The Nature Writers of Texas

The best nature writing from the newspaper, magazine, blog and book authors of the Lone Star State . . .

Sunday, February 04, 2007

Where Monarchs Reign
Carol Cullar, Eagle Pass News Guide, week of 2-4-07

Just a few miles due south of here, roughly six hundred fifty, to the Transvolcanic Mountains of Michoacan and some one hundred fifty miles west of Mexico City, there is a tiny region on the globe that is ideal for monarchs.

If this were a fairy tale, then at this point, the children would all know that this was a perfect place and they would chant together that this place was “Not too hot and not too cold!” In fact, it is just right. It has been “just right” for possibly the last ten thousand years—a region of pine and fir-covered peaks between nine and fourteen thousand feet high.

These peaks have their bases firmly set in the tropics and each has a sheltered southwest side where nighttime temperatures rarely go below 32º for any length of time.

On November 19th, 2006, a winter storm began and persisted for two hours. In the morning of Nov. 20th the north sides of the sanctuaries of Sierra Chincua and Sierra Campanario had light snows. The snow persisted only for the morning on the tips of the crowns of the trees where it had accumulated. The rains and the cloudy skies lasted for four days until November 23rd. On the dawn of the 20th and 21st the temperatures fell to 0.78C (33.4 fahrenheit); and during the dawn of November 25th, the temperature descended to -0.38ºC (31.4 fahreneheit.) Fortunately, the massive mortality of monarchs did not occur as was noted on two occasions in the last five years. In 2001 and 2005, freak ice storms cause the death of eighty percent of the population.

Coupled with the dry season currently persisting over Southern Mexico, these mild conditions are sufficient to cool the butterfly’s ardor and serve to keep their metabolism slowed below the temperatures needed for active reproduction. In an bug whose typical lifespan is between fourteen and twenty-eight days, this chill enables the brightly colored insect to hide out from the cold of its typical breeding grounds to the north and survive the lack of flowering plants on which to feed.

The winter’s population is measured each December by figuring the number of hectares covered by their mass, then counting the number of trees within that particular colony. This year researchers located eleven hibernating colonies: seven within and four outside the Monarch Butterfly Biosphere Reserve. Six colonies were located in Michoacan and five in the state of Mexico. The butterflies occupied a forest surface total of 6.67 hectares, which represented an increase of 12.7% with respect to the 5.92 hectares occupied during 2005, and more than three times the area occupied (2.19 hectares) during, 2004.

An exact nose count of the actual number of individual butterflies is impossible, since each tree is estimated to hold between 40-100,000 monarchs. Scientists vary in their estimations of how many monarchs exist per hectare (2.6 acres). Some go as low as 10 million; others as high as 50 million. So this year’s actual population could range between sixty-seven million and 334 million monarchs.

Now they are merely awaiting the mysterious signal that tells them when it is time to return. During the last week of February and the first two weeks of March something happens. Perhaps it is the arrival of the sun at a particular angle in the noon sky, perhaps other unknown factors govern their departure. Whatever the trigger, it is abrupt. Within a matter of days some 140 (or more) million monarchs cascade down the mountains into the warmer tropic regions and began their return journey toward the arriving spring in the north.

All of those survivors will develop adult reproduction and begin to mate as they fly north, searching for their host plant, the lowly milkweed. And where are the first signs of tender milkweed sprouting from the earth? Why, south and southwest Texas, of course!

Fields, pastures, roadsides in the region west and south of San Antionio are absolutely vital to the health and size of the entire monarch population of the United States and Canada for the coming year.

If they meet with breeding success, determined by low fireant counts and rain-fed host plants (hierba de zizotes, antelope horn, green milkweed, and Mexican milkweed in people’s yards), then the monarch populations will multiply and spread across the continent, until the fall brings their grandchildren back to Eagle Pass and their journey south.

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