depopulation
  人口減少
  De·pop·u·la·tion n.  The act of depopulating, or condition of being depopulated; destruction or explusion of inhabitants.
     The desolation and depopulation [of St.Quentin] were now complete.   --Motley.
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  depopulation
       n : the condition of having reduced numbers of inhabitants (or
           no inhabitants at all)