scientific research debunks the myth that tropical milkweed is harmful to monarchs.  Leiling Tao (8) studied monarch lifespans when they fed on a variety of milkweed species.  They looked at both resistance to monarch parasite (O. elktroscirrha) infection and tolerance once infected.  They found a complex interaction between species of milkweed the monarchs fed on and the amount of mycorrhizal fungi on the roots of the milkweed.  But one result was clear:  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.  In short, there is nothing about tropical milkweed as a host that is detrimental to monarch survival in the presence of parasites.

Native plant advocates also speculate that tropical milkweed can disrupt the migratory patterns of monarchs because it is available when native milkweed is not available.  Given that monarchs have persisted for 200 years all over the world, using exclusively non-native milkweed and without migrating, this seems an unnecessarily pessimistic concern.  Neither native milkweed species, nor migration are essential to the survival of monarchs as a species.

Peek under the cover story

As we often do on Million Trees, we have taken a peek under the cover story being used by native plant advocates to justify the eradication of non-native plants and trees.  Once again, we find a lot of pessimistic speculation, but little evidence that eradicating non-native plants will benefit wildlife, or conversely that wildlife can only survive in native habitat.  Yes, it was a tedious journey to that conclusion and we thank you for your patience if you have persevered to our optimistic conclusion that wildlife is far more resourceful and resilient than nativism wishes to believe.

(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) Jessica Griffiths and Francis Villablanca, “Managing monarch butterfly overwintering groves:  Making room among the eucalyptus,” California Fish and Game 101(1): 40-50; 2015. Summary also available HERE.

(3) 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.

(4) Constance Millar, “Reconsidering the Conservation of Monterey Pine,” Fremontia, Vol. 26, No. 3, July 1998

(5) Justine I. Lyons, et. al., “Lack of genetic differentiation between monarch butterflies with divergent migration destinations,” Molecular Ecology, (2012) 21, 3433-3444

(6) Art Shapiro, Field Guide to Butterflies of the San Francisco Bay and Sacramento Valley Regions, California Natural History Guides, UC Press, 2007.

(7) Amanda Pierce, et. al., “Serial founder effects and genetic differentiation during worldwide range expansion of monarch butterflies,” Proceedings of the Royal Society of Britain, 281: 2014.2230.

(8) LeilingTao, 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.

“Gardening for Climate Change”

The White House recently released the National Climate Assessment which was prepared by a panel of scientists convened by the federal government.  This report informed us that average temperature increase of only 2° Fahrenheit over the entire country in the past century has produced these changes in the environment:

  • “Summers are longer and hotter, and extended periods of unusual heat last longer than any living American has ever experienced.”
  • “Winters are generally shorter and warmer.”
  • “Rain comes in heavier downpours.”

If greenhouse gas emissions continue to increase at the same pace, the report predicts an increase in average temperature of as much as 10 degrees by the end of the century.  If an increase of only 2 degrees is capable of producing the extreme weather we are experiencing, it is difficult to imagine what we can expect if the temperature increases 10 degrees.

Climate Change Map
Climate Change Map

President Obama announced the report“This is not some distant problem of the future.  This is a problem that is affecting Americans right now.  Whether it means increased flooding, greater vulnerability to drought, more severe wildfires—all these things are having an impact on Americans as we speak.”

The impact of climate change on plant life (1)

Henry David Thoreau recorded the arrival of spring at Walden Pond in Concord, Massachusetts in the 1850s.  His data has been incorporated into the records of his successors, creating a continuous record across 160 years.  Spring arrives in Concord, Massachusetts about three weeks earlier than it did in the 1850s.  This pattern mirrors the changes occurring around the planet according to field studies and satellite images taken from space. 

The response of plants to this change in seasons has varied, according to a study published recently by the Proceedings of the National Academy of Sciences.  Some species of plants reach their flowering peak earlier in the year, while other species are extending their flowering into later in the fall.

Another study speculates that increased temperature isn’t the only factor influencing these changes in flowering patterns.  Increased levels of carbon dioxide also may be affecting plants.  Earlier snow melt may be another trigger for changes in timing of flowering.  The availability of pollinators at the time of flowering is assumed to influence the long term survival of flowering plants.  In other words, the affects of climate change on plants are complex and imperfectly understood.

The implications for gardeners who care about wildlife

The New York Times recently published an op-ed which offered an answer to this question:  “How do we garden in a time of climate change?”  There are probably many answers to that question, so we should understand the perspective of the author of the op-ed, James Barilla.  He describes his background on his website“… James Barilla held a variety of posts in wildlife research and management, crossing paths with wolves and mountain lions in remote wilderness and promoting “mini-beast” habitat in urban schoolyards. He first became intrigued by backyard wildlife while working in England for a land trust, where his job was to create wildlife habitat on the outskirts of a city.”  He has a Master’s Degree in Environmental Science from University of Montana and a Ph.D. in English from UC Davis.  He now teaches creative and environmental writing at University of South Carolina.  He has written a book about gardening to support urban wildlife and articles published by Atlantic and National Geographic Magazine. 

One of Mr. Barilla’s goals as a gardener is to provide habitat for wildlife.  Million Trees is therefore very interested in his answer to the question, “How do we garden in a time of climate change?”  because we are always responding to the perception of native plant advocates that the eradication of non-native plants will benefit wildlife.  Mr. Barilla shares our view that, particularly at a time of a rapidly changing climate, it no longer makes sense to limit ourselves to native plants if we are to provide useful habitat to wildlife:

“In [the] microclimate [of our backyards], extreme gardening means making the yard hospitable for as many species as possible, without worrying so much about whether they originally belonged here or not.  I used to think that tearing out turf and making room for native species like purple coneflower and switchgrass was the best thing I could do.  But things aren’t that simple anymore.  It doesn’t make sense to think in terms of native and nonnative when the local weather vacillates so abruptly.  A resilient garden is a diverse garden.”  (emphasis added)

Mr. Barilla also acknowledges the changing ranges of plants and animals in response to climate change and the need to accommodate those changes if species are to survive:  “…species are disappearing across their native range but flourishing outside it…This phenomenon of species movement and adaptation is likely to become commonplace as the climate changes.”

