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monarch-esa-petition-final_61585

monarch-esa-petition-final_61585

led to a drastic decline

led to a drastic decline in milkweed abundance in agricultural fields, as discussed in detail in the Threats section of this petition. In the western United States, milkweeds are distributed across the landscape (Figure 10). More research is needed to understand how milkweed availability may have changed over time in the west, and what impact that may have had on monarchs. Figure 10. Records of milkweeds (multiple Asclepias species) from 1860-2010 (blue and green) and records of monarch caterpillars on milkweed (orange). Note that records for Montana and Wyoming are not displayed on this map. Figure courtesy of the Xerces Society, available at: http://monarchjointventure.org/our-work/western-us-milkweed-survey The grasslands and prairies of North America were rapidly and almost completely converted to rangeland for domesticated animals and to agricultural fields after European settlers moved west beginning in the early to mid-1800s. Most milkweed species would have declined in abundance as a result. At about the same time that grasslands and prairies were being plowed under, forests east of the Mississippi were being cleared. Though most milkweed species declined following prairie conversion, common milkweed (A. syriaca), which thrives in areas of soil disturbance, increased in range and abundance in both agricultural and logged areas (Brower 1995). Monarchs thus would have been able to maintain high populations after European colonization of North America by shifting the center of their population east and north as formerly forested Monarch ESA Petition 34

land was invaded by common milkweed, and by substituting this one milkweed for most of the others as their main host plant in the northern and eastern breeding range. Based on the limited historical data that are available, monarchs were highly abundant in the mid- to late-1800s. Brower (1995 and references therein) discusses early observations of monarchs in the Midwest and east by naturalists, journalists, farmers, and scientists. D'Urban (1857) described monarchs appearing in the Mississippi Valley in “such vast numbers as to darken the air by the clouds of them” (in Brower 1995, p. 349). Scudder and Allen (1869) described monarchs gathered in groves of trees bordering the prairie in Iowa “in such vast numbers, on the lee sides of trees, and particularly on the lower branches, as almost to hide the foliage, and give to the trees their own peculiar color” (in Brower 1995, p. 306). In the 1870s swarms of monarchs were reported in New England and the Great Lakes. Saunders (1871) observed “vast numbers-- I might safely say millions” of monarchs clustering on trees on the Canadian shore of Lake Erie (in Brower 1995, p. 308). Scudder (1889) noted endless masses of monarchs migrating through Connecticut in 1871 (Ibid.). In 1872 an immense swarm of monarchs was observed in flight over Cleveland, Ohio (Brower 1995, p. 308). Prior to monitoring efforts that began in the 1980s, the historic distribution and size of the western monarch population was largely unknown. There are early accounts of overwintering masses of monarchs from Monterey, California in 1869 and 1873, and from Santa Cruz in 1888 (Lane 1993, Brower 1995). In May 1874 the Monterey Weekly Herald published an account from near Pacific Grove of “millions” of monarchs “fluttering around,” “while overhead stout branches of firs dropped with their weight” (in Lane 1993, p. 341). An 1881 letter describes trees near Monterey “over one and a half feet in diameter, and completely covered with live butterflies. To say that there were as many butterflies as leaves upon the trees would not be a very great exaggeration” (in Lane 1993, p. 341). Historic estimates of the western overwintering population size range from 1 to 10 million (Nagano and Lane 1985, Nagano and Freese 1987). Leong et al. (2004) used data from the California Natural Diversity Database (CNDDB) from 1990 to 2000 to estimate the maximum number of overwintering monarchs for a single season to be more than 2.3 million. Historic estimates of monarch population size that are available for a few overwintering sites suggest that the monarch population was larger prior to the onset of a large-scale yearly monitoring effort that began in 1997 (Figure 11.) Monarch ESA Petition 35

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