...单位时间内迁入或迁出种群的个体数占种群个体总数的比例，分别成为种群的迁入率和（immigration rate）迁出率（emigration rate）。迁入与迁出率在现代生态学对城市人口的研究中占有重要地位。
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It has a lower birth rate than the north and also has net emigration, particularly of the young and educated.
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The Mexican baby boom of the 1950s coincided with lowish emigration, whereas the exodus to the United States kicked off in the 1980s, just as Mexico's birth rate was plummeting.
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