The New York Times discusses dramatic declines in wintering monarch populations. Although the declines were noticed on the wintering grounds in Mexico, causes seem to be associated with climate change and habitat destruction in North America, where the butterflies breed.
“A rapid expansion of farmland — more than 25 million new acres in the United States since 2007 — has eaten away grasslands and conservation reserves that supplied the monarchs with milkweed, he said.
The monarchs’ migration is seen as a natural marvel and, for Mexico, a huge tourist attraction. But naturalists regard the butterflies as a forward indicator of the health of the food chain. Fewer butterflies probably means there are fewer other insects that are food for birds, and fewer birds for larger predators.”