The Marvelous and Mysterious Monarch

By Kip Hansen – Re-Blogged From WUWT


marvelous_monarchs-featuredThe marvelous Monarch Butterfly is an icon of biological mystery.  When I was born, circa 1950, monarchs were known to fly north into southern Texas every spring by the millions.  In the Fall, tourists would go to Cape May, New Jersey, timing their visits to watch the beauty and pageantry of the Monarch Migration south again as they are funneled down the southern New Jersey peninsula from New England and points north, headed south to an unknown destination, believed to be somewhere in Mexico.

It wasn’t until 1975, long after I had passed through university, that the location of the monarch overwintering site in the central mountains of Mexico was discovered.  For 40 years, Dr. Fred Urquhart had searched for the monarch’s overwintering site…finally discovering the location in the fall of 1975.  Urquhart wrote of his discovery in National Geographic magazine, which had funded his search, in the August 1976 issue.  He did not, however, reveal the actual location of the butterfly colonies at that time, not even to other scientists.

As this image at the start of this essay shows monarch butterflies begin to move south every August (the orange arrows).


“Unlike most other insects in temperate climates, Monarch butterflies cannot survive a long cold winter. Instead, they spend the winter in roosting spots. Monarchs west of the Rocky Mountains travel to small groves of trees along the California coast. Those east of the Rocky Mountains fly farther south to the forests high in the mountains of Mexico. The monarch’s migration is driven by seasonal changes. Daylength and temperature changes influence the movement of the Monarch.

In all the world, no butterflies migrate like the Monarchs of North America. They travel much farther than all other tropical butterflies, up to three thousand miles. They are the only butterflies to make such a long, two way migration every year. Amazingly, they fly in masses to the same winter roosts, often to the exact same trees. Their migration is more the type we expect from birds or whales. However, unlike birds and whales, individuals only make the round-trip once. It is their children’s grandchildren [or great-grandchildren and even great-great-grandchildren]  that return south the following fall.”  [ link ]

Migrating Monarch populations have been in trouble since the turn of the century as measured by the area occupied at their overwintering site in Mexico — and secondarily at their West Coast overwintering sites on the coast of California.


These two graphs end in 2013 – five years ago – and it looks pretty grim.

So, are monarch butterflies endangered, as so many activist organizations and their press releases claim, threatened with extinction?  Have they been more than decimated by pesticides?  Are they doomed by climate change?

SPOILER ALERT:  Clicking this link will give you the answer! (opens in a new tab)

Those of you not suffering from the desperate need for immediate gratification  can read on . . . . . the story of the monarchs is fascinating.

Let’s give that overall population statistic another more-up-to-date look to  include this winter’s stats from Mexico:


This population (above) represents 99% of the North American migrating population.  As we can see in this graphic, the lowest point was in the winter of 2013-2014 — this winter, 2018-2019, shows an almost ten-fold increase from that time.  The Western Migration population (not shown), however,  is at an all-time low this winter, as bad as that sounds, the Western migrating population (west of the Rocky Mountains) only comprises 1% of the overall population of migrating monarchs in the United States.

What’s that in numbers?

“Researchers have estimated that there are approximately 21.1 million butterflies per hectare, although this number most certainly varies with the time of the winter as the colonies contract, expand, and move. It also varies with the density and size of the trees in the colony. Based on this estimate the largest population of monarchs occurred in 1996-1997 when the colonies covered over 18 hectares and contained an estimated 380 million butterflies. To date the lowest population recorded was in 2013-2014 with 0.67 hectares and approximately 14 million monarchs.” [ link ]

Using Lincoln Brower’s formula for the current 2018-2019 winter gives us an estimated 127,655,000 monarchs hugging the oyamel firs in Mexico this year.  Yes, that’s 127 million.

 A million of anything is a lot — click this link to see an image (in a new tab) of a million dots, one of which is red.

Students of Population Dynamics should find the graph above instructive. The population of migrating monarchs is not part of a predator-prey dynamic but rather a carrying capacity dynamic (in which the carrying capacity of the environment is the limiting factor to population numbers).  The non-linearity of the population dynamic  is evident in the numbers, with typical boom-and-bust behavior,  on top of what appears to be a steady decline — which, when the information to follow here is taken into account, appears to be driven by a change in carrying capacity of the monarch’s North American range.

Are the Monarch’s Endangered by Climate Change?  No.

Naturally, the claims of pending doom for the monarchs includes a very large Climate Change factor.  Nearly every article on monarchs includes that they must be threatened by climate change, either in the present or certainly in the future.  Many of these claims use the RCP8.5 to claim that monarchs will be forced further north in range, looking for milkweed, increasing the length of their migration.  Another angle is that tropical milkweeds, which grow year around in the south of North America, will become more toxic with increasing temperatures harming the monarch caterpillar stage which feeds excusively on milkweed leaves and these imported, invasive tropical milkweeds have a higher toxicity which was shown to increase when grown under higher temperatures.  However, neither of these two hypotheses are well supported by evidence, rather they are fears for the future.

