Lots of young doesn't mean a population is growing
- The idea
- Predict
- See it
- Why
- Check
The idea
“A population that's producing lots of young must be growing.”
Births are the part of a population you can see, like nests, eggs and newborns. Deaths in the same year are easy to miss, and they subtract.
The short version
The mistake: A population that's producing lots of young must be growing.
What's actually true: With no migration, a population changes by births minus deaths. It grows only when births outnumber deaths.
- A population changes by births minus deaths, plus immigrants minus emigrants. With no migration, only births and deaths count.
- The per capita birth rate b is births per year divided by population size. The death rate d is deaths per year divided by size.
- The per capita growth rate is r = b - d. If r is positive the population grows. At zero it holds steady, and below zero it shrinks, even when births are high.
- To compare populations, find each one's b - d first. A high birth rate says nothing about growth until you subtract that population's deaths.
On the exam: With no migration, growth is births minus deaths. Births alone never tell you which way a population goes.