You might recall that this spring I reported seeing Monarch Butterflies in upland Chiapas, Mexico's southernmost state. Since arriving here I've seen dozens in the Yucatan.
This has surprised me because every magazine article and film I've ever seen about Monarchs focused on the story of their migration between North America and certain mountaintops in central Mexico. That doesn't explain the butterflys' occurence in Chiapas and here, far south of the central Mexican mountaintops.
I wrote Chip Taylor at http://www.monarchwatch.org/ and he exlained the situation with a single sentence: "Monarchs occur as far south as Venezuela."
He went on, " Monarchs can be found in most of lowland and mid elevation Mexico - wherever the tropical milkweed is found."
I'm glad to hear about this very large distribution area for Monarchs because I've visited some of those central Mexican mountaintops where overwintering Monarchs hang by the millions from trees, and my impression is that those "sanctuaries" are very mcuh endangered -- mainly by local firewood gatherers and timber cutters, farmers wanting land, and by changing microclimates inside the sanctuaries, because of forest removal outside the sanctuaries' boundaries. If North America's Monarchs depend on Mexico's sanctuaries, then either the sanctuaries need more protection or North Americans shouldn't plan on having Monarchs for long.
However, now it's clear that even if the central Mexican sancturies are destroyed, at least some Monarchs will survive. I'm unclear as to wehther those Monarchs who overwinter from here to Venezuela migrate to North America, or migrate at all.
You might enjoy reading a piece at Chip's site called "Understanding Mexico's Important Role in Preserving North America's Biodiversity." It's at http://www.learner.org/jnorth/tm/monarch/GeographyMigration.html.