Once the monarchs migrate north to Texas and beyond, they breed and grow throughout the entire northern range, following the blooming of milkweed, many of which grow and bloom all summer.  Monarchs that overwintered in Mexico breed in Texas as soon as they find milkweed plants on which to lay their eggs.  These butterflies then die, but their off-spring keep moving north, breeding generation after generation of new monarchs, each generation staying put if milkweed and nectar-flowers are available, or moving north to follow the seasons.  Monarchs, unlike many bird species, do not have a specific destination to which they head for breeding purposes…they breed all along the way.  Some keep moving — some settle in where they find themselves,  as long as conditions are favorable.  There is no reason to believe that they will not continue to do so,  adapting their migration [or even abandoning the migration as some non-migrating populations have] as the climate changes

The other Climate Change worry is that “the number and intensity of storms will increase”.  That is supposition and has not been found in the data to date. But, because butterflies are fragile, the Monarch Migration can and has been adversely affected by weather:

2002:   “Between 12-16 January 2002, a severe winter storm hit the monarch sanctuary region deep in central Mexico. Mexico’s over-wintering sites harbor all of eastern North America’s migratory monarch breeding stock. Dr. Lincoln Brower and colleagues released mortality estimates. Based on data collected from the two largest sanctuaries, over 75% of the population was killed by this single storm.”  [ The next overwintering season saw Monarch numbers only down by 20% — an astonishing recovery from a 75% mortality event! ]

2010 and 2016:  “Storms earlier this year (2016) blew down more than a hundred acres of forests where migrating monarch butterflies spend the winter in central Mexico, killing more than 7% of the monarchs, according to conservationists.”

“Rain, cold and high winds from the storms caused the loss of 133 acres (54 hectares) of pine and fir trees in the forests west of Mexico City, more than four times the amount lost to illegal logging this year. It was the biggest storm-related loss since the winter of 2009-10, when unusually heavy rainstorms and mudslides caused the destruction of 262 acres (106 hectares) of trees.”

“This year’s storm also appears to have frozen or killed about 6.2m [million] butterflies, almost 7.4% of the estimated 84m butterflies that wintered in Mexico, said Alejandro Del Mazo, the attorney general for environmental protection.”

2016 (recount):  “Approximately 30-38 percent of the monarch butterflies (Danaus plexippus) in the Sierra Chincua and Cerro Pelón overwintering colonies died in the storm, says a team led by Lincoln Brower, Ph.D., research professor of biology at Sweet Briar College, far more than the estimated 7 percent mortality rate cited in initial media reports after the storm that struck between March 7 and March 11, 2016.”

Hurricanes coming ashore in Texas in late-September and October would affect the migration as it heads for Mexico.  Only one hurricane since 1980 fills this niche:  “October 20–24, 2015 – Hurricane Patricia’s remnants bring heavy rain to Texas. The maximum rainfall total is reported to be 20.87 in (530 mm) in Corsicana. This rain causes the flooding and closure of Interstate 45 in that area.”  It does not seem to have affected the migration in 2015-2016 as that year’s overwintering population was actually 3 times larger than the preceding year.

But in the end, we have this cheering news:

“Even so, most monarch scientists don’t believe monarchs will become extinct.”

“The migration won’t last forever. The monarchs will,” said migration expert Andy Davis, Assistant Research Scientist at the Odum School of Ecology at the University of Georgia and editor of the journal, Animal Migration. “The monarchs will adapt just like they have in every other population around the world,” he said.”

Wait a minute!  What does he mean when he says “The migration won’t last forever. The monarchs will.”?

The venerable National Geographic reports:

Are Monarch Butterflies Endangered?

“The short answer is no. Monarch butterflies are actually quite common across the world, with populations occurring as far away as North Africa and New Zealand.”

“However, the subspecies known as Danaus plexippus plexippus is the only one that performs the great North American migration—and these butterflies are increasingly under threat.”

The Smithsonian Institute reports:

“Monarch Butterflies:  ORDER: Lepidoptera | FAMILY: Nymphalidae | GENUS: Danaus | SPECIES: plexippus

Status: Widespread common butterfly, best known as migrant.”

