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Monarch Butterfly Migration Update: February 14, 2002

Today's Report Includes:

Single Storm Kills Over 75% of Eastern North America?s Migratory Monarchs
Between 12-16 January 2002, a severe winter storm hit the monarch sanctuary region deep in central Mexico. Dr. Lincoln Brower and colleagues released mortality estimates this week. Based on data collected from the two largest sanctuaries, over 75% of the population was killed by this single storm.
  • Sierra Chincua Sanctuary: 74% of the butterflies were killed
  • El Rosario Sanctuary: 80% of the population was killed
  • At the two colonies combined, an estimated 200-272 million butterflies were killed.

Significantly, these two huge colonies are the winter sanctuaries of 2/3 of eastern North America?s migratory butterflies. The other 1/3 of the butterflies are spread among other smaller sites in the vicinity. While scientists have not yet visited these outlying sites, mortality rates are feared to be similar because the sites are small, their forest habitat is less pristine, and because the rain and cold were prolonged in the region.

All Photos by Dave Kust

Dr. Brower described the catastrophe in an interview with Journey North: ?The ground in these two colonies was littered with monarchs that had an eerie flat, pallid appearance that I have never before seen--like wet leaves. The heavily packed piles of butterflies were up to 13 inches deep, and even those that were still alive may not have been able to struggle out. The main survivors were buried alive, covered by dead butterflies that were insulating them (from the cold).?

In an interview with National Public Radio Brower said, ?The most macabre aspect of the whole thing was sticking my hands down in and realizing at the bottom of the pile there were a whole bunch of them still alive.?

Killing Conditions: Wet Followed by Extreme Cold
For monarchs, the weather pattern that occurred in mid-January was the recipe for disaster: Heavy rains were followed by clearing skies and plummeting temperatures. This storm was an historic extreme in that the rains were heavy during the peak of the dry season in Mexico, and the temperatures were exceptionally cold. Monarchs are essentially tropical butterflies and cannot tolerate sub-freezing temperatures for very long. When they are wet, they die at warmer temperatures than they would if dry.

Said Brower, ?Never in the 27 years we?ve been studying in Mexico have we known temperatures to have fallen this low. I think the temperatures dropped below -8C and froze all the wet butterflies...except those in the middle of the clusters...?

Discussion of Challenge Question #1
Like an Umbrella and Blanket

Last week we asked, "How do you think a forest can serve as an umbrella and a blanket for the monarchs? In your answer, explain how deforestation could change both kinds of protection."

Students from Ferrisburgh Central School in Vermont replied, ?The trees in the forest are like an umbrella because they have branches spread over the ground. The many leaves on the tree will protect what is under the forest. A blanket keeps your heat in so it doesn't escape into the air, and trees can keep the temperature warmer. If there aren't enough trees to keep the butterflies dry and warm, then they can't live.?

It?s important to emphasize that this year?s storm was an extreme, and a high level of mortality may have occurred even with the best forest cover. However, for scientists, conservationists and government officials alike, the historic storm of 2002 will serve as a cautionary reminder: If deforestation continues at its present rate, a single storm even less severe would be capable of decimating the over-wintering population.

?There's always a probability of a severe storm like this,? Brower told NPR, ?but at the hands of humans there's been a 44 percent degradation of the forest over the past 28 years. And the rate of forestry in the area is increasing. This storm shows that we need to really protect the central core area where their survival is really threatened.?

Meteorologist?s Perspective: Satellite Views of the Storm
We wondered whether satellite images were available to document the strength and scope of the storm and cold front. Dr. Dave Dempsey of San Francisco State University responded with these impressive animated video loops showing the jet stream, visible and infrared views. (The sanctuary region is marked with a red "+".) From Dr. Dempsey:

"A cold front swept across central Mexico from the northwest, followed by cold, clear weather. The infrared and visible satellite loops (below left and center) show the broad band of clouds that accompanied the cold front and produced rain and even snow. A cold front is the leading edge of a "tongue" of cold air that protrudes southward from its source farther north. High in the atmosphere, the jet stream blows around the edges of the tongue, forming a dip or "trough" in the jet stream pattern. The jet stream loop (below right) shows a narrow trough dropping unusually far south into central Mexico, indicating the arrival of a tongue of cold air." From here, the most useful addition would be a quantitative estimate of the relative rarity of the January event which requires a familiarity with the climatological data base that I don't have (though I have no doubt that someone out there does)."

