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Effect of Snow on Monarchs
Observations from the Sierra Chincua Colony
Contributed by Dr. Bill Calvert
After a snowstorm, I'd expect that most butterflies would
survive, even many of those stuck in the snow. Miraculously, once the snow
melts, I've seen them warm up and finally move off. They might even be buried
for up to a week!
However, the year of the devastating snow storm (1981), we were monitoring
mortality in the Chincua colony and documented tremendous mortality. To
do this we set a transect through the colony and then used random numbers
to locate the position of quarter-meter-squared quadrants. (To get random
numbers we literally picked from a hat.)
We next collected all the butterflies from the quarter-meter-squared
quadrant and placed them into categories. Then we counted the number in
Our categories were:
- preyed upon
- dead without apparent cause
- flight impaired
It was the "dead without apparent cause" category
that most intrigued us, because that was the group that most likely included
the butterflies that froze to death. "Flight impaired" and "moribund"
were those who could pass a flight test once the climate has warmed. (Moribund
were those that fell or glided directly to the ground. Flight impaired meant
that they landed within 10 meters.)
During the overwintering season, we monitored once a month
beginning in December. On December 6th we determined that about 4,000 butterflies
had died due to unknown causes. One month later in early January, this number
had risen to about 5,000 butterflies. But when we monitored after the snowstorm,
on February 8th, our count was over 2.5 million dead. The storm had devastated
the butterfly population!
- Compare regular mortality to that caused by the storm.
How big an impact did the storm have?
So here is a sample set of data so you can see how things
changed after the February snowstorm. (Preyed upon butterflies are not
On the basis of these data:
- What could you conclude about the safety of the
two positions (Bough Cluster and Ground) within the colony? You
may want to convert the raw data to percent of sample to get a better
idea about what is going on!
Science as Inquiry
Use data to conduct a reasonable explanation. (K-4)
Science investigations involve asking and answering a
question and comparing that to what scientists already know about the
Scientists use different kinds of investigations depending
on the questions they are trying to answer. Types of investigations include
describing objects, events, and organisms; classifying them; and doing
a fair test (experimenting). (K-4)
Different kinds of questions suggest different kinds of
scientific investigations. Some involve observing and describing objects,
organisms, or events; some involve collecting specimens; some involve
experiments; some involve seeking more information; some involve discovery
of new objects and phenomena; and some involve making models. (5-8)
National Math Standards
Number and Operations
Understand numbers, ways of representing numbers, relationships among
numbers, and number systems.
Data Analysis and Probability
Develop and evaluate inferences and predictions that are based on data.
Solve problems that arise in mathematics and in other contexts.
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