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Why Has The Magnificent Monarch Butterfly Migration Slowed To A Trickle?

Why Has The Magnificent Monarch Butterfly Migration Slowed To A Trickle?

Written by Joanna M. Foster

Every November, one very special forest in central Mexico is transformed into a quivering cathedral of monarch butterflies. The Oyannel fir forest in Mexico’s Sierra Madre mountains is the wintering ground for tens of millions of butterflies that migrate from as far north as Canada. It’s a natural wonder — the delicate bangled insects coat everything in orange and drip from the tree branches — but it’s also at risk of being lost forever.

The number of monarch butterflies that migrates across the United States each year to Mexico has dropped to an all-time low. World Wildlife Fund Mexico announced Wednesday that just 33.5 million individuals are wintering in Mexico this year. That may sound like a lot of butterflies, but back in 1997, there were more than 1 billion — covering 45 acres of forest with butterflies. This year’s migrants are in just 1.65 acres, or about one and a quarter football fields.

Although the number of butterflies varies from year to year — the long term average over the past 20 years of record keeping is 350 million — this year’s number is the 9th consecutive yearly measurement below the long term average.

Researchers have identified three major factors that are driving the decline: deforestation in Mexico, agriculture displacing key milkweed habitat in the U.S., and episodes of extreme weather along the migration route.

The deforestation in Mexico is leading to less dense tree cover in the monarch’s wintering area which makes the butterflies incredibly vulnerable to unusual weather. In 2002, for example, a single storm killed about 75 percent of the wintering population.

The ongoing drought in the Southwest has also taken a toll on the species. The butterflies depend on being able to fatten up on wildflower nectar in Texas to have enough energy reserves to make it through the winter. When the wildflowers can’t survive the drought, the butterflies are in danger of starving.

Other studies looking at the connections between climate change and monarch migration have found that monarchs need consistent cold triggers to continue migrating south to Mexico in the fall — without those cold conditions, monarchs in the midst of migrating south can actually reorient themselves and fly north where they cannot survive the winter.

The story of monarch butterflies confused by climate change was at the center of New York Times best-selling author Barbara Kingsolver’s most recent novel, Flight Behavior.

The third culprit in the monarch’s decline is the ubiquity of genetically modified crops that are resistant to the weed-killer, Round Up. Thanks to these plants’ engineered hardiness, Round Up is now liberally applied across agricultural fields. This kills milkweed plants, which would otherwise thrive around fields and in ditches. Milkweeds are the only type of plant that monarchs use for laying their eggs.

Few researchers fear that monarch butterflies will actually go extinct, but some do worry that the migration may collapse altogether. While the exact mechanism by which the monarchs make their astounding journey of 4,000 miles each year is not known, some scientists hypothesize that the butterflies rely on chemical cues left behind by last years’ migrants. If not enough butterflies use the route, those chemical signals may not be strong enough to lead the butterflies back and forth from their summer and winter homes.

The announcement of this year’s disappointing butterfly numbers comes on the heels of the 20th anniversary of the North American Free Trade Agreement, which saw the United States, Mexico, and Canada sign environmental accords to protect migratory species. The symbol they chose to represent that pledged cooperation? None other than the monarch butterfly.

 

This post was originally published in ThinkProgress

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Photo Credit: akumar

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107 comments

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3:30AM PST on Dec 7, 2014

thanks for sharing :)

1:49AM PST on Feb 14, 2014

Sadly noted. We have a lot fewer Monarchs this year and I was wondering what the cause may be.

11:51AM PST on Feb 11, 2014

We used the see lots of butterflies in our garden here in southeastern Pennsylvania. We built a small pond and planted lots of butterfly bushes. You could sit there and they would flutter around you. It was so beautiful and peaceful. The last several summers the butterfly bushes are still there ... the butterflies are not. Now, sadly, I know why. It absolutely breaks my heart.

8:03AM PST on Feb 6, 2014

This article was very informative. Hadn't factored in the drought or lack of cold signals - doesn't sound too promising, does it.

1:28PM PST on Feb 5, 2014

HUMANS !!!

10:00AM PST on Feb 5, 2014

Should be obvious, it's the pesticides that Monsanto & other chemicals force on farmers & others,

8:56AM PST on Feb 5, 2014

Their trees are cut down, places they were used to have probably been degraded (by man of course) etc. What I'm sure of is that man is responsible for the disappearance of those immensely courageous butterflies. They should deserve our respect, they only get death.
Heartbreaking?

6:35AM PST on Feb 5, 2014

I saw what was left of the monarchs that fly to Monterey [California],They cluster on the trees in a rather small area. The trees had been scheduled to be cut down [they call that development] but local citizens got together, pooled their money and bought the property to give the monarchs a place to mate. That year their numbers were down by, if I remember correctly, 90%.

6:16AM PST on Feb 5, 2014

I saw what was left of the monarchs that fly to Monterey [California],They cluster on the trees in a rather small area. The trees had been scheduled to be cut down [they call that development] but local citizens got together, pooled their money and bought the property to give the monarchs a place to mate. That year their numbers were down by, if I remember correctly, 90%.

4:14AM PST on Feb 5, 2014

Yep, something else that the human species is destroying. Ironically there was a tv programme on in the UK fairly recently about the migration of these butterflies, but it was an oldish film and everything was OK then.

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