3 years ago



larvae in laboratory

larvae in laboratory conditions (Rawlins and Lederhouse 1981, Zalucki 1982). Exposure to temperatures above 29˚C (84°F) can be detrimental to the development of monarch larvae, with effects being dependent on length of exposure (York and Oberhauser 2002, p. 290). Increasing lengths of constant exposure to high temperatures result in increasingly higher mortality, longer development time, and lighter adult mass (Ibid.). Increasing temperatures threaten to disrupt the monarch migration. Larvae could be subjected to high-temperature conditions of longer duration which could compromise fitness by increasing pre-adult mortality, increasing development time, or decreasing adult size (York and Oberhauser 2002, p. 297). Generally speaking, areas south of Ohio are already too warm to support optimal larval growth during summer months (Malcolm et al. 1987; Batalden et al. 2007). Increasingly high temperatures and more frequent, more intense, and longer duration heat waves threaten monarchs in both their eastern and western range (Christensen et al. 2007, IPCC 2013b). Increasing temperatures could make the monarch’s current summer habitat unsuitable (Batalden et al. 2007, p. 1371). In addition to threatening the migratory populations in North America, climate change could eradicate the peripheral monarch populations that are not part of the main eastern or western migrations such as the stationery population in south Florida and populations found outside the United States on Pacific Islands and in Australia. In Miami, Florida, for example, the mean monthly maximum temperature recorded at the Miami airport from 1961–1990 (April to September) was 31.7°C (Knight and Brower 2009, p. 821). This mean temperature is already high enough to have direct negative effects on monarch larvae (Rawlins and Lederhouse 1981, Zalucki 1982, York and Oberhauser 2002), and global climate change is expected to cause an increase in mean maximum temperatures throughout Florida (Alder and Hostetler 2013). From 2050-2074, the annual mean maximum temperature in Miami-Dade County is expected to increase by 4.1˚F, with models predicting mean temperature increases of 2.3˚F - 5.9˚F (Alder and Hostetler 2013). This increase would render the area unsuitable for monarchs and could eradicate the non-migratory resident monarch population. In many parts of Florida, temperatures may already often exceed the threshold that is lethal to developing monarchs (Knight and Brower 2009, p. 821). In Gainesville, for example, the mean monthly maximum temperature from 1961– 1990 from April to September was 32.6°C (Ibid.). Even increases at the lowest end of predictions would make the Gainesville area unsuitable for breeding monarchs, as temperatures in Alachua County are expected to increase by a mean of 5.0˚F, with models predicting increases ranging from 2.5˚F-7.4˚F (Ibid). Other outlying monarch populations could also be wiped out by climate change impacts. Australia, for example, has suffered from a decade-long severe drought and climate change is predicted to increase drought conditions on the continent (Van Dijk et al. 2013). In addition to threats from rising temperatures, island populations are likely to decrease in size as rising seas eliminate habitat. Increasing temperatures threaten monarchs with direct mortality, and also threaten to alter the distribution of milkweed, the monarch’s sole host plant. Due to increasing temperatures, the distribution of common milkweed will likely shift northward, but the plant may not be able to colonize northward as rapidly as monarchs will require if they are displaced from the southern parts of their range due to increasing temperatures (Batalden et al. 2007, p. 1371). Southern species of milkweeds generally become less nutritious or die back during summer and so are unsuitable host plants for the summer generations of butterflies, including those that will migrate in the fall. Monarch breeding and migration are coordinated with and dependent on milkweed Monarch ESA Petition 102

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

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