Per capita means per individual, not per breeder
- The idea
- Predict
- See it
- Why
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The idea
“Only adults breed, so I divide births by the females or the breeders.”
Only females give birth, so dividing by the mothers seems fair. The young and the males are part of the population too.
The short version
The mistake: Only adults breed, so I divide births by the females or the breeders.
What's actually true: A per capita rate divides by every individual in the population, young and nonbreeding ones included.
- Per capita means per individual. The birth rate b is births divided by N, the whole population.
- Births per female is a real number, but it describes the mothers, not the population.
- The growth rate r = b - d works only when b and d share the same N. Then r times N is the real change.
- That's why crowding that keeps adults from breeding lowers b. They still count in N but add no births.
On the exam: Per capita means divide by N. Young, males and nonbreeders all count.