James Hansen: A monarch butterfly resurgence?

  • Published on September 4th, 2018

In Quest of a Broken-Wing Butterfly I speculated in 2014 that the Monarch population may have fallen below a critical level.  We did not see more than a single Monarch on any day on our property.  The number of butterflies reaching the mountainous area in Mexico where they hibernate had fallen in the winter of 2013-14 to a few percent of levels two decades earlier.

Monarch butteflies in mexico

By James Hansen

Male and female Monarchs do not travel as pairs in their annual migration, so I wondered if a tipping point had been passed.  Chance encounters of male and female may have become so rare that the species was headed to extermination.  In Butterflies and Flamingos I cited empirical evidence in the summer of 2014 that added to my concern.

That concern was alleviated when the number of Monarchs reaching Mexico jumped in the next two winters, with the number in 2015-16 exceeding the minimum two years earlier by about a factor of five. A single female can lay 500 or more eggs, which helps explain the rapid rebound. Such reproductive capability is likely the ‘strategy’ that the species uses to survive natural weather and climate fluctuations that can greatly affect its food supply, the milkweed.

The number of Monarchs declined in the most recent two winters, but the number last winter (2017-18) was still much larger than the 2013-14 minimum (see Chart 1).

This summer has been remarkable. My grandson Jake and I counted 15 Monarchs as we walked two
blocks from a beach on Long Island. There was a fallow potato field on one side of the road with
occasional patches of milkweed. However, on this same stretch for the past decade, we never saw more
than 2 or 3 Monarchs. Similarly, we have seen Monarchs every day in recent weeks at our home in
eastern Pennsylvania, more than at any time in the 15 years we have lived here.

This abundance may be a regional fluke due to the wet weather this year, which caused an explosion of all weeds, including milkweeds. It will be interesting to see how many Monarchs make it to Mexico this
winter. The Western Monarchs, which mostly overwinter near the California coastline and in Baja
California, Mexico, may fare quite differently, given this year’s drought on the West Coast.

Monarch population area
Chart 1. Monarch population is estimated by the area of the colonies in which they cluster together for winter hibernation. (Graph updated from Brower et al., Insect Conser. & Diver., 5, 95, 2011.)

We had pulled the common milkweed out of our garden this year, because it attracted no butterflies last
year and there is lots of milkweed on the edges of our property. However, one or more of the female
Monarchs managed to find inch-high milkweed shoots in the garden and chose to lay eggs there! You
can see three larvae on a single shoot in the photo below.

monarchs on milkweed

I tried transplanting a milkweed to the garden, but it withered instantly. So instead I transferred the larvae to a healthy milkweed. I am going to try to minimize my interference in nature, but I hope they add to the population in Mexico this winter!

My next Communication should really be the promised “Acceleration of Global Warming.”

(James Hansen is Director of the Climate Science, Awareness and Solutions Program at the Earth Institute of Columbia University. He is the author of the Climate Change book “Storms of my Grandchildren”. Main image CC by Pablo Leautaud on Flickr.)

About the Author

Dr. James Hansen directs the Program on Climate Science, Awareness and Solutions of the Earth Institute at Columbia University. The Guardian calls him "The Father of Climate Science" - his 1988 Congressional testimony on climate change helped raise broad awareness of global warming. He is often in the news advocating for a price on carbon and other forms of climate action, divesting from fossil fuels, and helping his grandchildren sue the government over their future. He is the author of "Storms of my Grandchildren".
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