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The 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 each category.

Our categories were:

  • preyed upon
  • dead without apparent cause
  • moribund
  • flight impaired
  • normal

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 included.)

Location Butterfly status Date  
    Jan 22 Feb 8
Bough Cluster Dead 3 0
  Moribund 26 0
  Flight impaired 19 33
  Normal 61 183
Ground Dead 81 162
  Moribund 13 107
  Flight impaired 1 42
  Normal 48 16

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!

National Science Education Standards

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 world. (K-4)

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.

Problem Solving
Solve problems that arise in mathematics and in other contexts.

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