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Imagine a remote--and unique--island populated by 2,000 children,
half girls,
half boys. As they grow older they pair and have children.
By age 30 they
have had 2,000 children--half girls, half boys--and have no
more. The
island's birth rate is 2.0 (2,000 children
divided by 1,000 women) and its
population is 4,000.
Imagine the second generation of 2,000 children pair and
have children. By
age 30 they have had 2,000 children and have no more. The
island's birth
rate remains 2.0 and its population is 6,000.
Imagine the third generation pair and have children and the
first generation
begins dying at the same rate a fourth generation is born.
For each birth
there is a death. For each death, there is a birth. As a result,
the
population of our imaginary island remains stable at 6,000.
A birth rate of
2.0 is one element of a stable population.
Imagine another island populated by 2,000 children. As they
grow older they
pair and have children. By age 30 they have had 4,000 children
and have no
more. The island's birth rate is 4.0 (4,000
children divided by 1,000 women)
and its population is 6,000.
Imagine the second generation of 4,000 children pair and
have children. By
age 30 they have had 8,000 children and have no more. The
island's birth
rate remains 4.0 and its population is 14,000.
Imagine as the third generation pair and have children, the
first generation begins dying. When the first generation is
gone, the population is reduced by 2,000, but with the fourth
generation of children added, the population is 28,000. The
birth rate is twice that of our first island, but its population
is more than four times as large --and it continues to grow!
Birth rates matter! Think Population! |
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