Animals moving in and out count too
- The idea
- Predict
- See it
- Why
- Check
The idea
“Population change is just births minus deaths, so animals moving in or out don't change it.”
dN/dt = B - D has only births and deaths in it, so it looks complete. It assumes nothing moves in or out. And immigration and emigration sound alike, so they're easy to swap.
The short version
The mistake: Population change is just births minus deaths, so animals moving in or out don't change it.
What's actually true: Births and immigrants add to a population. Deaths and emigrants take away from it.
- A population is the members of one species in one area. Births and immigration, or moving in, add members. Deaths and emigration, or moving out, remove them.
- So the change is births + immigrants - deaths - emigrants. To keep the names straight, immigrants come in and emigrants exit.
- dN/dt = B - D describes a closed population, one nobody enters or leaves. Use it alone only when movement is too small to matter.
- A population can hold steady or even grow while deaths outnumber births, if enough individuals arrive. Its count alone can't tell you whether it sustains itself.
On the exam: Count four flows. Births and arrivals add, deaths and departures subtract.