End of a migration? Monarch levels at worrying low in Mexico

Monarch butterflies cover only 1.65 acres of the forests west of Mexico City, a new report says - compared with 2.93 acres last year and more than 44.5 acres at their recorded peak in 1996.

After finishing Tuesday's story about the extinction of the passenger pigeon -- the species left the planet forever 100 years ago this September -- it was disheartening to learn of yet another creature's precipitous decline.

Yesterday, the World Wildlife Fund and the Mexican government announced that the population of wintering monarchs in Mexico declined to their lowest level since scientists started keeping records in 1993 -- 20 years ago.

Officials said that while the monarch is not in danger of extinction at the moment, its migration is. 

Every year, monarchs leave their summer homes in the U.S. and Canada and fly up to 2,800 miles to mountain forests in central Mexico for the winter. It's a two-month journey that the WWF says is the second-longest migration of all known insects. (The longest belongs to a dragonfly in Africa.)

It is one of the marvels of nature. No monarchs make the full round trip. So somehow, an intergenerational memory tells the insects where to go.

Since there are still millions of monarchs, scientists count not the individuals, but the size of the territory they occupy.

The yearly totals, listed here, vary widely.  The area was at a high of nearly 45 acres in 1996; this year, the monarch occupy just 1.65 acres, down from 2.94 acres in 2012.

Officials blamed a decline in milkweed plants, due to herbicides used in U.S. agriculture. Plus, the insects experienced higher-than-normal temperatures and storms during their migration. Also, they face habitat loss throughout their range, including in Mexico, where the trees they live in are illegally logged, officials said.

Monarch expert Lincoln Brower, an entomologist at Sweet Briar College in Virginia, wrote in an email to the Associated Press that "the migration is definitely proving to be an endangered biological phenomenon." He blamed genetically modified "herbicide-resistant corn and soybean crops and herbicides in the USA," which "leads to the wholesale killing of the monarch's principal food plant, common milkweed."

There are monarchs in many parts of the world, so they would not go extinct, the AP reporter wrote. "The butterflies can apparently survive year-round in warmer climates, but populations in the northern United States and Canada would have to find some place to spend the bitter winters. There is also another smaller migration route that takes butterflies from the west to the coast of California, but that has registered even steeper declines."

Every fall, monarchs are a common sight along the New Jersey coast as they migrate southward. I've sat on a deck and watched waves of orange and black wings coast by.

New Jersey Audubon's Cape May Bird Observatory has been monitoring their numbers since 1992, and they, too, have seen a decline. In 2013, the average number of monarch seen per hour over the migration season fell to 12.74.  Only two other years saw lower numbers.  The highest number was 359.8 in 1999. 

It must have been amazing to watch, that year. At the height of the season, 1,500 monarchs were flying by every hour.

Many gardeners in this region have begun planting milkweed to help the monarch.  The sign of success is to have a glorious stand of orange-ish flowers at the beginning of the year, and raaged plants at the end.  That means the monarchs have come and laid their eggs, and the larvae have gotten the food they need!

Here's a monarch blog with more information.

And if you want to read a wonderful novel about monarchs, you can't do better than Barbara Kingsolver's Flight Behavior.