3 years ago



Drought has also

Drought has also contributed to declining abundance of monarch’s west of the Rockies. Stevens and Frey (2010) attribute declining western monarch populations to increasing drought conditions in the western United States (p. 733). They found that variation in moisture availability, as measured by Palmer’s drought severity index (PDSI), across the western region predicted monarch abundance patterns, and that moisture regimes act as a strong bottom-up driver of monarch abundance via resource availability in the western United States (p. 731). Furthermore, climate change models predict that drought severity will increase in large areas of temperate western North America, with 10-30 percent less precipitation and water availability by 2050 (Stevens and Frey 2010, p. 732). Extreme weather conditions that impact monarchs have become much more frequent and intense in recent years and have contributed to significant reductions in monarch population size (Vidal et al. 2013, p. 179). Moreover, the frequency of severe weather events is expected to increase with climate change (Brower et al. 2012a, p. 98). Barve et al. (2012) used ecological niche estimates and future climate projections to estimate future monarch overwintering distributions and predicted that regional climate change in the monarch’s overwintering grounds would result in increased monarch winter mortality because climate conditions in Mexican overwintering areas are trending consistently toward conditions inimical to monarch survival and extreme events appear to be increasing in frequency (Barve et al. 2012, p. 820). Models developed by Oberhauser and Peterson (2003) also predict increased winter season mortality and a likelihood of the monarch’s entire current winter range becoming climatically unsuitable habitat for monarchs (Oberhauser and Peterson 2003, p. 14063). Invasive Species The spread of invasive species also poses a threat to monarch butterflies. Invasive fire ants prey on monarch eggs and larvae (Calvert 1996), as discussed above in the petition section on predation. Invasive exotic plants threaten monarchs by acting as a sink when oviposition occurs on plants that are unsuitable for larval development, and when invasive plants displace milkweeds. Tropical or scarlet milkweed (Asclepias curassavica) is a non-native milkweed that is often planted in backyard gardens. In parts of the southern United States, A. curassavica has become naturalized and is considered invasive (Harvey et al. 2009). Its leaves do not die back at the end of summer as do native milkweeds, and this can have several negative effects on monarchs (McCord and Davis 2010, p. 415, Monarch Joint Venture 2014). When migrating monarchs encounter tropical milkweed in the fall, they may stop migrating, break diapause and lay eggs, a common occurrence in Florida where tropical milkweed is continuously available (Knight and Brower 2009). Another negative consequence of tropical milkweed is that in the absence of winter dieback, spores of the parasite Ophryocystis elektroscirrha accumulate on leaves over time and spread infections to monarch larvae (Altizer et al. 2004). The non-migratory south Florida monarch population is thus heavily infected with the parasite (Altizer et al. 2000). Invasive swallow-wort species also threaten monarchs by outcompeting and displacing native plant species, including milkweed, and by acting as a sink for monarch oviposition. There are Monarch ESA Petition 106

two highly invasive swallow-wort species that are widely distributed in the United States—black swallow-wort (Vincetoxicum nigrum, synonym Cynanchum louiseae L.) and pale swallow-wort (V. rossicum, synonym C. rossicum). Black swallow-wort is found from Maine through Kansas and in California. Pale swallow-wort is discontinuously distributed from the Great Lakes through New England and the Mid-Atlantic States. Both swallow-wort species out compete milkweed and also serve as dead-end hosts for monarch oviposition because monarchs lay eggs on them due to chemical cues similar to milkweeds, but larvae do not survive (DiTommaso and Losey 2003, p. 205, Casagrande and Dacey 2007, p. 632, 635). The threat posed to monarchs by invasive species is likely to be exacerbated by climate change, which is expected to facilitate the spread of exotic species (e.g. Dukes and Mooney 1999). Mortality at Solar Energy Facilities Monarch butterflies are threatened by mortality from solar arrays, particularly in southern California and the southwestern United States. In a study of bird mortality at three solar energy facilities in California, Kagan et al. (2014) documented significant monarch mortality at a solar site in Ivanpah, California (p. 2). The authors observed “hundreds upon hundreds” of dead butterflies and concluded that it appears that Ivanpah acts as a “mega-trap” for insects and in turn, insect-eating birds (p. 2, 20). Some butterfly carcasses were singed. Researchers deduced that the butterflies were attracted to a brightly lit area around the boiler at the top of facility (p. 20). Based on the large numbers of monarch carcasses observed at the facility, the authors conclude that solar power towers could have a significant impact on monarch populations in the desert southwest (p. 21). The threat posed to monarchs from solar facilities will likely increase in the future as more facilities are constructed. Electromagnetic Noise Monarchs may potentially be threatened by electromagnetic noise. Recent research has demonstrated that monarchs possess an internal magnetic compass, located in their antennae, which may help guide their migration (Guerra et al. 2014). In a recent paper, Guerra et al. (2014) note the possibility that electromagnetic noise emitted from various electronic devices could possibly impair the monarch’s migratory ability: Taken as a whole, our study reveals another fascinating aspect of monarch butterfly migratory behaviour. Greater knowledge of the mechanisms underlying the fall migration may well aid in its preservation, currently threatened by climate change and by the continuing loss of milkweed and overwintering habitats. Another vulnerability to now consider is the potential disruption of the magnetic compass in monarchs by humaninduced electromagnetic noise, which can apparently disrupt geomagnetic orientation in a migratory bird (Engels et al. 2014) (Guerra et al. 2014). Electromagnetic noise from AM radio signals and some electronic equipment can disrupt the magnetic compasses that migratory birds use to navigate (Engels et al. 2014). Human-induced electromagnetic noise presents a potential threat to the monarch migration and should be further investigated. Monarch ESA Petition 107

Parks for Monarchs
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