当population表示人口时可数吗,表示人的时候可数吗?

有时population可用作可数名词,其前可用不定冠词.例如:
China has a population of about 1.3 billion.(=There is a population of about 1.3 billion in China.) 中国大约有十三亿人口.
New York is a big city with a population of over 10 million.纽约是一个有一千多万人口的大城市.
在表示多个地区的人口时,population要用复数形式populations.例如:
Many parts of the world,which once had large populations and produced plenty of crops,have become deserts.世界上很多地区一度人口众多,种植大量的农作物;现在,这些地区已经变成了沙漠.