monarch-esa-petition-final_61585

availability (Cockrell et al. 1993, Malcolm et al. 1993, Brower 1995, Howard and Davis 2004),
making disruption in milkweed distribution a dire threat to their survival and reproductive
success.
Climate change is also expected to cause increased frequency and intensity of drought, which
threatens monarchs in several ways. Climate change models predict increasing drought and
reduced water availability across much of temperate western North America by 2050
(Christensen et al. 2007; IPCC 2013b). Moreover, it is generally expected that the duration and
intensity of droughts will increase in the future (Glick et al. 2011, p. 45). Drought has already
been identified as a primary contributing factor in population declines of western monarchs
(Stevens and Frey 2004, Stevens and Frey 2010, p. 733). Stevens and Frey (2010) found that
variation in moisture availability (as measured by Palmer’s drought severity index) predicted
monarch abundance patterns across the western United States, and determined that moisture
regimes act as a strong bottom-up driver of monarch population dynamics; essentially, years of
severe drought across the western monarch breeding range were associated with the lowest
monarch population estimates in the western United States (p. 731). Stevens and Frey (2010)
suggest that drought reduces the abundance and quality of milkweed, thus leading to lower
monarch populations. Milkweed quality for developing larvae deteriorates at high temperatures
(Batalden et al. 2007, p. 1365). Drought reduces milkweed germination, survivorship, growth,
and seed production (Stevens and Frey 2010, p. 740). Reduced water availability can also cause
changes in the properties of milkweed plants. Milkweed plants with low water availability may
cause declines in larval survival because the latex is more viscous and can make leaf-eating more
difficult (Stevens and Frey 2010, p. 740).
Climate change also threatens monarchs in their winter ranges in California and Mexico.
Monarchs east of the Rockies migrate to Mexico each fall where they overwinter in conifer
forests in the Trans-Mexican Volcanic Belt. The monarchs require very specific habitat
conditions in these forests so that they do not freeze or become too warm and break diapause.
The climate change models for the monarch’s overwintering habitat predict that the currently
occupied habitat will become unsuitable for monarchs by the end of the century.
Saenz-Romero et al. (2012) found that, by the end of the century, the climate will no longer
support the forested habitat conditions upon which monarchs depend for overwintering in
Mexico. In this study, the authors projected the monarch’s contemporary Mexican overwintering
climate niche into future climates provided by three General Circulation Models and two
greenhouse gas emission scenarios and found that the area occupied by the niche will diminish
rapidly over the course of the century. They predicted a decrease of suitable conditions of 69.2
percent by the decade surrounding 2030, a decrease of 87.6 percent for that surrounding 2060,
and a decrease of 96.5 percent by 2090 (p. 98). In Mexico by the end of the century,
temperatures are expected to increase by an average of 3.7˚C, and precipitation is expected to
decrease by 18.2 percent (Ibid.). By 2100, suitable habitat for the monarch butterfly may no
longer occur inside the Monarch Butterfly Biosphere Reserve (Ibid.). Drought is already causing
tree loss and increased susceptibility to forest diseases within the Reserve (Saenz-Romero et al.
2012, p. 99).
Monarch ESA Petition 103