The seasonal migrations that occur in the animal kingdom annually vary in scale on many levels and cover nearly all classifications and all ecosystems of land, sea and air. Climactic conditions and other environmental changes impact these movements on a regular basis. In the case of the monarch butterfly migrations from North America to Mexico, the fragile balance is especially evident as populations are noticably dropping due to habitat loss, insecticide/herbicide use and extreme weather. (An interesting note in this case is that the migration itself was initially caused by human impacted landscape changes when the great North American forests began to be cleared for agriculture in the 1600s.)
Milkweed is the sole food plant for the monarch caterpillers. Adult butterflies feed on many nectar plants, but eggs are layed and caterpillers grow only on milkweed, which is less previlent due to the strong herbicides required in the monoculture agriculture currently practiced in North America. Consequently, monarch populations are declining dramatically. Individuals can participate in numerous ways to coordinate within their communities, at home and in school to both monitor the migrations and assist with habit reconstruction.
The Mexican site Soy Monarca is an excellent source for further information on how to get involved.