Why isnt it just 2? What factors make up the 0.1?
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Future biologist here I’ll explain in a more scientific way and then in a more ELI5 way in a comment
When looking at population growth in ecology we just account for the members of the reproductive sex because it gives us a more precise outlook on the future of the population and the growth trend (females for humans), so the the factor to sustain a population is a little different than simply 2.1 babies/female.
I’ll skip the maths but basically the factor takes into account:
– Mortality rate of the specie per age range;
– Reproductive success (fertility) of a female that survives long enough to reach sexual maturity (how much females offspring they have on average each year);
– Life expectancy of a sexually mature female;
– Reproductive window (past a certain age the reproductive success is close to 0)
– The male to female at birth ratio
So the factor to sustain a population is 1, which essentially means that each female that can reproduce will in average have one female offspring (before dying) that will also have on average one female offspring that will reproduce
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