The phrase "to sustain the population" means to keep enough resources, such as food, water, and shelter, available to meet the needs of all the people living in a particular area, and to maintain their well-being and survival. Full definition
This planet is plenty big enough to sustain a population of humans who never die, right? (religion.blogs.cnn.com)
The environment has no trouble sustaining the population we have. (ilrdb.com)
Even before agriculture, human societies required technologies and social strategies to sustain their populations well above what natural ecosystems could provide. (dotearth.blogs.nytimes.com)