POPULATION ECOLOGY Ecological Niches in Sequential Generations of Eastern North American Monarch Butterflies (Lepidoptera: Danaidae): The Ecology of Migration and Likely Climate Change Implications

Eastern North American monarch butterßies (Danaus plexippus L.) show a series of range shifts during their breeding season. Using ecological niche modeling, we studied the environmental context of these shifts by identifying the ecological conditions that monarchs use in successive summer months. Monarchs use a consistent ecological regimen through the summer, but these conditions contrast strikingly with those used during the winter. Hence, monarchs exhibit nichefollowing among sequential breeding generations but niche-switching between the breeding and overwintering stages of their annual cycle.Weprojected their breeding ecological nicheontomonthly future climate scenarios, which indicated northward shifts, particularly at the northern extreme of their summermovements, over thenext 50yrs; if bothmonarchs and theirmilkweedhost plants cannot track thesechangingclimates,monarchs could losedistributional areaduringcritical breedingmonths.