monarch-esa-petition-final_61585

The North American monarch population is biologically significant because without it, the
redundancy of the species would be drastically curtailed. North America is the core of the
monarch’s range and the North American population as recently as the mid-1990s numbered
nearly one billion butterflies. The loss of milkweed due to increased spraying of particular
herbicides and development and the degradation of overwintering sites has reduced the
population to approximately 35 million butterflies as of winter 2013-2014. The migratory
populations in eastern and western North America still represent the vast majority of all
monarchs in the world. Though monarchs are found in relatively small, peripheral, and
introduced populations in tropical and subtropical locations such as Bermuda, the Canary
Islands, and Australia (see Appendix A), these non-migrating populations cannot conserve the
spatial distribution of the species over the core of its range in North America, and are limited in
population growth potential such that they cannot substitute for the abundance of the continental
North American population.
In terms of resiliency, the North American monarch population is biologically significant
because if it were to be lost, the resiliency of the species would be so reduced that the monarch
would be at risk of extinction. North America is home to nearly all monarchs. Within North
America, the population from east of the Rockies that overwinters in the mountains of Mexico is
the largest monarch population in the world representing by far the majority of all monarchs.
Within the eastern population, in the spring most monarchs breed in Texas, Oklahoma, Arkansas,
and Kansas. Summer breeding occurs mainly in the Corn Belt states (Iowa, Illinois, Indiana,
northern Missouri, Ohio), the eastern portions of the Northern Plains states (Kansas, Nebraska,
South Dakota), and the southern parts of the Lake States (Minnesota, Michigan and Wisconsin)
(Wassenaar and Hobson 1998, Miller et al. 2011, Flockhart et al. 2013). The Corn Belt states are
particularly important for production of butterflies that will overwinter (Wassenaar and Hobson
1998, p. 15439). In population models, Flockhart et al. (2014, p. 15) found that at a regional
scale total monarch abundance was most sensitive to changes in vital rates in this central eastern
breeding region, and in the Corn Belt in particular (p. 18). They concluded (Flockhart et al. 2014,
p. 16) that the loss of milkweed due to the increased use of pesticides on herbicide-resistant
crops in the Midwest has increased the current and future extinction probability for monarchs.
The Corn Belt region is pivotal to monarch resiliency because it is a source population for
monarchs in other regions including along the East Coast and Florida, and also provides genetic
influx to the western monarchs that migrate to Mexico in lieu of overwintering in California, and
presumably to many of the peripheral populations (Appendix A).
Numerous scientific studies have identified the importance of the eastern monarch population in
supporting other monarch populations in North America. Miller et al. (2011, p. 43) used isotope
measurements to estimate natal origins of monarchs collected from 17 sites along the East Coast
and found that 88 percent of the coastal monarchs had originated in the Midwest and Great Lakes
regions before completing a west to east longitudinal migration across the Appalachian
mountains. The Florida monarch population is also apparently supplemented by monarchs with
Midwestern origins. Though non-migratory monarchs reside in southern Florida throughout the
year, this population too receives an influx of individuals each fall from the eastern migratory
population (Knight 1997, Altizer 2001). In addition, the demographic success of monarchs in the
Corn Belt region is directly linked to overwintering population size in Mexico (Wassenaar and
Hobson 1998, Oberhauser et al. 2001, Brower et al. 2012a, b; Flockhart et al. 2013, Pleasants
Monarch ESA Petition 110