There are many factors that can cause a population to increase. One of them being reproduction. Another one is having immunity to certain diseases, or health problems. There are more than just these two, like control of outside factors that are a threat to the population such as predators like bears, wolves, etc; as well as being a safe community without murder, fires, and natural disasters. These are factors that are true not only for human populations but other species as well.
The two factors that can cause a population to increase are birth/death rate and immigration/emigration.
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