How does population growth in other countries affect the US?
People tend to move (legally or illegally) to the USA.
Population growth in other countries cause accelerated immigration to the USA because of better income, health care, job opportunities, etc. there. Since white race in the USA does not have a significant population growth rate, the USA would like to accept some skilled workers (scientists, artists, businessmen, who have lots of money) to the country.
But mostly poor, unskillful people tend to move to the USA from other developing countries. In high population growth rate countries some wars or conflicts raise. This is also a factor for emigration from those areas.