Dry lands are particularly affected by these trends, as livestock are often the only source of livelihoods for the people living in these areas.
干旱土地尤其受到这些趋势的影响,因为畜牧业往往是生活在这些地区的人口的唯一生计来源。
Agriculture provided a more dependable source of food, causing populations to increase; eventually excess population migrated to new lands.
农业提供了更加可依赖的粮食来源,使人口增长;最终过多的人口迁移到新的土地。
Now genetic studies show that the southern United States, not these native lands in South America, is the source of multiple new invasions around the globe.
现在的遗传学研究表明,这些美国南部的火蚁并不是南美原产的那种,而是来自世界各地的生物入侵。
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