Are Monarchs an “invasive species” in California?

The much-loved butterfly might be a recent arrival

Monarch butterflies on Eucalyptus trees, California. (Photo by USFWS Pacific Southwest Region)

[This was originally written as part of a chapter in the book I am co-authoring with Nikki Hill, which is tentatively entitled, “Don’t Blame the Messenger: A critique of the ‘invasive plant’ narrative.” This section might get dropped from the final draft so I’m presenting it here because it definitely took some time to research and write.]

The massive migrations of the Monarch butterfly (Danaus plexippus) are an iconic and much loved phenomena. Many people are familiar with the marvelous images of groves of trees in Mexico and California covered with thousands and thousands of the beautiful insects, their orange and black wings so densely packed they look like the trees’ foliage. Because of plummeting Monarch numbers since 1997, the Union for the Conservation of Nature has designated the Monarch’s migration a “threatened phenomena” that should be protected, and the plight of the butterfly has become, quite deservingly, a cause celebré.

What is less known is that in California, the annual event could be a recent historical development connected to European colonization. At the very least, its scale has been much enlarged by the introduction of Eucalyptus trees, which are the main roosting sites for overwintering Monarchs, and have extended the range of winter roosting south along the coast. It’s possible that Monarchs did not arrive in California until almost 100 years after the 1769 cut-off that demarcates “native” species from “introduced” species, and, by some definitions of the word, Monarchs would be “invasive.” After all, an introduced plant with an equivalently widespread range that is still expanding would certainly be subject to that label.

“The Columbus Hypothesis,” presented by Dick Vane-Wright in 1993,ⁱ proposes that Monarchs did not arrive in California until the mid-19th Century, when the butterfly was also expanding its range to other, more far flung locales. The spread of the Monarch across the Pacific and Atlantic Oceans in the 19^th century is thoroughly documented, in part by museum specimens going back to 1856,ⁱⁱ and is well-known among Monarch researchers and some enthusiasts. The butterflies reached Hawai’i in the 1840s, Bermuda by 1850, Australia and the Caroline Islands by 1860, the Azores, Tonga, Samoa and Vanuatu/New Caledonia by 1870, Indonesia and England by 1880, France and Spain by 1890, and China by 1900.ⁱⁱⁱ Since that initial pulse, they have since spread to other places, though not as far or as fast. A 2019 review identified “90 countries, islands, and island groups where monarchs occur (74 with recent documented sightings.)”^iv

Vane-Wright theorizes that this range expansion was caused by the wide-scale deforestation of North America by European colonists in the late 18^th and 19^th centuries for agriculture which created the sunny, disturbed habitat in which Common Milkweed (Asclepias syriaca) thrives. With this increase in their food supply, Monarchs underwent a population explosion and responded by seeking new homes, in what Vane-Wright calls “overspill.” He further suggests that the annual migrations to and from Mexico also date to this period, rather than being a long-term practice.

If Vane-Wright is correct, this boom could have followed a previous period of greater Monarch abundance. The forests that were deforested by Europeans were the product of reforestation after the cessation of Native American landscape management practices that utilized fire to enlarge prairies and keep woodlands in check. This tending activity was stopped by the massive depopulation of Native Americans from European diseases in the 16^th and 17^th centuries. (Further, it has been suggested that this reforestation was significant enough to stimulate climatic cooling and bring about the “Little Ice Age” in Europe, although this is debated.^v)

A 2003 paper, “Monarchs across the Pacific: the Columbus hypothesis revisited,” questions some of Vane-Wright’s premises and conclusions. Their analysis of records does not suggest the simple east-to-west wave or front that Vane-Wright posits. They also note that in most cases, sightings of Monarchs in given locations were preceded by the human-introduction of some species of Milkweed, which could have served as the vector of entry. Though, once established on one island, they spread to others in independent movements and events by riding trade winds, following ships, or hitching along with plants again. They have also been blown long distances by weather events, such as in 1870 when hundreds of Monarchs were carried to Australia from New Caledonia by cyclone winds. Numerous such cases have been reported in both the Pacific and the Atlantic up to the present, which sometimes result in the establishment of new populations and sometimes not.^vi

The case of New Zealand as an exceptional case is worth mentioning. Though Monarchs were reported there by Europeans by 1840, the Maori claim the butterfly is a long-time resident: “Early New Zealand writers [in 1874 and 1878] considered the butterfly endemic to their Islands, with Fereday [a naturalist] referring to older Maoris who were insistent that the butterfly was present before Europeans arrived.” In fact, the Maori have a traditional name for the insect, kãkâhû.^vii How and when the Monarch arrived there is unknown, but illustrates how the concept of “native range” should reserve space for such anomalies.

Genetic data has been released that tells yet another story. As described in 2014’s “The genetics of monarch butterfly migration and warning coloration,” Zahn, et al., used genome sequencing to compare Monarch populations around the world, and identified three different lineages outside North America: Pacific, Atlantic, and Central/South American. The North American samples showed gene flow throughout, with the exception of south Florida’s year-round population, which is distinct. Their modeling also suggested something that surprised them:

[O]ur PSMC analyses [a modeling method] suggest Pacific and Atlantic dispersal events may have occurred as early as 2-3000 years ago. Because this timing was unexpected, we performed a follow-up analysis... and this also yielded split times of 2-3000 years ago between North America and the Atlantic and Pacific populations. Interestingly... analysis further indicated recent bottleneck recovery in the past 200-500 years, perhaps pointing to trans-oceanic dispersal events that were initially seeded thousands of years ago but which spread widely only within the last 200 years.

