article reports that the overwintering grounds for monarch butterflies in Mexico has dwindled to 2.94 acres. A number of factors are at play here. Increasingly erratic weather due to changing climate, the switch to herbicide-resistant crops in the midwest, which allows eradication of weeds that used to serve as food and habitat, and past logging of the high altitude forests in Mexico.
The warm weather last year sped the Monarchs' travel northward, which correlates with my memory of seeing them in Princeton last summer much earlier than usual. Their multi-generational travel northward in spring/summer is timed with the milkweed, so any change in schedule can affect whether the habitat is ready for their arrival.
In preparation for an upcoming trip, I was surprised to learn that the biosphere reserve where the Monarchs overwinter is just a couple hours west of Mexico City. There are several locations where one can hike up to see them, though El Rosario is the easiest to access. Given the multiple locations, it's strange to hear that the total acreage occupied by the overwintering butterflies is less than three acres.
Though the butterflies cling to the tree branches at night, in February they become active during the day, flying down the mountain slope to a water source, then back up in the evening to roost overnight. Tourists typically stay over night in the small mountain town of Angangueo, then head to the overwintering sites in the morning. Having remained sexually immature during their long fall migration to Mexico, and through much of the winter, the Monarchs finally mate just prior to heading north in late March.
As farm country becomes increasingly an ecological desert, due to reduction in fallow areas and increased spraying of weeds, urban areas become all the more important for sustaining species like the Monarch. Looks like starting new plants of the local swamp milkweed will be part of my "managing water in the landscape" course at the Princeton Adult School this spring.
Carolyn Edelmann, who has a far-ranging nature blog, sent me this link to a 6 minute video about the Monarchs' migration and overwintering site.