Populations
What you must know
Track population size, density, dispersion, age structure, and sex ratio. Distinguish exponential (J-shaped) growth, which occurs with unlimited resources, from logistic (S-shaped) growth, which levels at carrying capacity (K). Know biotic potential, environmental resistance, and density-dependent (disease, competition, predation) versus density-independent (fire, drought, storms) factors. Interpret survivorship curves (Type I late loss, Type II constant, Type III early loss). Use the rule of 70 (doubling time ≈ 70 / growth rate %). Understand demographic transition stages and how age-structure diagrams (pyramids vs. columns) predict future growth. Know r- vs. K-selected strategies.
Traps
- Calling a population "growing exponentially" when it is logistic and slowing near K.
- Mixing density-dependent with density-independent factors.
- Forgetting a broad-based age pyramid means high future growth (population momentum).
- Miscalculating growth rate: it is (births − deaths + immigration − emigration) per capita, often expressed as a percent.
Worked example / rule of thumb
A population grows 2% per year. Doubling time ≈ 70 / 2 = 35 years. Rule: 70 divided by percent growth rate gives doubling time; higher r means faster doubling.