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Population Count is in...
February 16, 2017 by Elizabeth Howard

Mexican officials have announced the size of this year's population. What do the results mean for monarch recovery efforts?

Monarch Butterflies at El Rosario Sanctuary in Mexico
A single sanctuary — El Rosario — contains 40% of this winter's population..
Image Estela Romero

Down 27% but Up from Record Lows
According to this year's official population estimate, this winter's monarch colonies cover 2.91 hectares (7.19 acres) of forest. This is down from last years 4.1 hectares, but still higher than the record low of .67 hectares in 2013/2014.

Higher Than Predicted
Dr. Chip Taylor of Monarch Watch expected the population to be quite low, perhaps at the 2014/2015 level of only 1.13 hectares.

"The count is much higher than I expected and that I had been led to believe by those that had visited a large number of the sites. The fall migration was really low in the east and the far west (west of the eastern Dakotas to the front range of the Rockies) and there was no place where the numbers were recorded as high. All the fall counts - roosts, etc. appeared to be low."

Several factors led to low predictions: the March 2016 storm at the overwintering sites devastated 5-10% of the population, low numbers were reported during the summer breeding season, and the weak fall migration.

Why Higher?
Some reasons the population estimate is higher than expected could include:

Warm fall: Did unusually warm temperatures give more monarchs time to complete their life cycle and fall migration?

Warm December in Mexico: Monarch don't form clusters as tight and dense in warm temperatures as they do when it's cold. The butterflies spread out so the colony occupies more area. The population estimate is a surface area measurement. Could the warm December result in an artificially high estimate?

Goal is 6 Hectares
The target for monarch recovery is a sustained population of 6 hectares. The level must be high enough so that the population's natural yearly fluctuations don't drop below an extinction threshold. For long-term viability, the population must be resilient during conditions in an off-year.

"We can only save the monarch butterfly by working together. Through continued research, education, and protection, we can give future generations the same opportunity my children have: to look in wonder at the beautiful orange, black and white butterfly," said Catherine McKenna, Canada’s Minister of Environment and Climate Change.

 
Monarch butterfly population in Mexico: Chart
Down 27%

Map Showing Distribution of Monarch Butterflies between the 12 Major Sanctuaries in Mexico
Distribution
 
Which of the 12 major monarch sanctuaries in Mexico have the butterflies selected in the past? Pie Chart
Compare Years
 
How many millions of monarch butterflies in Mexico?
Millions? Do the Math

How the Population is Measured

Monarchs migrate to Mexico from across eastern North America. They cluster by the millions in 12 mountain sanctuaries. Because the butterflies are so concentrated, scientists can measure the size of the entire migratory population. How could they count so many butterflies?
How Many Monarchs? Monarchs migrate to Mexico from across eastern North America. They cluster by the millions in 12 mountain sanctuaries. Because the butterflies are so concentrated, scientists can measure the size of the entire migratory population. How could they count so many butterflies?

It's impossible to count individual butterflies, so scientists measure the area of forest covered with monarchs. This picture shows a monarch colony from the air. The trees look orange because they are covered with butterflies. How large an area does this colony cover? Count the trees and estimate the area.
Measuring the Area Covered With Monarchs It's impossible to count individual butterflies, so scientists measure the area of forest covered with monarchs. This picture shows a monarch colony from the air. The trees look orange because they are covered with butterflies. How large an area does this colony cover? Count the trees and estimate the area.

Scientists walk through the forest and decide which trees have enough butterflies to be considered part of the colony. They mark the edge of the colony with flagging tape.
Identifying Trees in the Colony Scientists walk through the forest and decide which trees have enough butterflies to be considered part of the colony. They mark the edge of the colony with flagging tape.

Next the scientists walk around the border of the colony and measure the distance between all of the marked trees. They must visit and measure all 12 of the monarch wintering sites. Their goal is to calculate the total area of forest covered with butterflies.
Measuring the Perimeter of the Colony Next the scientists walk around the border of the colony and measure the distance between all of the marked trees. They must visit and measure all 12 of the monarch wintering sites. Their goal is to calculate the total area of forest covered with butterflies.

The red line marks the border of this colony. All of the trees inside the polygon are occupied by monarchs. The area of the polygon would be calculated for this colony.
Area Occupied The red line marks the border of this colony. All of the trees inside the polygon are occupied by monarchs. The area of the polygon would be calculated for this colony.

Scientists measure all 12 of the monarch wintering sites. Their goal is to calculate the total area of forest covered with butterflies.
Total Area of 12 Sanctuaries Scientists measure all 12 of the monarch wintering sites. Their goal is to calculate the total area of forest covered with butterflies.

The scientists always measure the colonies in December or January. This is the coldest time of year, when the monarchs form their tightest clusters.
Measuring During Cold Temperatures The scientists always measure the colonies in December or January. This is the coldest time of year, when the monarchs form their tightest clusters.

Population data have been collected consistently in Mexico since 1994. The graph shows the estimated area of forest the monarchs covered each winter. Look for trends. Why do you think it's valuable to collect population data every year?
Historic Population Data Population data have been collected consistently in Mexico since 1994. The graph shows the estimated area of forest the monarchs covered each winter. Look for trends. Why do you think it's valuable to collect population data every year?

 

Companion Resources

Monarch Butterfly Winter in Mexico

Journal

Photo Gallery

Text Only Article

Teaching Suggestions

Driving Question

Why is it important to collect and analyze monarch population data from year to year?

Report Your Sightings
 
Report all monarchs you see — adults, eggs, larvae.
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