Population Biology
What to know
Population biology examines how populations grow, interact, and change over time. Understand population size, density, dispersion, and the factors that affect them (births, deaths, immigration, emigration). Know the difference between exponential growth (J-shaped, unlimited resources) and logistic growth (S-shaped, limited by carrying capacity K). Recognize density-dependent factors (competition, disease, predation) and density-independent factors (weather, natural disasters).
Study species interactions: competition (including competitive exclusion and resource partitioning), predation, parasitism, mutualism, and commensalism. Understand energy flow through trophic levels, the 10% rule, food webs, and the roles of producers, consumers, and decomposers. Review ecological succession (primary versus secondary), biodiversity, and the effect of human activity on ecosystems. Population genetics topics include allele frequencies, the Hardy-Weinberg principle, and the conditions required for equilibrium.
Traps
Questions often mislabel logistic growth as unlimited, or confuse density-dependent with density-independent factors. Watch for items that reverse the direction of energy flow or claim energy is recycled (nutrients cycle, energy flows). A frequent distractor is that a population at carrying capacity continues to grow exponentially, or that Hardy-Weinberg equilibrium applies when selection is acting.
Worked example
A population of 1,000 grows by 5% per year with unlimited resources. After one year it is 1,050. Rule of thumb: exponential growth uses the multiplier (1 + r) each period, while logistic growth slows as the population approaches K.