What are two ways that the size of a population can increase?

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Two ways a population can increase is more births than deaths, and immigration to the area.

One way is by medicine. You see medicine means that somebody will be able to live longer. This could then mean that more people on Earth would be living longer, causing the population of the Earth to increase. 
The second way is by birthrate. Because of birthrate, the Earth is being overpopulated. (That is why long ago, China has a strict rule about having one child per family. China was already overpopulated and needed to kind of stop the increase of population there.)
For more information, it was told that by 2020 that the Earth would have about 8 million people in the Earth.

Hope this helps!!!

Plants take energy from the sunlight with the of chlorophyll so the plant starts the food chain. then an herbivore eats the plant and gets its energy.then a carnivore or a human eats the herbivore and gets its energy. : )