3 years ago



Figure 14. The total

Figure 14. The total number of monarchs counted at 17 monarch overwintering sites during the Western Monarch Thanksgiving Count from 1997-2012. The solid line represents the actual survey data. The dotted line represents the regression function. Figure from Griffiths and Villablanca (in preparation). Though their numbers have been drastically reduced, monarchs are still widespread in appropriate habitat in the continental United States. Flockhart et al. (2013) predicted where eastern monarchs are most likely to be found during the breeding season by determining the probable range based on amount and kind of vegetation, geographical limits (latitude, longitude, altitude, and slope), temperature, precipitation, and records from Journey North citizen scientist observations collected between 1997 and 2011 (Flockhart et al. 2013, Fig. 1). They determined that the majority of monarchs are found from east- and mid-Texas north into the Midwest, and then at a somewhat lower density throughout the east from southern Canada south to the Gulf. Some monarchs also occur much further west and north. Although monarchs are distributed throughout the eastern United States during the breeding season, their reproductive success is not uniform across regions. Wassenaar and Hobson (1998) analyzed stable hydrogen and carbon isotope profiles from wings of butterflies overwintering in Mexico to determine the host plants and latitude where the caterpillars had developed. They determined that half of the overwintering monarchs had “originated from a fairly restricted part of the breeding range, including the states of Kansas, Nebraska, Iowa, Missouri, Wisconsin, Illinois, Michigan, Indiana, and Ohio, corresponding to an area of intense corn, soybean, and dairy production in the Midwestern United States” (Figure 15, below). It is important to note that the butterflies they analyzed developed during the 1996 breeding season, and overwintering monarchs from that year covered the largest area in Mexico recorded in the last 20 years, 20.97 hectares. Using the standard estimate of 50 million butterflies per hectare (Slayback et al. 2007), Monarch ESA Petition 40

almost a billion individuals were in the population at that time, half of which metamorphosed on common milkweed in regions dominated by agriculture, particularly corn and soybeans. Figure 15. Natal origins of monarch butterflies in Mexico from the 1996 breeding season based on isotope data. The dark and light-shaded areas show the natal origins of 50% and 95% of the one billion monarchs that overwintered in 1996/97. The dashed line approximates the eastern breeding range. The Mexican monarch overwintering colonies are denoted by the solid circle. Figure 3 from Wassenaar and Hobson (1998), original caption omitted. Flockhart et al. (2013) extended the monarch natal origin studies by measuring isotopes in butterflies collected throughout eastern North America at different times during the 2011 breeding season. Researchers collected monarchs as they arrived in the southern United States from overwintering in Mexico, and then continued to sample butterflies throughout the summer and into fall to determine where each successive generation had originated. They determined that the overwintered generation in 2010 – 2011 had natal origins throughout much of eastern North America, but that most individuals came from a swath running from the northeastern states through the lower Midwest into northern Texas, and that fewer overwintered butterflies had originated in the heart of the Corn Belt as compared to the 1996 season (Flockhart et al. 2013, Fig. 2, panel a: “overwintered generation”). Notably, fewer overwintered butterflies originated in Monarch ESA Petition 41

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