These images show a mountainous region of central Mexico in dry season. In the east of the images lies gigantic Mexico City, one of the largest agglomerations of people in the world at about 20 million. In mountainous forest reserves 75 miles west, Monarch butterflies likewise live in dense agglomerations of their own, totaling perhaps 60 million in a much smaller area. Conservationists worry that the fir forest (appearing vibrant red in the Landsat images) may suffer too much clearing and thinning (appearing as greenish tan), and the butterflies may perish.

When the last ice age ended, about ten thousand years ago, this area's climate grew warmer and drier. Some species responded by retreating uphill; because these mountains are so high (3,000-3,500 meters), their peaks are much cooler and wetter than the valleys below. Other species simply retreated north. Monarch butterflies did both in a way; in the summertime they now live in the eastern United States and southern Canada. For winter they survive by flying south, all the way back to the high mountains of central Mexico.

Monarchs are admired for their beauty, but also for this migration. Not only is it an impressive distance for such a flimsy animal, but we know that every individual butterfly is making the trip for its first and only time; those who return the next year will be its grand- or great-grandchildren. Biologists call this migration an "endangered phenomenon." If this Canada-to-Mexico population were wiped out we would still have a few Monarchs elsewhere in the world-- though only a few-- but the amazing annual journey would end. ¹

For decades, scientists painstakingly tagged thousands of Monarchs' wings with tiny stickers, in the hope of retrieving a few and tracking their movement. But until 1975 they did not know exactly where the Monarchs ended up for the winter. Local people saw the colonies but did not fully know their significance, or that they were being searched for. Two volunteers, a retired American machinist and his Mexican wife, finally found the colonies, with the guidance of local woodcutters and faraway zoologists. They were amazed to see how many millions of butterflies congregated in such tiny areas. ²

This concentration makes the Monarchs vulnerable. They require oyamel fir forest, which is found in only about 40-50,000 mountaintop acres in Mexico, all above 3,000 meters elevation or so. And even within this small range they pile thickly on a few trees. So millions can die from a very local disturbance-- a windstorm, snow, or even logging. ³

Above all, wintering Monarchs must have a proper, constant temperature. They must not be so cold that they freeze or can not fly to food and water. But they must not be so warm that they dry out or burn all their energy reserves. From about November to March, the fir forest provides such an environment. The forest canopy shades them by day and blankets them by night. Even a partial thinning of this canopy exposes them to heat and cold. In a thinned forest, birds and other predators also eat them in greater numbers, and wind can blow them off their branches to freeze on the ground. This thinning is primarily what threatens the Monarchs. ⁴

Monarch habitat loss In 1986 the President of Mexico decreed five reserves for the Monarchs, four of which are shown here. We can compare a 1986 Landsat image to a 1973 image (the oldest available) and a 2000 (the newest), to look for effects of this designation.

A few things to remember when looking at the images:

• Red signifies healthy vegetation; we chose dry-season scenes so that grass and crops would look a dead, greenish tan.
• We overlaid the reserves' boundaries; each reserve has a central core area surrounded by a less restrictive buffer area.
• We also overlaid points where colonies were observed between 1977 and 1998. (These are missing for the southernmost reserve.)
• Landsat images are always taken in midmorning, so shaded northwest slopes look darker. Shadows vary a bit between one month and the next.
• The resolution of Landsat images improved between 1973, 1986, and 2000. The old images are not out of focus, just blown-up.
• You can select different years, click in brackets to zoom in, and click anywhere else to zoom out.

The Chincua reserve shows some rashy tan areas around the its core, especially to the south, in 2000 but not 1986. This may be evidence of a fire, or some other kind of clearing.

The Campanario reserve, southeast of Angangueo, shows some clearings appearing in the forest in and near the reserve. (It also shows a large cloud shadow in 2000.) You can see the forest retreating on the reserve's west side, especially if you compare 1973 to 2000. People live all along this area, and they are often dependent on this land for subsistence, through farming, grazing, and woodcutting.

The Huacal reserve, as shown above, shows a large area of tan in 2000, possibly thinning or clearing, with a very artificial-looking border to the east. Some form of deforestation also appears to be progressing west of the reserve.

Finally, the southernmost reserve, Pelon, again shows in 2000 a rashy tan color absent in 1986.

