Danaus plexippus

 

The monarch butterfly has a regular migratory pattern in North America which is completed over multiple generations (Brower, 1986). There are two different monarch populations in North America: (1) the western migratory population which overwinters along the California coast (Dingle et al., 2005), and (2) the eastern migratory population which overwinters in Mexico (Brower, 1989). The latter forms a larger migratory population, and is the focus of this study.

 

The locations of these overwintering sites is largely attributable to Dr. Fred Urquhart and associates, as many of these colonies were unknown to the scientific community until 1975 (Urquhart 1976). More recently, scientists have found several more colonies in the state of Michoacán, Mexico, which is the study area selected for this study. Inside Michoacán, the largest overwintering colony is found within the Monarch Butterfly Biosphere Reserve. Outside this reserve, colonies are found in Pizcuaro, Puerto Bermeo, San Andres, Puerto Morillo, Palomas, Piedra Herrada and San Francisco Oxtotilpan (Garcia et al., 2004). Although scientists have learned much about overwintering colonies in the past few years, there are several basic questions unknown. Currently there is a need to understand the characteristics of the overwintering sites that are most valuable to the species survival, one such factor is habitat connectivity.

 

Abies religiosa

 

North American monarchs overwinter in oyamel fir (Abies religiosa) forests which are located in the Transvolcanic Mountains of central Mexico. Monarchs migrate to specific overwintering sites because they require specific environmental conditions which is characterized by these fir trees. The ultimately balance the macro-and microclimatic factors that monarchs need to survive throughout the winter (Calvert and Brower 1986; Alonso et al. 1992). Temperatures that are typical of these forests are between 3˚C and 18˚C, with high humidity. Thinned forests have been shown to be detrimental to survival as they are more likely to get wet during winter storms, and become colder at nigh which both are deadly conditions to the monarch (Brower et al., 2004).

 

Modelling efforts conducted by Missrie (2004) and Bojorquez-Tapia (2003) show that preferred overwintering sites share four attributes: (i) high elevations, most located at altitudes greater than 2890 meters, (ii) sites close to streams, (iii) steep slopes between 23˚ and 26˚, and (iv) south-southwest orientation.