3 years ago



Figure 11. Western

Figure 11. Western monarch population estimates from November 1 - December 15 at four sites: Ellwood Main (Santa Barbara County), Morro Bay State Park Campground (San Luis Obispo County), Purple Gate (Marin County) and Natural Bridges (Santa Cruz County); figure from Jepsen and Black in press. Thus it is clear that historically monarchs were highly abundant, though annual population sizes were not quantified prior to the late 1990s when monitoring began Though monarchs are still widely distributed, their abundance has declined drastically across their U.S. range, as discussed in detail below. Very recently, the number of monarchs from east of the Rockies has declined from occupying an overwintering area of 7.8 hectares in the 1994-1995 overwintering season (the first year data are considered to be reliable), to occupying an area of only 0.67 hectares in the 2013-2014 overwintering season, a decline of more than 90 percent from the 20-year average, and a decline of 97 percent from the 1996-1997 population high (Rendón-Salinas and Tavera-Alonso 2014). Monarchs from west of the Rockies have also undergone recent significant decline. In the winter of 1997, which is the year that monitoring began, there were more than 1.2 million monarchs overwintering in California (or an average of 12,232 monarchs per site), but in 2013 there were only about 200,000 monarchs counted (an average of 2,151 monarchs per site), representing a decline of 90 percent from the 1997 high and a 51 percent decline from the 17-year average (Monroe et al. 2014, Figure 13). Western monarch numbers have not reached the highs recorded in the late 1990s since that time, and have fluctuated around 200,000 butterflies since 2001 (Monroe et al. 2014). Historical estimates of the overall California overwintering population size range up to 10 million butterflies (Nagano and Lane 1985, Nagano and Freese 1987). There are several research and citizen science programs that provide data on current monarch distribution and abundance, including the World Wildlife Fund Monarch Monitoring Project in Monarch ESA Petition 36

Mexico, the Monarch Larva Monitoring Project, Peninsula Point Migration Monitoring Project, Cape May Migration Monitoring Project, the Western Monarch Thanksgiving Count, annual censuses of monarchs in the Central Valley and Sierra Nevada by Dr. Art Shapiro, the North American Butterfly Association annual breeding adult surveys, and state-level programs (Monarch Net 2014, see: To estimate overall abundance of monarchs that overwinter in inland Mexico, scientists rely on the combined area of overwintering colonies because it is a direct measure of the entire migratory population (Brower et al. 2012b, p. 328). On-the-ground counts have resulted in estimates of 10 to 60 million butterflies per hectare of trees occupied, with 50 million monarchs per hectare being used as a standard estimate of overwintering butterfly numbers, since measurements are taken at a time of year when butterflies are likely to be most tightly packed, and since the higher density numbers are from more recent and standardized studies (Slayback et al. 2007). Monarch numbers in winter roosts generally correlate with numbers produced during breeding in a given season, although variable mortality does occur during migration. Reliable information on colony sizes and locations in Mexico is available since the 1994–1995 overwintering season for eastern North America; earlier information is considered less reliable because it was gathered on increasing numbers of colonies as they were discovered by diverse groups of investigators with variable expertise. The overall abundance of monarchs that overwinter on the California coast is estimated from counting the actual number of butterflies at each site; 76-162 overwintering sites have been counted each year, and 17 sites have been consistently monitored since 1997 (Figures 13 and 14). The number of monarchs overwintering in Mexico, primarily representing the eastern migratory population, shows a statistically significant decline over the past twenty years (Figure 12). In winter 1994-1995, monarchs occupied 7.81 hectares of oyamel forest. The highest number observed was in winter 1996-1997 when monarchs occupied 20.97 hectares. By 2004-2005, the number of hectares had dropped to 2.19, and has not since risen to 7.0 hectares, the area covered when standardized counts began in 1994-1995. Regression analyses show statistically significant monarch population decline even when the highest and lowest measurements are removed (linear model, P = 0.032 or 0.042; exponential model, P = 0.040 or 0.049; Brower et al. 2012a, p. 96). We extended the Brower et al. (2012a, Fig. 1) graph to include the results of the three most recent winter surveys (Figure 12). Regression analysis of the extended data continues to show a statistically significant decline in monarch abundance (P = 0.01). In summary, there has been a 91 percent decline in overwintering eastern monarch numbers over the past twenty years, with numbers in winter 2013-2014 being the lowest ever recorded. Monarch ESA Petition 37

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