Click to View Full-sized Images
Courtesy of San Francisco State University



Jet stream

Natural Selection: Meet the Survivors
From an evolutionary perspective, scientists are interested in this event as an example of natural selection at work. Just think: The monarchs that migrate across Eastern North America in the future--perhaps to your own backyard--will be the offspring of those that survived the storm. Any characteristic that helped them to survive will be in the gene pool of future monarchs. For example, might these survivors:
  • Be more tolerant to cold than the ones that died?
  • Be stronger, fatter, or otherwise more physically fit than those who died?
  • Can you think of other possible differences, physical or behavioral, that enabled some to survive?

...or was it just chance!?

Outlook for the Future
Scientists are now left wondering how long it will take for the monarch population to rebound. Mexico's over-wintering sites harbor all of eastern North America's migratory monarch breeding stock. What long-term effect will this storm have on monarch populations, given such a high mortality rate?

First, it?s important to remember that monarchs, as insects, are capable of high reproduction rates. A single female can lay hundreds of eggs. Second, scientists simply don?t know enough about monarch population dynamics to make predictions with confidence. Dr. Orley Taylor, Director of Monarch Watch, wrote an excellent analysis of the storm (see link below) and made this important observation:

"This is the lowest known number of butterflies at these sites over the last 27 years. In retrospect, it appears to be fortunate that the January freeze occurred this year, a year in which the overwintering population was robust (about 100 million butterflies). Even though estimated mortality due to the January freeze is extremely high (>80%) the number of surviving butterflies may be sufficient to recolonize the breeding areas without a long-term depression of the population. Had this storm occurred last season when the overwintering population was at an all time low (28.3 million), it is likely that it would have taken the population many years to return to normal levels of 60-120 million overwintering butterflies.?

Storm in Monarch Sanctuaries: Links to Additional Information

Spring Migration Data Takes on New Importance
In the aftermath of this event, scientists will watch with interest as the remaining butterflies move north this spring to breed.

?It will be important to follow the population closely this spring as it moves north," said Dr. Karen Oberhauser, who encourages volunteers to participate in Journey North's migration study and her own study of monarch reproduction, the Monarch Larva Monitoring Project. "...this will give us realistic estimates of the cascading effects of the storm mortality.?

Journey North: Spring Monarch Migration Study
Observers report the first sightings of adult monarchs, the emergence of first milkweed (the host-plant monarchs need to reproduce), and the first evidence of reproduction (first eggs, first larva).

Monarch Larva Monitoring Project
The overarching goal of the project is to better understand how and why monarch populations vary in time and space, with a focus on monarch distribution and abundance during the breeding season in North America. On a weekly basis throughout the breeding season, observers monitor monarch reproduction at local sites.

Thinking on Your Feet
Discussion of Challenge Question #2

Challenge Question #2 asked, "How do you now if a butterfly is dead, or just paralyzed by the cold?"

Warm it up! thought Ryan in Miss Bailey's Third Grade Class at Citrus Elementary School in Vero Beach, FL: "You can put the dead or paralyzed butterfly in warmer weather. If it moves, it was just paralyzed. If it doesn't, it's dead."

?The scientists should randomly take some of the butterflies and warm them up to see if they are still alive,? said students in the Ecology Class Port Washington High School in Port Washington, WI.

Estimating Monarch Mortality: The Scientists? Methods
And this exactly what the team did. As these pictures show, Dr. Brower, Dave Kust, his family and visiting friends, collected hundreds and hundreds of butterflies from the sanctuary floor. They selected butterflies by making quadrants measuring .25 meters square, chosen randomly in the colony. They gathered all of the butterflies from inside each measured area. These are called ?measured area samples.?







Twenty nine random samples were taken from the Rosario and Chincua colonies. The team then carried the butterflies down to a lower a elevation where it?s warmer--the Kust?s house in downtown of Angangueo. For several days they worked through the samples, separating the butterflies as alive, dead, or ?moribund.? (Moribund means the butterfly is not yet dead, but is flight impaired even when warm, and may die in the future.)







