A population has properties that an individual does not: density, birth rate, death rate, sex ratio and age distribution.
How it's asked: Per capita birth and death rate calculations, the shape of expanding, stable and declining age pyramids, the factors that change density, and when biomass or cover is better than numbers.
Why this is true
An individual is born once and dies once, so 'birth rate' and 'death rate' only make sense for a group; that is why these are population attributes. Expressing them per capita (per individual per unit time) lets populations of different sizes be compared. Density changes only through four processes: natality and immigration add individuals, while mortality and emigration remove them, so the net change is the balance of these inputs and outputs. The age structure tells us the future: a population with a broad base of pre-reproductive individuals will grow, one with roughly equal classes is stable, and one with a narrow base will shrink.
Key points
Key formulae
Change in density
Nₜ₊₁ = Nₜ + [(B + I) − (D + E)]
B births, I immigrants, D deaths, E emigrants in the interval
Age pyramids
| Shape | Population trend | Base (pre-reproductive) |
|---|---|---|
| Triangular | Expanding | Very broad |
| Bell-shaped | Stable | About equal to reproductive group |
| Urn-shaped | Declining | Narrower than reproductive group |
Common traps
- Per capita birth rate is divided by the original population, not by the population after the births are added.
- Immigration raises density; emigration lowers it. Students reverse their signs in the density equation.
Worked example
A pond had 30 lotus plants last year. Through reproduction 9 new plants were added this year.
High-yield
Density ↑ by natality and immigration, ↓ by mortality and emigration; expanding = triangular, stable = bell, declining = urn.
Quick check
Q1.An urn-shaped age pyramid indicates a population that is:
Q2.Of 200 individuals of a species, 20 die in a month. The death rate is: