The population of a country can decrease for various reasons: fewer people are born, more people die, or migration out of the country is large enough to counter the usual population growth. Japan currently shows no population growth because of their balance between fertility and mortality. For the countries shown in the graph here, the story is mostly demographic and thus economic. Bulgaria, Moldova, and Romania are poor countries from Eastern Europe. In the early 1990s, it became possible to emigrate from these countries to look for better opportunities. Many residents chose to do so, and this trend continues to this day. For Greece and Portugal, the story is different. For example, the economic turmoil in recent years prompted a sufficient number of locals to leave for jobs elsewhere, which also led to a reduction in population. The data sample available in FRED shows that this happened twice before in Portugal.
How this graph was created: Search for total population and the respective countries. Convert units to “Percent Change from Year Ago” (if frequency is not annual) or “Percent Change.”
Suggested by Christian Zimmermann