Many areas in the southern United States support year-round populations of Monarch Butterflies, particularly southern Florida.  There are some year-round local populations in southern California, where there are also some western monarchs which migrate to Mexico instead of the California coast.  “In the Americas, the monarch ranges from southern Canada through northern South America. It has also been found in Bermuda, Cook Islands, Hawaii, Cuba, and other Caribbean islands, the Solomons, New Caledonia, New Zealand, Papua New Guinea, Australia, the Azores, the Canary Islands, Gibraltar, the Philippines, and North Africa. It appears in the UK in some years as an accidental migrant.” [ link ]

It is the Monarch Migration, as a phenomena, that is possibly endangered.  And it would be sad to see such a fantastic, unexpected, mysterious phenomena disappear.  No one knows how the super-generation of monarchs knows to fly south and manages to find the same little patch of firs in far off Mexico.


California Monarchs overwintering on eucalyptus. [photo credit – charles the moderator]


What  do experts really think is causing the decline in migrating monarchs?

Monarch populations are limited by the carrying-capacity of their range  — this means that the major factor limiting numbers is the availability of the necessary breeding plant (milkweed species) and adequate food sources — nectar filled flowers.

That said, there are lots of nectar filled flowers in the North American environment but not so with milkweed.    Milkweed is a common weed that loves disturbed soils — plowed fields, roadside ditches, hedgerows, highway verges.  It used to be a major competitor in the corn belt’s millions of acres of corn and soy beans.  What happened to all that milkweed?

Public enemy #1:  Round-up.  A herbicide that just works as designed — it kills plants that are not genetically engineered to withstand it.


Newly introduced “round-up ready” corn and soy beans now allow the between-the-rows milkweed to be killed without damaging the intended crop.  In 2018, 94% of soybeans, 91% of cotton and 90% of corn planted in the US was an herbicide-resistant strain.  All of that acreage used to be prime milkweed habitat, growing between rows and on the edges of farm fields.  Almost all of that historically available milkweed has been lost.

This represents a case where an agricultural practice change brought about by a technological advance in biology has had the intended effect — fewer weeds competing with crops boosting crop yields — but which has been accompanied by an unintended effect — the loss of milkweeds subsequently suppressing populations of Monarchs.

This loss is seen not only in the corn, cotton and soy fields, but on the roadsides and waste land where weeds are being mowed or treated with herbicides.  In my area, the land alongside railroad tracks sports wild native milkweed, but just as it begins to mature, it is mowed down.  The same is happening all over America – plants that we consider as weeds are being eliminated.

Monarchs need milkweed to reproduce, they lay eggs only on milkweed and the young caterpillars eat only milkweed.  More specifically, they need native milkweeds, not the often prettier non-native tropical milkweeds sold to gardeners.

Loss of native milkweed in the North American breeding grounds — Texas to the Great Lakes — is the primary factor that has limited monarch populations and is believed to be the major factor leading to their steady decline since the turn of the century.

There are lots of groups that are promoting milkweed planting (remember to insist on native milkweeds) and butterfly-friendly home gardens that provide ready sources of nectar for the monarchs and other butterflies.    An internet search for butterfly-friendly gardens and milkweed sources will point you to the right places.  (Many of the links in this essay go to monarch-specific sites that contain information.)

The other limiting factor is loss of overwintering habitat in Mexico due to illegal logging.  To correct this, the government there has created a Monarch Butterfly Biosphere Reserve.  There are ongoing reports of continuing illegal logging in the oyamel fir forests of central Mexico, inside the reserve.  There have been damaging storms since the turn of the century (detailed above) [and in all probability there have always been storms]  that have felled trees there and caused mudslides.  But as we see in the population graphs above, monarch populations seem to be able to bounce back.

Note that there are some myths and superstitions about the oyamel — one is that monarchs return to the “same tree” each year, which is utter nonsense.  It is not the same butterfly that left which is returning but a great-great-grandchild.  But it may be true that the monarch swarms do tend to form on some of the trees used in previous years — whatever it is that attracts them to a specific tree may well attract them again — they may leave a scent marker on trees when they use them — research continues on this fascinating subject.

Take Home Messages: 

  1. Monarch butterflies represent a nearly unique phenomena in their North American migration behavior — a beautiful thing to observe — and well worth conserving.
  1. Our North American Migratory Monarchs face challenges in our modern world — loss of available milkweed being the primary problem.
  1. You can help. You can plant native milkweeds (many of which are gorgeous) and butterfly-friendly flowers in your yards and gardens.  You can encourage local authorities to leave milkweed patches unmowed on public land.  You can participate in Monarch Citizen Science programs all across the US:  Monarch Joint VentureUS Forest Service,  the Monarch Larva Monitoring Project, Journey North’s monarch migration tracking program, and several opportunities for large and small project with Monarch Watch.   The Xerces Society itself has programs and links to other monarch programs, including sources of milkweed seeds.
  1. Monarchs are not threatened by climate change — but can be adversely affected by weather phenomena. It is beyond our capability to control the weather, but we can support monarchs and add to their resilience by protecting their overwintering sites (well underway) and act to increase available milkweed in their breeding ranges.



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