Numbers Down
This graph shows population data collected every December for the last 18 years. Look how far this year's population was below the 18-year average. What factors could cause the year-to-year changes you see on this graph?
![Monarch Butterfly Population Estimate: Graph](https://s3.us-east-2.amazonaws.com/journeynorth.org/images/graphics/mexico/population_estimate.gif)
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