Quantitative changes in forest quality in a principal overwintering area of the monarch butterfly in Mexico, 1971-1999

Abstract

Degradation of the oyamel fir-pine forest ecosystem in central Mexico is a threat to the overwintering and migratory phenomenon of the eastern North American population of the monarch butterfly (Danaus plexippus). Because a lack of quantitative data has hindered effective conservation policy, we photogrammatically analyzed the changing state of a major overwintering forest area. We analyzed stereographic aerial photographs of a 42,020-ha area taken in 1971, 1984, and 1999 with GRASS, a geographic information system. What in 1971 was a nearly continuous high-quality forest is now fragmented and severely degraded. Between 1971 and 1999, 44% of conserved forest (forest with >80% cover) was degraded, and the largest patch of high-quality forest was reduced from 27,115 ha to 5827 ha. The annual rate of degradation from 1971 to 1984 was 1.70%, and this increased to 2.41% during the next 15 years. At the latter rate, <10,000 ha of high-quality forest will remain in 20 years and <4,500 ha in 50 years. A subset of the analysis quantified changes in a 6596-ha area on the Sierra Chincua, Sierra Campanario, and Cerro Chivati Huacal massifs that were declared protected by presidential decree in 1986. Corresponding rates of degradation of these reserves more than tripled, from approximately 1.0% between 1971 and 1984 to more than 3% between 1984 and 1999. Passage of the 1986 decree failed to protect the forest. Our data provide irrefutable evidence that successful implementation of a more inclusive presidential decree issued in November 2000 will require (1) effective enforcement against logging within the oyamel-pine forest ecosystem and (2) restoration of areas that have been degraded. All indications are that the rate of logging is increasing throughout the area. The grandeur of the monarch butterfly overwintering phenomenon in this tiny area of Mexico is too great a cultural and biological treasure for this rampantly destructive process to continue.