population表示什么意思的情况下加s?

表示人数的数量时不加s

见此短语
the populations of the west European countries （牛津英汉词典）
表示多个团体的人口的时候加s

这样：
population是一个集合名词,它的用法有时较为特殊,所以很容易用错.下面谈一下它的用法.
一、population常与定冠词the连用,作主语用时,谓语动词常用第三人称单数形式.例如:
The world\'s population is increasing faster and faster.全世界的人口增长得越来越快.
At the beginning of the twentieth century,the world\'s population was about 1,700 million.在二十世纪初,全世界的人口大约是十七亿.
二、当主语是表示\"人口的百分之几、几分之几\"时,谓语动词用复数形式.例如:
About seventy percent of the population in China are farmers.中国大约有百分之七十的人口是农民.
三、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.
四、表示人口的\"多\"或\"少\",不用\"much\"或\"little\",而要用\"large\"或\"small\".例如:
India has a large population.印度人口众多.
Singapore has a small population.新加坡人口少.
五、询问某国、某地有多少人口时,不用\"How much...\",而用\"How large...\";在问具体人口时用\"What...\".例如:
-How large is the population of your hometown?你们家乡有多少人口?
-The population of our hometown is nearly twice as large as that of yours.我们家乡的人口是你们家乡人口的将近两倍.
-What is the population of Canada?加拿大的人口有多少?
-The population of Canada is about 29 million.加拿大的人口大约有二千九百万.
六、population还表示\"某地、某类的动、植物或物品的总数\".例如:
In India,however,the population of tigers has increased,from 2,000 in 1972 to about 5,000 in 1989.然而在印度,老虎的总数已从1972年的2,000只增长到了1989年的大约5,000只.
我想应该知道了吧?
再见!