Population change and its limits
A population grows by births and immigration and shrinks by deaths and emigration, until limiting factors cap it.
A worked example
A herd of 600 with 90 births, 40 deaths, 20 arrivals and 10 departures in a year ends at 600 + 90 - 40 + 20 - 10 = 660.
The mistake people actually make
Movement in and out counts as much as births and deaths. Density-dependent factors such as disease and competition bite harder as crowding rises; a hailstorm does not.