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Try This!

How Many Monarchs in Mexico This Year?
As a rule of thumb, monarch biologist Dr. Bill Calvert estimates that there are 13,000,000 monarchs per hectare. Using this figure, you can estimate the number of monarchs that over-wintered in Mexico this year. You can also compare the population size from one year to the next.


Hectares of butterflies

x 13 million butterflies/hectare

Estimated number of monarchs that overwintered in Mexico this year


Graph the population data from the winter of 1994/1995 to the present. Then answer these questions:

  • In which year was the population the highest during the past 7 years?
  • In which year was it the lowest?
  • Calculate the average population size, based on 7 years' data?
  • How does this year's population compare the average?
  • Describe this year's population as a percentage of the highest and lowest years.
  • Look at data from the two largest sanctuaries, Chincua and El Rosario. How much do the sizes of the colonies vary from year to year?
  • Which sanctuary would you say in the largest, based on 7 years' data?

Low numbers of monarchs make the migration very difficult to track this year. Here are some comments from people who are watching for monarchs:

"The monarch activity here is definitely below what I have observed over the past 5 years. The egg counts and adult sightings continue to be low. The few eggs we've found have all disappeared within 24 hours, presumably to fire ant predation."
Dr. Bill Stark, Clinton, MS

"I have not seen another monarch after the first one on April 9th!! No eggs or caterpillars, either, on any milkweeds that I have checked. Strange year!!"
Dr. Gary Ross, Baton Rouge, LA

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