Western monarch population trends (winter-arrival):

  • Highlighted subregion: California Coast. Groves along the CA coast where monarchs congregate in the winter.
  • Season: Winter arrival (11/1-11/30). The arriving population at the overwinter grounds. Mortality occurs throughout the winter and that is excluded from estimates.
  • Featured program(s):
    • Xerces: Once a year counts at each of the known overwinter groves. Raw numbers are the estimate of total monarchs observed in the grove.
00.250.50.751199719992001200320052007200920112013YearRelative abundance IndexProgramsWinter-arrival California Overwintering Grounds (Thanksgiving Counts)

All trend graphs show “relativized” abundance indices.  This means that “raw” scores that measure some index of abundance are transformed so the highest value in the series is equal to 1, and all other values are then relative to the highest value.  This allows scores to be overlaid among programs so that yearly dynamics can be compared.  The “raw” score measured varies and is described separately for each program.  The length of the time series (start and end year) vary depending on how long the programs have run and how much data we have received from each program.