Butterflies like the Monarch have always been one of my favorite parts of the prairie.
MonarchWatch has some interesting prairie-as-habitat information on their web site and have even started a blog. For example, in "Status Report on the Monarch Butterfly (Danaus plexippus) in Canada" by Crolla and Lafontaine they state:
In the last 150 years there has been a major shift in the North American distribution of the eastern population of the Monarch (see Brower 1995). Until the 1880’s, the prairie region of central North America appears to have been the main breeding area of the eastern Monarch population. The native prairie flora includes about 22 species of habitat-specific milkweeds (Asclepias), many of which can serve as larval hostplants, and an abundance of flowering plants that provide a diverse array of nectar resources for adult Monarchs.