Final answer:
The population of Europe was reduced significantly during outbreaks of bubonic plague, such as the Plague of Justinian and the outbreaks between 1347 and 1351. The demographic transition in the twentieth century also contributed to declining birth rates in developed countries.
Step-by-step explanation:
From 541 to 750 A.D., an outbreak called the Plague of Justinian eliminated, by some estimates, one-quarter to one-half of the human population. The population in Europe declined by 50 percent during this outbreak. Bubonic plague would strike Europe more than once.
Another significant reduction in the population of Europe occurred during the outbreak of bubonic plagues between 1347 and 1351. Over half of Europe's population died during this period. These outbreaks continued to strike Europe nearly every year until 1782. Europe's population began to recover in the fifteenth century but did not return to its pre-plague levels until the seventeenth century.
The demographic transition in the middle of the twentieth century also led to declining birth rates in developed countries, resulting in a slowing of population growth. Some developed countries, such as Russia and Japan, are currently experiencing a decline in population.