Monarch Butterfly Population Increases Dramatically

Monarch Butterflies

Monarch Migration News: March 3, 2016
By Elizabeth Howard

Inspired by the monarch population rebound, people are celebrating the good news and continuing with conservation efforts. What caused the increase?

Monarch Butterflies in Mexico
A Time for Celebration! Photo by Heather Jersild

Good News from Mexico: Population Rebound

The number of monarchs overwintering in Mexico has more than tripled from last winter's level and has increased dramatically from the record lows of the past 3 years. The clustering butterflies cover 4.01 hectares of forest. The population contains 200 million monarchs compared to a long-term average of 300 million and a peak of 1 billion.

Why the Increase?
Favorable breeding conditions in summer 2015 are credited for the population increase according to Dr. Karen Oberhauser of the Monarch Joint Venture.

"Good weather in the growing season this year allowed us to produce about the maximum number of monarchs as possible from the habitat we have, but we know that this amount of habitat can also produce many fewer monarchs in a bad year. All we need to do is look back to 2013 when approximately the same amount of habitat produced less than one hectare of occupied forest in Mexico. This suggests more habitat is needed to reach our goal of 6 hectares."

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.

"I'm ecstatic to see the increase, but cautious that it is still low by historic standards, and virtually equal to 2011, which dipped again afterwards. So... let's celebrate and then continue our efforts!" said Kit Goodwin.

Get Ready for Monarchs
The first signs of migration are expected any time. Welcome the monarchs back to the breeding grounds for the 2016 season with the flowers and milkweed they'll need to produce a new generation.

"I'm doubling up my milkweed garden this spring. If everyone did, maybe we'd have 8+ hectares next winter," tweeted Keith Chasteen from Kentucky.

Monarch butterfly population in Mexico: Chart

How the Population is Measured

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?

What does the graph reveal about the monarch population?

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Next Update March 10, 2016