Spaying and neutering prevent unwanted litters, overpopulation, and inbreeding. The effects of overpopulation are compounded by overconsumption. Most shelters also provide medical care that includes spaying and neutering to prevent overpopulation. Global food security is being threatened by desertification and overpopulation. Spaying or castrating dogs helps keep overpopulation down. This suggests that Third World poverty and famine are caused by underdevelopment, not overpopulation. The country continues to face challenging problems, including overpopulation, terrorism, poverty, illiteracy, and corruption. Donella Meadows argued a correlation between overpopulation and poverty. Many new residents come from rural areas with chronic underdevelopment and overpopulation issues. Overpopulation and land shortages stimulated a Vietnamese expansion south. Emigration was encouraged by political discontent at home, overpopulation and impoverishment. Keller also expressed concerns about human overpopulation. Controversy about life extension is due to fear of overpopulation and possible effects on society. Concern about overpopulation is an ancient topic. Population growth that exceeds the carrying capacity of an area or environment results in overpopulation.