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Monarch Butterfly Population Graph

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January 27, 2015
Population Decline
Monarch numbers are down and people are wondering why. This article examines the factors that contributed to the recent decline, and the downward trend over the past decade.

Recent Years: Dramatic Decline
What factors have caused the monarch population to decline so dramatically during the past three years? Habitat loss has been accelerating, and weather events have played a role.

1. Summer 2012: Drought
Drought and excessive heat in the mid-continent during summer 2012 resulted in low reproduction. Historically, the U.S. Corn Belt has produced half of the monarchs that migrate to Mexico. Drought in that region is cited as major factor that contributed to the decline.

2. Winter 2012/2013: Record Low
The area of forest covered with monarchs was only 3 acres, compared to the 17-acre average.

3. Spring 2013: Cold Temperatures
Monarchs migrate from Mexico into the southern U.S. in March. There they produce the first spring generation, then die. The new generation was slow to develop and late to migrate northward. Unusually cold temperatures across the mid-continent was the cause.

4. Summer 2013: Unproductive Breeding
Low winter numbers, cold spring temperatures, and a delayed spring migration meant few monarchs entered the northern breeding grounds in June. Monarchs were scarce all summer. Many observers didn't see a monarch until August, other people saw none at all. Observers also saw little evidence of reproduction. Population recovery depended on a productive breeding season.

5. Winter 2013/2014: New Record Low
The area of forest covered with monarchs plunged to only 1.65 acres, 90% below to the 15-acre average. This record low compares to a peak of 51.8 acres recorded in 1996. At 20 million butterflies per acre, this year's population is estimated at 33 million monarchs compared to the peak of 1 billion in 1996.

6. Summer 2014: Ideal Conditions
Following two unfavorable summers that were partially responsible for the population crash in 2012, weather conditions during last summer's breeding season were ideal. If the population had encountered poor breeding conditions in summer 2014, scientists were concerned that the migration would reach its extinction threshold.

7. Winter 2014/2015: Still Dangerously Low
Although the number of monarchs overwintering in Mexico has increased from last year's record low, the population remains 80% below the historic average. There are 57 million monarchs compared to a long-term average of 300 million and peak of 1 billion.

Monarch Butterfly Population Graph
Dr. Chip Taylor
Conservation Challenges and Opportunties
Monarch Butterfly Population Graph
Why the Decline?

"Numbers are really down, but the monarchs will come back."

Dr. Chip Taylor
Monarch Watch


Past Decade: Downward Trend
The population graph shows the size of the population overwintering in Mexico since record-keeping began in 1994. What factors have caused the monarch population to decline so dramatically during the past decade?

1. Challenges on Breeding Grounds
The monarch cycles through 3-5 generations during the breeding season. Only the final generation migrates to Mexico. Monarch caterpillars feed exclusively on milkweed. Historically, the U.S. corn belt has produced half of the monarchs that migrate to Mexico. However, milkweed habitat has been greatly reduced in the region due to:

New agricultural practices
Milkweed is being eradicated from corn and soybean cropland by the increased use of herbicides on genetically modified, herbicide-tolerant crops (GMO's).

More acerage is being planted in herbicide-tolerant corn to meet increased demands for biofuels. High corn prices push more land into production.

Weather extremes
Drought and excessive heat on the breeding grounds in summer 2012 is cited as a key cause of the record-low population in Mexico the following winter, and reinforces concerns about the potential effect of climate change.

2. Challenges on Wintering Grounds
Monarchs migrate to a unique forest habitat in central Mexico. The butterflies depend on the forest microclimate to survive the winter. The entire population overwinters together in a very small region.

Illegal logging continues to threaten/deplete the monarch's forest and disrupt its delicate microclimate.

Concentration of population
The fact that the monarchs concentrate in one small region for the winter makes the entire population vulnerable to a single storm or drought. Fire and disease are an added risk in years when the population is small; in winter 2013/2014, 78% of the population was concentrated in a single sanctuary (El Rosario).

High-volume, unregulated ecotourism threatens the integrity of the monarch's winter refuge

Other socio-economic pressures
The needs of the people who live in the region must be balanced with the needs of the monarchs. Humans and monarchs have competing needs for food, water, shelter and space.

3. Challenges of Migration
Migration is inherently risky. Monarchs must find habitat to meet their needs every day of the journey. Like links in a chain, the loss of one habitat component could break the monarch's annual cycle.

Need for International Cooperation
People of separate countries and different languages must join together to address these conservation challenges.

"The Monarch butterfly unites the three countries of North America in peace. It is an ambassador of peace which requires protected areas and ecosystems that are preserved through sustainable agricultural and forestry practices. Let us continue to work together to maintain a healthy and balanced ecosystem for all North America." President Jimmy Carter

Elizabeth Howard
Director, Journey North

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