Dr. Chip Taylor at Monarch Watch has been tracking monarch butterfly populations for about the last 20 years. Every year, monarchs from the part of North America that is east of the Rocky Mountains overwinter in a concentrated mass in some forests in Mexico. The butterflies will migrate north in the spring, and that population repopulates the continent.
Every year there is an estimate of populations, based on the area of the forests occupied by the butterflies. This winter the area was lower than has ever been recorded. The area covered by butterflies is about 0.67 hectares. For perspective, that is about 1.25 times the area of a football field. That is 31 times lower than the highest area recorded.
Doctor Taylor points to several factors that have contributed to the decline—cool weather this spring and loss of habitat. The loss of habitat can be specifically broken down into three factors: continuing development, the use of herbicide tolerant crops (and spraying broad spectrum herbicides in the crop rows), and the conversion of grasslands to row crops due to the ethanol mandate.
Loss of habitat for those three factors was calculated to be an area the size of Texas, or about 31% of the total monarch butterfly habitat in the continent.
Christopher Taylor, (a different Dr. Taylor), who blogs at Catalog of Organisms points out that a large population size does not guard against extinction. In North America, Rocky Mountain locusts, passenger pigeons, and Carolina parakeets all went extinct, in spite of large population sizes.
The consensus of experts seems to be that the monarch butterfly itself is not in too much danger of extinction, but that the phenomenon of monarch migration might be. That would mean that monarchs would be gone from the Midwest and most of North America. They would not be extinct, but we would not have them here.
And that would be a shame.