What causes a population to grow?
More and more of that species to be born and species migrating into that area.
Population - The amount of a certain species in a certain area. If there is a disorder in the food chain or web it would cause on species in the community to grow larger than another.When one species in that community or ecosystem grows larger then another it makes the population grow bigger in size.
If you look at the demographics of the Japanese people it is most likely that the population will be in decline in the future. Tokyo is currently the world's largest city in terms of population and a large percentage of the population is advancing in age to the point where it will be very difficult for Tokyo to grow in population.