3 years ago



The North American

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

and Oberhauser 2012). There is also strong evidence for significant mixing of eastern monarchs with the western monarch population in California (Lyons et al. 2012, p. 3341). The productivity of monarchs as a species is thus largely dependent on the monarchs in the eastern North American population. Monarchs from the east, and some from the west, overwinter in a small area in the mountains of central Mexico where they are highly vulnerable to severe weather events and predation from birds and mice. In fact, winter storms and predation in some years have killed the majority of overwintering monarchs. If the overwintering population were lost due to stochastic events, climate change, or high levels of predation, the majority of the monarch’s habitat in North America would be unoccupied the following summer, as the entire breeding range east of the Rockies would not be repopulated by remigration. The western population and resident southern populations are likely too small to provide this function, and are themselves vulnerable to development, disease, climate change, and other factors. Also, non-migrating populations in southern areas are not subject to environmental cues that would cause them to migrate long distances in spring to re-populate the full range (e.g. Guerra and Reppert 2013). Moreover, there is no question that the resiliency of monarchs as a species would be at risk if the North American population overall were to be lost. Without the North American population, the survival of monarchs as a species would be dependent on isolated, introduced, vulnerable populations that are themselves threatened with extinction. In Australia, for example, the monarch population has declined below the 1960s population size and is threatened by coastal development, active removal of milkweed by ranchers, severe drought, and record heat waves. Monarchs have narrow thermal tolerance, and populations in tropical and sub-tropical areas are vulnerable to rising temperatures from climate change and also to severe storm events, drought, and sea-level rise. In terms of representation, the North American monarch population is biologically significant because the spatial distribution and diversity of the species would be severely disrupted without it. The continental North American population harbors high genetic diversity and the migrations and intermingling of the eastern and western populations maintain genetic diversity that has been lost in peripheral and isolated populations from other areas. For example, Hawaiian monarchs are smaller than North American migratory monarchs, and microsatellite markers show that Hawaiian monarchs are genetically distinct from those in North America and New Zealand (Pierce et al. 2014, p. 2). The range of variation, spatial distribution, and diversity of monarchs as a species are dependent on the survival of North American monarchs. The overall North American population of monarchs is biologically significant, and within this population, the eastern migratory population is also biologically significant. The redundancy, resiliency, and representation of the monarch species would be gravely compromised without North American monarchs. After determining that the North American monarch population constitutes a significant portion of the species’ range, the Service must then examine whether the North American SPR is threatened by any of the five listing factors. As discussed in detail in previous sections of this petition supra, monarch butterflies in eastern and western North America have undergone precipitous decline and are threatened by modification or curtailment of habitat and range, Monarch ESA Petition 111

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