Why do you think population changes?
Population changes take place due to a number of factors, such as food and resource availability, predation and environmental factors. When the availability of food reduces, population decreases. For example, a decrease in number of deers will reduce the population of tigers, etc. Resource availability, such as, drinking water, hideouts, etc., also determine the size of population that can be supported. Predation by higher organisms is another factor that can change the population. An increase or decrease in number of predators can cause a variation in population. Environmental factors, such as temperature, humidity, weather, etc., can have drastic effects on population. An increase in temperature will increase burden on existing water resources and will result in deaths from heat or lack of water, etc.
In case of human population, emigration, immigration, political crisis, wars, technology, etc. are additional factors that may affect the population.