It’s hard to know what to make of this. As they themselves ask, if their estimates are accurate, “Where has the monarch been for all this time?” especially given that known host plants were not available in these areas until later. They add: “We stress that the ancient Atlantic and Pacific dispersal scenarios we outline here are speculative, but plausible, and they would be in line with our genomic results.” Given that this work is such an outlier, we’re going to consider the jury to still be out.

Regardless, whatever the mechanism or its ultimate historic causes, a rapid range expansion of the Monarch butterfly occurred in the second half of the 19^th century and then slowed. As a 2019 global survey illustrated: “While the monarch now resides in many worldwide locations, they do not reside in all climatically suitable locations.”^viii That is, the inertia of the 19^th century pulse depleted itself before it carried them everywhere they could have gone.

All of this still leaves the question of California. The first reported incidence of overwintering Monarchs in the state was not until 1864. The Spanish had founded Monterey as the capital of California in 1770, and though they recorded many other flora and fauna, they don’t mention the mass roosting. In an alternate timeline where Spanish missionaries discovered the yearly gathering, they might have dubbed it the Rebaño de María del Rosario (rebaño = “flock”) since it occurs around the Memorial of Our Lady of the Rosary in the Catholic liturgical calendar.

Also telling is the fact that famed botanist David Douglas spent two winters in Monterey, from 1830-1832, and never mentioned the butterflies despite the fact that he explored the woodlands where they now roost. In fact, Douglas was the first European to “discover” the Monterey Pine, a native tree that hosts overwintering Monarchs. Botany geeks familiar with Douglas might take this as final and clinching proof that the Monarchs weren’t there yet, because if he had found them, wouldn’t he have named them Danaus douglasii? <jk>

Eucalyptus trees were introduced to San Francisco in 1853, and by 1860, these first trees were 50 feet tall. People were impressed by their fast growth and their usefulness for timber, shade, and windbreaks and by the 1870s, the trees were being planted widely.^ix This timing is too late to suggest that the Monarchs followed the Eucalyptus into the state, as they might have followed introduced Milkweed to Pacific islands. Nor could they have followed Tropical Milkweed into California, because it wasn’t planted in the state until the first decade of the 20^th century. (California also has over a dozen native species of Milkweed that are utilized by Monarchs as larval hosts and nectaries, but some localized populations of the butterfly prefer Tropical.)

Regardless, the fact that sightings of overwintering Monarchs in California become regular starting in the mid 1860s after no records at all, has the appearance of a newly established phenomena, especially in the context of the simultaneous spread of Monarchs to so many other places. We are curious whether any California Native Americans have stories about Monarchs and their mass roosting, which they surely would have noticed, or if, as the Maori do, they have any traditional names for the butterfly.

If it is accepted that Monarchs did not arrive in California until nearly a century after the 1769 cut-off date, are we concerned about them being labeled as “invasive” and subjected to an extermination campaign as are so many plants in the state? No, we think that would be a bridge too far even for the most fervent invasion ideologue. As far as our research found, Monarchs are not labeled “invasive” in any of their adopted non-North American homes, even in places like Hawai’i and Australia, where the dogma is a really big deal for some people. It’s reassuring to know that, at least in this one case, the anti-“invasive” fanaticism stops somewhere.

References:

iVane-Wright, Dick. “The Columbus hypothesis: an explanation for the dramatic 19^th century range expansion of the monarch butterfly.” Migration 1993. https://www.pnas.org/doi/10.1073/pnas.2001283117

iiFreedman, Micah G. “Two centuries of monarch butterfly collections reveal contrasting effects of rage expansion and migration loss on wing traits.” PNAS Nov. 17, 2020 vol. 117 no. 46 28887-28893 https://www.pnas.org/doi/epdf/10.1073/pnas.2001283117

iiiZalucki, Myron P. and Clarke, Anthony R. “Monarchs across the Pacific: the Columbus hypothesis revisited.” Biological Journal of the Linnean Society, 2004, 82, 111-121. https://www.researchgate.net/publication/247673833_Monarchs_across_the_Pacific_The_Columbus_hypothesis_revisited

ivNail, Kelly R., Drizd Lara, Voorhies Kristen J. "Butterflies Across the Globe: A Synthesis of the Current Status and Characteristics of Monarch (Danaus plexippus) Populations Worldwide" Frontiers in Ecology and Evolution 7 2019 DOI=10.3389/fevo.2019.00362 https://www.frontiersin.org/articles/10.3389/fevo.2019.00362

vAlexander Koch, Chris Brierley, Mark M. Maslin, Simon L. Lewis, Earth system impacts of the European arrival and Great Dying in the Americas after 1492, Quaternary Science Reviews, Volume 207, 2019, Pages 13-36, ISSN 0277-3791, https://doi.org/10.1016/j.quascirev.2018.12.004. https://www.sciencedirect.com/science/article/pii/S0277379118307261

viZalucki & Clarke.

viiZalucki & Clarke.

viiiNail.

ixBracewell, Ronald. “Eucalyptus globulus, Tasmanian Blue Gum” Trees of Sanford. https://trees.stanford.edu/ENCYC/EUCglo.htm