Ecosystems, populations and land use
Population numbers come from births and deaths, arrivals and departures, and from scaling up a count made in a small sample plot.
A worked example
A herd of 800 with 210 births, 130 deaths, 30 arrivals and 55 departures ends the year at 800 + 210 - 130 + 30 - 55 = 855.
The mistake people actually make
Density needs the area of the sample plot, not the length of one side. A 5 m by 5 m quadrat is 25 m2, not 5 m2. Immigration is arriving and emigration is leaving; swapping them flips the sign on two terms at once.