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supporting materials, p. 30). The model is based on the assumption that increasing temperatures
from climate change will decrease the risk of severe winter storm events, yet this assumption is
not supported by other climate models. The model also fails to take into account the influence of
predicted warmer temperatures on lipid depletion during overwintering which reduces butterfly
fitness (See Threats…Other Factors, Global Climate Change).
Thus, the Flockhart et al. (2014) model demonstrates that the monarch is threatened, yet certainly
still underestimates extinction risk. The model demonstrates that ongoing population declines
will be driven by land-use change and global climate change, and identifies as a top priority for
slowing future population declines the need to reduce the loss of milkweed host plants in the
Midwest and Southern U.S. breeding grounds, which they determine is the primary driving force
behind the current population decline (p. 3, 14). The model also demonstrates that the drastically
reduced current population size of monarchs makes the species even more vulnerable to
catastrophic events. The overall population of monarchs in North America is exhibiting a
significant decline and the butterflies are facing high magnitude, imminent threats from multiple
factors across their range.
THE MONARCH BUTTERFLY WARRANTS ESA PROTECTION
The Endangered Species Act states that a species shall be determined to be endangered or
threatened based on any one of five factors (16 U.S.C. § 1533 (a)(1)). In this case, the monarch
is threatened by all five of these factors and warrants protection under the Act. The monarch is
threatened by the first factor, the modification and curtailment of habitat and range, due to the
drastic reduction of milkweed in its summer breeding habitat that has occurred due to increased
herbicide spraying caused by the widespread adoption of genetically-engineered, herbicideresistant
corn and soybean crops (Pleasants and Oberhauser 2012, Flockhart et al. 2014).
Monarch habitat has also been reduced due to increased production of ethanol since 2007 that
has resulted in conversion of grasslands to corn and eliminated milkweed from those habitats
(Brower et al. 2012a), and by other factors such as urban development and aggressive
management of roadside vegetation (Commission on Environmental Cooperation 2008). East of
the Rockies, it has been very roughly estimated that approximately 167 million acres of monarch
habitat, an area about the size of Texas, may have been lost since the mid-1990s due to
agricultural changes and development, including nearly one-third of the monarch’s total summer
breeding range (Taylor 2014). The monarch’s wintering grounds are threatened by illegal
logging, legal wood gathering, water diversion, and agricultural conversion of forest land in
Mexico, and by development, aging forests, and other threats in California. The butterfly is
potentially threatened by the second factor, overutilization, due to commercial production and
release of large numbers of butterflies, which threatens to spread disease and undesirable genetic
traits to wild populations. The monarch is also threatened by the third factor, disease or
predation. High levels of predation are a significant threat at all life stages, especially in synergy
with habitat loss and declining populations. Disease further threatens the monarch, and the
spread of one protozoan parasite in particular may be reducing the proportion of females in the
population and thus reducing the monarch’s potential for population growth and recovery (Davis
and Rendón-Salinas 2010). The fourth factor, inadequacy of existing regulatory mechanisms, is a
threat because voluntary efforts undertaken have not been able to stop and reverse population
decline. Finally, monarchs are also threatened by the fifth factor, other natural and manmade
factors affecting their continued existence, including pesticides, invasive species, global climate
Monarch ESA Petition 44