Tracking multi-generational colonization of the breeding grounds by monarch butterflies in eastern North America

Abstract

Insect migration may involve movements over multiple breeding generations at continental scales, resulting in formidable challenges to their conservation and management. Using distribution models generated from citizen scientist occurrence data and stable-carbon and -hydrogen isotope measurements, we tracked multi-generational colonization of the breeding grounds of monarch butterflies (Danaus plexippus) in eastern North America. We found that monarch breeding occurrence was best modelled with geographical and climatic variables resulting in an annual breeding distribution of greater than 12 million km 2 that encompassed 99% occurrence probability. Combining occurrence models with stable isotope measurements to estimate natal origin, we show that butterflies which overwintered in Mexico came from a wide breeding distribution, including southern portions of the range. There was a clear northward progression of monarchs over successive generations from May until August when reproductive butterflies began to change direction and moved south. Fifth-generation individuals breeding in Texas in the late summer/autumn tended to originate from northern breeding areas rather than regions further south. Although the Midwest was the most productive area during the breeding season, monarchs that re-colonized the Midwest were produced largely in Texas, suggesting that conserving breeding habitat in the Midwest alone is insufficient to ensure long-term persistence of the monarch butterfly population in eastern North America.

Monarch Butterflies Use Regenerating Milkweeds for Reproduction in Mowed Hayfields in Northern Virginia

“…Some mating occurs at these winter roosts before spring dispersal Frey 1997, Brower et al 2007). Most individuals that overwinter colonize northern Mexico and the southern tier of the United States as milkweeds develop, although a few migrate directly to more northern areas (Miller et al 2012, Flockhart et al 2013. Because breeding monarch adults typically only live from two to five weeks, successive generations continue north and east as southern areas get too hot and milkweeds decline in number and quality.…”

“…In late April the first-generation butterfliesoffspring of the migrants from Mexico-continue to move north, laying eggs throughout the mid-South into the Midwest and North. Then in June, the main colonization of the Midwest and North takes place with two to three more generations being produced there before migration south begins in August , Howard and Davis 2004, Flockhart et al 2013). …”

“…Though their numbers have been drastically reduced, monarchs are still widespread in appropriate habitat in the continental United States. Flockhart et al (2013) predicted where eastern monarchs are most likely to be found during the breeding season by determining the probable range based on amount and kind of vegetation, geographical limits (latitude, longitude, altitude, and slope), temperature, precipitation, and records from Journey North citizen scientist observations collected between 1997 and 2011 (Flockhart et al 2013, Fig. 1).…”

“…Notably, fewer overwintered butterflies originated in northern Indiana, Illinois, Iowa, Michigan, or the upper Midwest compared to the 1996 breeding season. Flockhart et al (2013) went beyond study of the overwintering generation to determine the natal origins of successive monarch generations produced in the east throughout the 2011 breeding season. The natal origins showed a broad spatial distribution that encompassed the entire breeding range in eastern North America, though the preponderance of individuals originated from northern Texas to western Ohio, in a region extending from the southern Great Plains through the Midwestern Corn Belt (Figure 16).…”

“…There were few indications of natal origins from Mississippi, Alabama, Georgia and Florida despite the fact that areas located north of these locations were sampled extensively. When butterflies were collected for the Flockhart et al (2013) study, the overwintering monarch population size was drastically reduced from the 1996-1997 level. During the winter of 2010-2011, the estimated population size was 200 million individuals ( Figure 12, above), compared to the estimated billion butterflies at the time of the earlier study.…”