monarch-esa-petition-final_61585

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: http://monarchnet.uga.edu/).
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