Partnering to conserve the monarch butterfly migration

Analysis of Western Monarch Population Trends

Data collected by Monarch Alert will be used to determine if the peak western overwintering population size can be estimated by the Thanksgiving Count, or if the population of overwintering monarchs increases in size after the Thanksgiving sampling period. Monarch Alert has conducted weekly counts for three falls (2009‐2011), and three winters (2010‐2012) at a subset of the sites that are sampled for the Thanksgiving count. These 17 sites are located within San Luis Obispo and Monterey counties. Through this analysis, they anticipate being able to determine if the annual Thanksgiving counts from the last three years were conducted during a time of year that overlaps with the maximum population size of overwintering Monarchs in San Luis Obispo and Monterey Counties. From this they will recommend whether these counts should remain as the single best estimator of the population size, or that they be supplemented by an additional survey period. The additional survey period, if any, would likely be after the winter solstice.

Xerces Society staff will use the Thanksgiving Count data set to apply the International Union for Conservation of Nature (IUCN) Red List criteria to the overwintering western subpopulation of monarchs in order to understand their conservation status and extinction risk. This process will be informed by the analyses of the western monarch population trend conducted by Monarch Alert (described above), and will be completed in collaboration with Monarch Alert. 

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