A steady count doesn't mean nothing is changing
- The idea
- Predict
- See it
- Why
- Check
The idea
“If a population stays the same size, the same individuals are just staying alive.”
A number that never moves looks like a population at rest. Behind it, individuals are born and die all the time.
The short version
The mistake: If a population stays the same size, the same individuals are just staying alive.
What's actually true: A steady count means births and arrivals balance deaths and departures, and all four can be large.
- A population grows with births and immigration, and shrinks with deaths and emigration. A flat count only tells you the gains and losses cancel.
- In a closed population, where no one arrives or leaves, a flat count means births equal deaths. Both can be high, so individuals keep getting replaced.
- When individuals can move, births and deaths don't have to match. Arrivals can make up for extra deaths.
On the exam: A flat count only says gains equal losses. Both can be big.