Factors affecting Growth of Natural Population

1. Competition for food /water/oxygen: Population will thrive when these are in abundant supply. When in short supply, the population number will decrease. Some organisms may die due to starvation

2. Effect of disease and pests: This may cause death of organisms within the population. E.g. Black death/Bubonic plague killed 30-60% of Europe's population in the 14th Century. 

3. Natural Disasters:  Fires, flood, hurricanes and oil spills for example can wipe out several populations at once in a short time (overnight maybe). E.g. Tsunami (cause by earthquake) in Indian Ocean in 2004 killed more than 230,000 humans and many other species of living organisms.

4. Competition: Organisms (both plants and animals) compete for space, food, light and water and other resources. The resources can however, only support a finite number of organisms within an ecosystem.  (Will the earth have enough resource to maintain its growing population?). Competition helps to keep the number of organisms in check (prevent overpopulation of one type of species). This is an advantage in some cases e.g. in human digestive system.

Reference

1.CXC, Biology Resource Materials for Teachers, Book 5, 1997.

2.http://en.wikipedia.org/wiki/Black_Death

3. http://en.wikipedia.org/wiki/2004_Indian_Ocean_earthquake_and_tsunami