Whatever these images say about the effects of the 1986 decree, it should be remembered that even the higher-resolution images from 2000 can really only show patches of clearing, not forest thinning by removal of single trees. Because this kind of selective logging is common here, aerial images with even-higher resolution are very helpful in studying the forest. One recent study, which used aerial photographs to estimate forest canopy cover, suggested bad news for the Monarchs. Its findings for the northern reserves are shown on this map. You can see intact forest in 1971 giving way to thinned and largely-cleared forest by 1999. A forest "blanket" looking solidly red in the Landsat images may on closer inspection have some holes in it. ⁵

Question Did you notice the apparent clearing, about a quarter by a half mile, right on the northern border of the Pelon reserve in 2000? Do you have a good idea what caused it?

(See the answer below.)

Footnotes Thanks to Dr. Chip Taylor and the staff of Monarch Watch (www.monarchwatch.org) for ground photos and help with this article. Many thanks to Dr. Lincoln Brower for his advice and thinning maps.

1. Stephen B. Malcolm, "Conservation of the Monarch Butterfly Migration in North America: An Endangered Phenomenon," in Stephen B. Malcolm and Myron P. Zalucki, eds., Biology and Conservation of the Monarch Butterfly (Los Angeles: Natural History Museum of L.A. Co., 1993), p. 357-361.

2. Fred Urquhart, "Found at Last: The Monarchs' Winter Home." National Geographic 150, no. 2 (August 1976), p. 161-173. Monarch Watch 1998 Season Summary 7, No. 1 (May 1999), p. 24-25.

3. Laura C. Snook, "Conservation of the Monarch Butterfly Reserves in Mexico: Focus on the Forest," in Stephen B. Malcolm and Myron P. Zalucki, eds., Biology and Conservation of the Monarch Butterfly (Los Angeles: Natural History Museum of L.A. Co., 1993), p. 363-375.

4. William H. Calvert and Lincoln P. Brower, "The Location of Monarch Butterfly (Danaus Plexippus L.) Overwintering Colonies in Mexico in Relation to Topography and Climate," Journal of the Lepidopterists' Society 40, no. 3 (1986), p. 164-187.

5. L. P. Brower, G. Castilleja, A. Peralta, J. Lopez-Garcia, L. Bojorquez, S. Diaz, D. Melgarejo, and M. Missrie, "Quantitative changes in forest quality in a principal overwintering area of the monarch butterfly in the states of Michoacan and Mexico:Ê1971 to 1999," 2001, in review by Conservation Biology. Carol Kaesuk Yoon, "Monarch Butterflies Lose Much of their Wintering Grounds," New York Times 12 September 2000.

Other references Educational materials on Monarchs and tagging are available from Monarch Watch at monarchwatch.org.

Satellite images The reserve boundaries over these images are from the Presidential decree of 9 October 1986 as shown at http://www.mbsf.org/decree.html.

1973: Landsat 1 MSS: LM1028046007311190 and LM1028047007311190 (21 April 1973), LM1027047007311190 (20 April 1973).

1986: Landsat 5 TM: LT5027046047086073 (14 March 1986) and LT5026046047086082 (23 March 1986).

2000: Landsat 7 ETM+: LE7027046000010450 (13 April 2000) and LE7026046100008150 (21 March 2000). These images were sampled to 15 meters and sharpened with panchromatic band 8.

Maps

Forest thinning maps: from a manuscript in review by Conservation Biology, "Quantitative changes in forest quality in a principal overwintering area of the monarch butterfly in the states of Michoacan and Mexico:Ê 1971 to 1999," courtesy of the World Wildlife Fund-- Mexico, and the authors, L. P. Brower, G. Castilleja, A. Peralta, J. Lopez-Garcia, L. Bojorquez, S. Diaz, D. Melgarejo, and M. Missrie. The dots on the Landsat images signifying Monarch colonies are taken from this map. They are all known locations from January 1977 through March 1998. They do not represent colonies that form every year.

Hungarian National Office of Lands and Mapping, 1968, Mexico, N D-F 13-15 sheet 87, scale 1:2,500,000.

Instituto Nacional de Estadistica Geografia e Informatica, 1976, Angangueo E14A26, scale 1:50,000.

Instituto Nacional de Estadistica Geografia e Informatica, 1974, Villa de Allende E14A36, scale 1:50,000.

Photographs

All photographs are courtesy of Monarch Watch.

Answer to the question above

Well, did you notice the fire burning in the northeast of this image? You can see its smoke, and you can tell by the map that it is a mountaintop. Compare the land around the fire to that clearing we asked about. Both are darker and grayer than the tan rashes. These are probably both scars from fires recent enough that the black ash still covers the ground. Typically as the ash disappears the area brightens.

This article was released on 12 January 2001.