Monarch Butterfly Migration Update: February 21, 2002
Today's Report Includes:
First: A Quiet Walk Through the Monarch?s Forest
Take time, before reading on, to enjoy these images from the overwintering sites in Mexico.
Try This! What Makes You Wonder?
Why Don?t Cold Butterflies Fall? Challenge Question #4
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Field Notes from by Dr. Bill Calvert
Wednesday, February 20, 2002
A Butterfly Blizzard
The monarchs performed on Tuesday, perhaps the first truly warm day after a long period of cool, rainy, windy weather. At Chincua, millions upon millions were in the air, filling the valley of Zapatero Canyon. Many millions more were basking in the surrounding treetops and wherever exposed to the sun. Surrounded by butterflies, one had the impression of a great, orange snowstorm. Witnessing this spectacular sight were members of the St. Hubert School in Chanhassen, MN, the Prairie Wood School of New London, MN, the Nettleton School and the Ordean Middle School of Duluth, MN. I didn?t realize you could see such a phenomenon this early in the season--it was truly, truly remarkable.
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But What If...Challenge Question #6
The annual mid-winter population estimates were made by Eligio Garcia, of Mexico?s Instituto Nacional de Ecolocia. We estimated the post-storm population size by assuming 20% of the population survived. Much more information is needed to validate this estimate; it?s based on observations at only two colonies, for example, and the mortality rate might be much higher at the smaller, outlying colonies.
However, whatever the precise number, it leaves people concerned: What if this storm had hit last year, when the population was so low?
How Many Monarchs/Hectare?
A Surprise for Scientists After the Storm
Imagine the impossible prospect of counting the millions of butterflies in their winter sanctuaries! To estimate their numbers, Drs. Brower and Calvert did a ?mark, release and recapture? study at one of the sites back in January, 1986. On the basis of that study they estimated 13 million monarchs per hectare. Last May, Dr. Calvert reviewed the data and revised the estimate to 10 million per hectare.
Then came the storm of 2002! Butterflies cascaded to the ground by the millions, and along came Dr. Brower and Dave Kust to count them...
In last week?s update, Dr. Brower shared preliminary mortality data from the samples he collected from the forest
floor at Chincua. At that site, they found 2,241 dead monarchs per meter squared. This works out to be 22.41 million
dead monarchs per hectare. He was astonished. This was more than twice the 10 million/hectare density they had
originally estimated! And remember, this doesn?t even include the living butterflies that still remained in the
Estimates are Estimates! Challenge Question #7
When people hear numbers we tend to take them as fact. But let?s play with these two different density estimates and see how different the numbers of butterflies become. Just think: Nobody really knows how many butterflies there are! We can only estimate.
Are you ready? Before the storm this winter, monarchs covered a total area of 9.35 hectares in all colonies. (See graph above.) How many monarchs were there?
Discussion of Challenge Question #3
Why Count Males and Females?
"Why do you think the scientists were interested in knowing the number of male and female butterflies?"
These students are thinking along the same lines as scientists do! Whenever studying a population, knowing the number of males vs. females is important. What?s more, their size, condition, and lipid content might reveal differences between males and females that affected their survival.
Dr. Brower told us that the sex ratio is generally close to 50:50 in the eastern monarch population. (Remember, you can?t just assume this for all species!)
There are slight differences in the physical condition of males and females in a population. Dr. Brower explained, ?We have it documented, based on 5,211 monarchs collected throughout the year from the Eastern population, that:
(By the way, 2 and 3 are probably related to the fact that the males have to fly faster in order to court females
successfully. Females probably need the extra lipid to go into egg production.)"
Map of Over-wintering Monarchs
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:
1. Address an e-mail message to: email@example.com