核心问题
Many parts of the world, which once had large populations and produced plenty of crops, have become deserts.
英语教学辅导书大都把 population 说成是不可数名词。不可数名词能有复数形式吗？
词典解答
作“人口”解的 population 一般用作不可数名词,没有复数,但有时可与不定冠词连用:
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Population ... increases in a geometrical ratio, subsistence in an arithmetical ratio. 人口……按几何级数增加，而生活资料按算术级数增长。（马尔萨斯）
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New Zealand is an important agricultural country with a small population. 新西兰是一个重要的农业国，人口很少。
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The island has a population of about 78,000. 这个岛上有 78,000 人。
作“居民”解的 population 通常用作不可数名词,但偶尔亦用作可数名词(如你所提问的句子中的 populations):
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The population in these villages has to get its water from wells. 住在这些乡村的人必须自井中汲水。
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Entire populations were enslaved. 全部居民都作了奴隶。
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the populations of Western Europe countries 西欧各国的全体居民