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.

Understanding a migratory species in a changing world: climatic effects and demographic declines in the western monarch revealed by four decades of intensive monitoring

“…In studies on eastern monarchs, it has been posited that dwindling host plant populations (Flockhart et al 2015;Pleasants and Oberhauser 2012;Zalucki and Lammers 2010;Brower et al 2006), the use of insecticides (Krischik et al 2015;Pecenka and Lundgren 2015), and overwintering habitat destruction (Vidal and Rendon-Salinas 2014;Saenz-Romero et al 2012;Brower et al 2002) are primary drivers of decline. To our knowledge, a decline in milkweed abundance has not been reported within the range of the western monarch.…”

“…For the overwintering monarchs in Mexico, severe weather and forest degradation are further stressors that compound habitat and host loss on breeding grounds in the USA (Brower et al 2012;Flockhart et al 2015). Despite apparent stressors and declines in monarchs at their overwintering grounds (Vidal and Rendon-Salinas 2014;Saenz-Romero et al 2012;Brower et al 2002), numbers have not declined at some of the fall stopover sites in the Eastern USA (e.g., Davis 2012) or summer breeding grounds (Ries et al 2015). Also, weather has not been considered to have a significant effect on the eastern Monarch population during spring and summer (Zalucki et al 2015).…”

Fire creates host plant patches for monarch butterflies

“…This combination of breeding, long-distance migration and overwintering behaviours and locations means that monarchs are potentially threatened not only by the elimination of their host plants (Asclepias spp.) throughout their breeding range, but also by the degradation and loss of their overwintering habitat [4][5][6]. In the southern Great Plains, monarch butterflies are uncommon or absent after the initial spring migration, suggesting that they move farther north in advance of high temperatures before passing back through the area during autumn migration [7 -10].…”

Are female monarch butterflies declining in eastern North America? Evidence of a 30-year change in sex ratios at Mexican overwintering sites

“…This discovery was certainly unexpected and is not readily explainable. At first glance, one might think that the thinning of the sanctuary's forest over the past three decades from illegal logging (Brower et al 2002) has gradually changed the microclimate, making it somehow less favourable for female survival in recent years. However, evidence to date does not indicate this species has any sex-related differences in cold tolerance to natural temperatures (i.e.…”