monarch-esa-petition-final_61585

Severe Weather and Catastrophic Events
Periodic extreme weather conditions and catastrophic events have been identified as a primary
threat to monarch butterflies (Slayback et al. 2007, p. 38, Brower et al. 2012a, p. 95, Vidal et al.
2013, p. 178). Monarchs are threatened by severe weather conditions and catastrophic events
including high and low temperatures, drought, winds, storms, fires, and flooding. To complete
their multi-generational migration, monarchs depend on moderate temperature conditions during
the various stages of their life cycle, and aberrant temperatures can kill larvae and adults. Severe
cold threatens the survivorship of overwintering monarchs, and spring and summer weather that
is too cold or too hot lowers breeding season survivorship and fecundity and alters larval growth
rates (York and Oberhauser 2002, p. 294, Brower et al. 2012a, p. 97).
Milkweed host plants are also sensitive to temperature extremes. Late frosts can kill milkweed
shoots during the early breeding season for monarchs (Brower 2009). Droughts also harm
milkweed both by resulting in fewer milkweed plants and by causing plants to be of lower
quality (e.g. Craig, as quoted in Mulvaney 2013; see also Climate Change section of this
petition, supra).
A series of severe weather conditions in recent years demonstrates the significant threat that
stochastic weather poses to monarch survival, especially in light of drastically reduced
population sizes due to other threat factors. For example, above normal temperatures in Texas in
spring 2009 reduced first-generation migrating monarch numbers. Then below normal
temperatures in the Midwest limited numbers of summer generations produced in the Corn Belt
region. Combined, these aberrant climate factors severely reduced the number of butterflies in
the migrating fall generation (Brower et al. 2012a, p. 97). Already at reduced numbers, the 2009-
2010 overwintering monarch population in Mexico was subjected to record-breaking
precipitation levels and heavy winds which blew down hundreds of oyamel fir trees in the core
of the Monarch Reserve. In addition to habitat loss from flooding, landslides, and freezing
temperatures, large numbers of butterflies were killed by the winter storms (Brower et al. 2012a,
p. 98).
During winter when monarchs are clustered, stochastic events can kill the vast majority of the
population. Calvert et al. (1983), Brower et al. (2004), and Brower et al. (2012a) document storm
events that killed very large numbers of overwintering butterflies. A winter storm in 2002 killed
an estimated 468-500 million monarchs representing colony losses of 75 percent (Brower et al.
2004, p. 162). When considering that a single winter storm event killed more than 450 million
butterflies, it is important to note that the 2013-2014 eastern monarch overwintering population
numbered only 35 million butterflies. The drastically reduced current population size of
monarchs now makes the species even more vulnerable to catastrophic events.
Stochastic weather events kill monarchs directly and cause habitat degradation. Vidal et al.
(2013) found that 115 ha of monarch overwintering grounds in Mexico were degraded by floods,
winds, droughts, and fires from 2009-2011 (p. 182). Extreme drought in the monarch reserve
from 2008-2011 is thought to have contributed to greater susceptibility to forest diseases and
parasitic plants (Ibid.).
Monarch ESA Petition 105