The population size is being affected by different factors. In general, these factors are divided into two: density dependent and density independent factors.
Density dependent factors are have a positive or negative relationship to the size of the population. These factors are diseases, competition, and predation. Outbreaks may lead to reduction of the population if it will cause direct mortality. Competition will affect birth and survival of the population. Predation will act as a top-down control of the population.
Density independent factors are food limitation, presence of pollutants, and changes in climate. These are considered as limiting factors. They are also called as physical factors of the environment. If there is food crisis that will result to limitation of its supply, a part of the population will not be able to survive. Population that cannot survive the presence of pollutants in the environment will die. The changable cllimate will not let human survive a certain environment.