The monarch migration is an endangered phenomenon. Logging in the overwintering sites in Mexico reduces the area available for the monarchs. The Monarch Butterfly Biosphere Reserve, established in 1986, is intended to protect the overwintering areas. But studies have shown that deforestation has occurred within the reserve. Some of that deforestation can be tracked from space with Landsat imagery.
According to Monarch Watch, during the 2012–2013 season, monarch colonies occupied the lowest number of hectares of forest in the previous 20 years. But the 2013–2014 season was even lower. Recent seasons have seen a slight increase in area of forest occupied, but monarch populations this low are extremely vulnerable. Just one winter storm could severely decrease their numbers.
Landsat sensors use infrared reflectance. One of the wavelengths of light they use is called near-infrared. Actively growing vegetation reflects this wavelength, so when this wavelength is assigned the visible color red, vegetation is displayed as red in these false color images. As you take a closer look at the zoomed in areas in the other sections, watch for gray patches in areas that were once red. This indicates a degraded forest.
Not all of the reduction in the monarch population can be attributed to oyamel forest loss in Mexico. In the United States, expansion of agriculture and herbicide use are reducing the milkweed that the caterpillars need. Extreme weather conditions have also been harming the monarchs. Satellite imagery continues to be useful in tracking the monarch population in these overwintering sites and can help recover this remarkable migration.