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

Finally, Mr. Barilla appeals to us on moral grounds:  “we humans are responsible for the current changes.  So we must also be responsible for helping other species survive them.”  He uses the needs of the monarch butterfly as an example of a species that has been particularly hard hit by both climate change and the agricultural practices of humans.  He urges gardeners to plant milkweed—the host plant of monarchs—in their yards.

Scientists in Ohio have concluded that episodes of extreme heat have reduced the population of native butterflies.   Here in California, we can help monarchs by stopping the many projects that are destroying eucalyptus because monarchs use eucalyptus in several hundred locations along the coast of California as their overwintering roost.   

Native plant advocates are putting their heads in the sand

We (and thousands of people with whom we have collaborated in the past 15 years) have made every effort to inform native plant advocates that they are mistaken in their assumption that native plants provide habitat superior to non-native plants.  We have provided them with the many empirical studies that prove otherwise, including one cited by Mr. Barilla in his op-ed:  “One study in Davis, California, found that 29 of 32 native butterflies in that city breed on nonnative plants.  Thirteen of these butterfly species have no native host plants in the city; they persist there because nonnative plants support them.”  This study by Professor Arthur Shapiro and his graduate student was published over 10 years ago.  (1)  It is only one of 5 local studies that report similar findings for every taxon of wildlife:  benthic microorganisms, insects, amphibians, reptiles, birds, mammals.

Anise Swallowtail butterfly in non-native fennel
Anise Swallowtail butterfly in non-native fennel

There are similar studies elsewhere in the country and around the world that also find equal numbers of insects in native and non-native vegetation.  The British Royal Horticultural Society is conducting a 4-year study of insect use of plants.  Their preliminary findings are that insects are equally likely to use native and non-native plants.  Even Doug Tallamy was unable to find evidence to support his mistaken assumption that more insects use native plants than non-native plants.

Yet, native plant advocates refuse to consider the damage they are doing to both the environment and wildlife that is struggling to survive the destruction of their habitat.  They demand the destruction of thousands of healthy trees, storing millions of tons of carbon dioxide that is released into the atmosphere when the trees are destroyed, thereby contributing to climate change.  They demand that herbicides be used to eradicate non-native vegetation and kill the roots of the trees that are destroyed to prevent them from resprouting.

Here are a few specific examples of native plant advocates– and the environmental organizations that support them– refusing to consider the damage being done to the environment and wildlife:

  • Neighbors of Mount Davidson in San Francisco have been trying for several years to discuss plans of the Natural Areas Program to destroy 1,600 trees on Mount Davidson with the Bay Area Chapter of the Sierra Club, which supports those plans.  The Chapter Sierra Club leadership has repeatedly refused to even discuss the issue with the neighbors who are members of the Sierra Club.  The final response came from the Sierra Club Executive Director, Michael Brune who supports the refusal of Chapter leadership to discuss the issue with Club members.
  • The Sierra Club recently announced in its newsletter, The Yodeler, that it has asked the East Bay Regional Park District to destroy 100% of all eucalyptus trees on over 1,200 acres of park land.  East Bay Regional Park District has estimated the average density of the eucalyptus forest on their properties at 650 trees per acre, which means that the Sierra Club is demanding that over 780,000 trees be destroyed in the East Bay.
  • The Sierra Club recently announced that it has asked UC San Francisco to implement its original plan to destroy over 30,000 trees on Mount Sutro in San Francisco.  In making this request, they claim that such destruction will benefit native plants, although the original plan did not propose to plant any native plants.  (These plans are presently on hold, although UCSF is now in the process of destroying about 180 trees they consider hazardous, in the height of nesting season.)
  • San Francisco’s Department of the Environment has submitted an application for funding to create a Biodiversity and Ecology Master Plan which proposes to treat all open space in San Francisco as “natural areas” using the Natural Areas Program as its model.  The Natural Areas Program is presently restricted to 1,100 acres of city-managed park land.  If implemented, this plan could eradicate non-native plants on all city-owned open space as well as private backyards.

The changing climate requires that we reconsider the commitment to native plants in historic ranges because they are probably no longer adapted to those ranges.  They must move if they are to survive and we must accommodate that movement if we want them to survive.  Likewise, we must reconsider everything we are doing to contribute to climate change, including our use of fossil fuels and deforestation.

Taking action

If you are a member of the Sierra Club, please tell them your opinion of their recent demands to destroy more trees in San Francisco and the East Bay than is presently planned by the owners of those properties.  Also, urge them to listen to the concerns of their members regarding the plans for tree removals by San Francisco’s Natural Areas Program.  Their address is:  San Francisco Bay Chapter Sierra Club, 2530 San Pablo Ave. Suite I, Berkeley, CA 94702-2000

If you live in San Francisco and don’t want all open space in the city to be treated as native plant museums, please write to Polly Escovedo (who is considering the grant application to create a Biodiversity and Ecology Master Plan) by May 14, 2014:




(1)    This section is from:  Carl Zimmer, “Springing Forward, and Its Consequences,” New York Times, April 23, 2014

(2)    SD Graves and AM Shapiro, “Exotics as host plants of the California butterfly fauna,” Biological Conservation, 110 (2003) 413-433