What are the four processes that change population density? Write the equation that links them.

1. 1 · Define population density. Population density (N) is the number of individuals of a species per unit area at a given time.
2. 2 · Name and explain the four processes.
   • Natality (B): births in the period.
   • Mortality (D): deaths in the period.
   • Immigration (I): individuals of the same species arriving.
   • Emigration (E): individuals leaving the habitat.
3. 3 · Write the equation. Nₜ₊₁ = Nₜ + [(B + I) − (D + E)], explaining each symbol.
4. 4 · State when density rises or falls. It rises if (B + I) is more than (D + E), and falls if (D + E) is more than (B + I).
5. 5 · Give the special case. Births and deaths matter most normally; immigration matters most when a new habitat is being colonised.
6. 6 · Link to real life. Indian cities grow partly by people moving in from villages, which is immigration.
7. 7 · Close in one line. So the balance of additions and losses decides whether a population grows or shrinks.

Scoring tip: Write the equation exactly, with brackets, and define B, I, D and E. One unexplained symbol can cost half a mark.