All of the factors affect population size.
Birth rate, death rate, immigration, emigration, disease, competition and predation, human activity and natural disasters and occurrences.
Population size is the total number of individual organisms in a population. A population size can be increased by birthrate (number of individuals born per 1,000 individuals per year), and immigration (movement of an individual to a new location), and it can be reduced by death rate, disease, emigration, competition and predation, and natural disasters and occurrences.