They also counted the number of male and female butterflies in each sample.

Challenge Question #3
?Why do you think the scientists were interested in knowing the number of male and female butterflies??

(To respond to this question, please follow the instructions below.)

Here are the data from the six samples from Chincua. Together they represent a combined area of 1.5 square meters. They counted 4,288 butterflies of which:

  • 599 were still alive (13.9%).
  • 327 were moribund (7.6%)
  • and the rest, 3362 were dead (78%).

Thus the density of dead butterflies was 3,362 per 1.5 meter squared. This works out to be 2,241 dead per meter squared, or 22.41 million dead monarchs per hectare.

Population Estimates Before and After the Storm
Warming the butterflies would resolve the question about WHETHER the grounded butterflies were truly dead, and counting them revealed the percent killed. But what about the living butterflies? How many remained? To estimate the damage they realized they needed to know:
  • The population size BEFORE THE STORM
  • The population size AFTER THE STORM

Fortunately, every winter Eligio Garcia, of Mexico?s Instituto Nacional de Ecologia, measures the colonies, and had done so before the storm. The size of the colony is estimated in two different ways: 1) by measuring the area the colony occupies, and 2) by counting the number of trees filled with butterflies. After the storm the Chincua and El Rosario were revisited. (Other sites are still to be measured.) Here are the results:
Sanctuary BEFORE Storm AFTER Storm

Percent Killed

Chincua 2.96 hectares 0.76 hectares


El Rosario 2.69 hectares Not Measured*


El Rosario 942 trees filled 187 trees filled


* In El Rosario it was not possible to measure the area occupied by the butterflies after the storm, due to dispersal of the colony into loosely aggregated clusters. For this reason, tree counts were used.

These are the percentages the scientists and Mexican government reported to the press, and probably appeared in the news coverage you read. Think about the two different methods that were used to estimate mortality. List all the challenges of each method. Does information from one method help interpret data from the other method?

Field Notes from Dr. Bill Calvert:
Post-Storm Visit to Sierra Chincua Sanctuary

Wednesday, February 13, 2002
We arrived at the Rincon Villa Lobos, watered up, a transportable lunch of quesadillas made with blue corn tortillas, and started walking. The scene in the Sierra Chincua was rare. Cold, wet, wind laced with clouds blew in from the west. At times the mountains were completely obscured. The ground was so wet that our dust masks, normally needed at this dry time of year, was useless. There was no dust. This is a wet, wet winter. We?ve seen this before many times, but this year seems especially wet, the ground is wet, the trees were dripping as the clouds condenced on the boughs, there were pools of water on the trails. There was no dust!

The Chincua butterfly colony was very remote, an hours? walk, but the walk was incredible through high altitude managed forests. Things were different. Many plant species were brown or yellow, with no green foliage. They had frozen. They looked the way your tropical plants look when you forget to bring them inside. Millions of small branch-lets from the fir trees littered the ground, brought down by the combination of snow, ice and wind. At the colony the first impression was there was little mortality. Nowhere could be seen the piles of dead butterflies described in the press. But in the air there was a faint stench of decaying butterflies. Presumably the ground is littered with them, but they were in the ravine where we were not taken.

The Chincua colony was located in Zapatero Canyon at the boundary line between the municipalities of Sengio and Angangauo. The usual sight of trees completely festooned with butterflies was apparent. The colony was approximately 600 meters long by 30 meters wide but with many thin spots. Using the estimate of 10 million butterflies per hectare there were about 10 million butterflies in the colony. (So 1 hectare, that?s a very rough estimate.) Plenty of butterflies for all who wished to see them. But rumor has it that things were much worst at Rosario, the neighboring colony that I won?t see until I return next week.

Map of Over-wintering Monarchs

More Sightings Needed Before Migration
This map shows where monarchs have been reported this winter. Please send your observations NOW if monarchs are present in your area.

How to Respond to Today's Challenge Questions:

IMPORTANT: Answer only ONE question in each e-mail message.

1. Address an e-mail message to:
2. In the Subject Line of your message write: Challenge Question #3.
3. In the body of EACH message, give your answer to ONE of the questions above.

The Next Monarch Migration Update Will Be Posted on February 21, 2002

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