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habitat than thinned forest and provides important temperature buffering effects, especially
during severe weather events (Brower et al. 2011, p. 27, 42).
The integrity of the high-elevation cloud forest that supports the monarchs depends upon an
extensive and dense forest structure to capture moisture (Brooks et al. 1997). Ongoing logging
and canopy loss threatens to undermine the hydrological integrity of the ecosystem, which
threatens the continued survival of the overwintering monarchs (Calvert et al. 1979, Slayback et
al. 2007, p. 39). Small canopy openings also increase edge effects which increase the risks of
wildfire, tree mortality, changes in plant and animal species, and increased human use of the land
(Vidal et al. 2013, p. 8).
In addition to small-scale logging, the monarch’s overwintering forest habitat is threatened by
senescence and forest diseases. There has been a recent increase in the level of bark-beetleinduced
tree mortality in the overwintering grounds. Several species of beetles are causing tree
mortality including Scolytus mundus Wood, Psuedohylesinus variegatus [Blandford],
Pityopthorus spp., and Dendroctonus mexicanus Hopkins (Steed and Willhite 2011, p. 12). Most
tree mortality in the core area is in oyamel firs that have been attacked by P. variegatus, which
was “observed in the lower bole of every examined dead and dying fir greater than 5 inches in
diameter at breast height” during a recent forest health assessment (Steed and Willhite 2011, p.
3). Although only a small area has been affected, the beetle outbreak is occurring in multiple
sites within the reserve. In an attempt to stop the spread of the beetle, 9,000 trees were felled in
2009 alone. It is estimated that 15 years of continued beetle population growth could decimate
the fir trees in the reserve (COSEWIC 2010, p. 12).
Other disease agents are also contributing to increased levels of mortality of firs, pines, and other
trees in the reserve including annosus root disease (Heterobasidion annosum, P-group [now H.
occidentale]) and dwarf mistletoes (Arceuthobium abietis-religiosae Heil, A. globosum Hawksw.
and Wiens) (Steed and Willhite 2011, p. 12). In field visits from 2011-2012, Vidal et al. (2013)
identified 14 ha of forest that had been impacted by drought and parasitic plants (Arceuthobium
spp. and Psittacanthus calyculatus) and an additional 7 ha that had been logged for disease
control (p. 181). In addition to tree loss due to disease and disease-control activities, natural
forest aging also threatens the reserve because monarchs typically form colonies in mature
forests and as forest patches age, it is unclear whether they will be replaced (Keiman and Franco
2004).
Water diversion for human and domestic animal use may also pose a significant threat to
overwintering habitat in Mexico (Commission for Environmental Cooperation 2008). At one major
water source for monarchs—the Ojo de Aqua ravine on the south side of Cerro Pelόn—water has
been diverted so extensively that the stream is now dry for more than 1 km. Monarchs now have
to fly farther distances to obtain water, which may deplete the lipid reserves needed to survive
the winter and sustain the spring migration (Ibid).
As discussed in more detail in the Other Factors Affecting the Monarch’s Continued Existence
Section of this petition, severe weather events threaten the monarchs with direct mortality and
with habitat degradation when trees fall down due to ice, wind, fire, floods, or drought. From
2009-2011, 115 hectares of forest were impacted by floods, strong winds, droughts, and fires,
Monarch ESA Petition 70