These images show a mountainous region of central Mexico, around the town of Angangueo. 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.

Monarch butterflies

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 retreated north. Monarch butterflies did both; 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 species, but we know that each butterfly is making the trip for the first 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, but the amazing annual cycle 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 knew the colonies but did not fully know their significance, or that they were being searched for. It was two volunteers, a retired American machinist and his Mexican wife, who finally tracked down 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 forty to fifty thousand 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 through 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, about a decade after these wintering sites were discovered, the Mexican government declared five Monarch reserves, and in 2000 these reserves were expanded. We can compare images from the Landsat archive to get some of idea of the area's changing landscape over these years.

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.
• The 1986 reserves around Angangueo are overlaid as white lines (red on the white maps). The 2000 reserves around Angangueo are overlaid as blue lines. Each reserve has a central core area surrounded by a less restrictive buffer area.
• We also overlaid points around Angangueo where Monarch colonies were observed between 1977 and 1998.
• 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 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 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: 1971 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.

Satellite images

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). The images labeled "bands 4 3 2 8" were sampled to 15 meters and sharpened with panchromatic band 8.

2001: Landsat 7 ETM+, 31 March 2001, path 27, rows 46-47. The images labeled "bands 4 3 2 8" were sampled to 15 meters and sharpened with panchromatic band 8.

Maps

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

American Geographical Society of New York, 1957, Ciudad De Mexico , scale 1:1,000,000.

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.

Instituto Nacional de Estadistica Geografia e Informatica, 1978, Morelia E14-1, scale 1:250,000.

Instituto Nacional de Estadistica Geografia e Informatica, 1977, Queretaro F14-10, scale 1:250,000. (North end of Altamirano map.)

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: 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.

The 1986 reserve boundaries over these images (shown in white) are from the Presidential decree of 9 October 1986 as shown at www.ine.gob.mx/ucanp/data/decretos/decmarmon.html. The Altamirano reserve is not outlined.

The incomplete 2000 reserve boundaries over these images (shown in blue) are from the Monarch Butterfly Sanctuary Foundation. Dashed lines are approximate.

Photographs

All photographs are courtesy of Monarch Watch.

Answer to the question above

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.

How to cite this article

Robert Wellman Campbell, ed. 2001. "Angangueo, Mexico: 1973, 1986, 2000, 2001." Earthshots: Satellite Images of Environmental Change. U.S. Geological Survey. http://earthshots.usgs.gov. This article was released on 12 January 2001 and revised on 14 August 2001.