How 10 Million Became 50 Million Monarchs
The Story Behind Changing Scientific Estimates

Scientists really don't know how many individual butterflies are in a monarch colony. For almost twenty years, their best estimate was 10 million monarchs per hectare* of forest. Then something happened that caused them to raise their estimates 5 times higher, to as many as 50 million monarchs per hectare. Here is the story.

The First Estimate: 10 Million
From the moment scientists saw the monarch colonies in Mexico, they have wondered how many millions of monarchs were hanging from the trees. They tried many clever ways to satisfy their curiosity without disturbing the butterflies. They counted and weighed the butterflies on a single branch, then counted the branches on the trees and made a colony-wide estimate. They tagged and released
butterflies, then recaptured some, and figured out a mathematical way to estimate the total number in the colony. After many years, their best estimate was 10 million monarchs per hectare.

A Bad Storm—But a Chance to Count Butterflies
In January of 2002, a terrific storm struck the monarch sanctuary region. Frozen butterflies fell to the ground and most of Mexico's monarchs died that winter. Dr. Lincoln Brower was one of the scientists on the scene. He and teacher Dave Kust had the first and only chance to actually count thousands of butterflies and make a more direct estimate.

 Over 2,000 monarchs were counted in one square meter! Photos Courtesy of Dave Kust

The New Estimate: 50 Million
In one place, 2,241 dead monarchs were counted in a single square meter. This would make 22.41 million monarchs per hectare. Dr. Brower was astonished. This was more than twice as high as the 10 million per hectare estimate—and they were only counting butterflies on the ground. Many more living butterflies still remained in the trees. Dr. Brower made counts at other sites and found an even higher number. Now scientists estimate there could be as many as 50 million monarch butterflies in a hectare.

* One hectare equals 2.47 acres.

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