• A number of factors cause shifts in population growth.world population image by Marvin Gerste from Fotolia.com
  
  Billions of people live on Earth, and the population continues to grow. The population of the Earth was 1 billion in 1804; it surpassed 6 billion in 1999. At this rate of growth, some wonder if the Earth can continue to sustain a growing population. To determine where the population will go in the future, scientists look at factors that cause shifts in population growth.
  
  

Fertility

• Fertility affects population growth by adding to the population. The number of babies born is also referred to as birthrate or natality. The rate of fertility depends on the location in the world, economics and human aspirations. More developed countries tend to have a lower birthrate, while less developed countries tend to have a higher birthrate. A University of Michigan professor cites the reason for this in a lecture called, "Population Growth over Human History." People in less developed countries need more people (children) to accomplish jobs with less technology. On the other hand, having more children in a developed country decreases your standard of living.
  
  

Mortality

• Mortality, or the death rate of individuals, also affects population growth. Mortality in developed countries has steadily declined since the start of the industrial revolution. This is a result of improvements in medicine, public health and sanitation. In pre-modern times, the birthrate and death rate were fairly even, resulting in hardly any population growth. During pre-modern times, infectious diseases spread quickly and children died at a young age. In the late 18th century, the death rate began to decline while the birthrate was still high. This resulted in high population growth.
  
  

Immigration/Emigration

• Immigration and emigration also cause shifts in population growth. Immigration is when people move into an existing population. Emigration is when people leave an existing population. Immigration causes a population to grow, while emigration causes a population to shrink in a particular region. Many people immigrating to an area can cause an increased birthrate in that area. If large numbers of people emigrate from an area, the birthrate may decline because of a decreased number of people.
  
  

Environment

• The ability of the surrounding environment to support specific populations also causes shifts in population growth. In developing countries that are growing rapidly, concerns exist about the sustainability of the land. In India, the amount of water withdrawn is double the amount of water replenished. This causes water tables to drop each year. Rising populations in sub-Saharan Africa and India are decreasing the amount of grain land available to each person. This may lead to countries needing to import crops, and they may not have the economic capabilities to afford that.