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Biology 3% exam weight

Population Ecology and Biogeography

Part of the WAEC WASSCE study roadmap. Biology topic bio-16 of Biology.

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Population Ecology and Biogeography

🟢 Lite — Quick Review (1h–1d)

Rapid summary for last-minute revision before your exam.

Population ecology studies groups of one species in a defined area, while biogeography maps where species and ecosystems occur and why. Five numbers drive every population problem: size (N), density, distribution, natality, and mortality, all modified by immigration and emigration.

The two growth curves you must draw on demand:

CurveShapeConditionFormula
ExponentialJ-shapedUnlimited resourcesN(t) = N₀ × e^(rt)
LogisticS-shapedLimited by KN(t) = K / (1 + ((K−N₀)/N₀) × e^(−rt))

Three distribution patterns appear in every paper: random (dandelion seeds), uniform (desert shrubs competing for water), clumped (schooling fish, herd animals) — clumped is the most common in nature.

🟡 Standard — Regular Study (2d–2mo)

Standard content for students with a few days to months.

Core Population Metrics

Population density is the counting unit examiners love: D = N / A, where N is the number of individuals and A is the sampled area (individuals per m² or per hectare). Comparing absolute N between two habitats is meaningless; only density allows fair comparison.

Population growth rate (r) balances four flows:

r = (Births + Immigration) − (Deaths + Emigration), all divided by N to give the per capita rate.

A positive r means the population grows, negative means it shrinks, zero means it is stable.

Two Growth Models Compared

  • Exponential (J-curve): resources are unlimited; population multiplies by a constant factor every generation. Rare in nature but seen briefly when species colonise a new habitat.
  • Logistic (S-curve): growth slows as carrying capacity (K) is approached. K is the maximum population the environment can sustain given food, space and water limits. The inflection point lies at K/2.

Density-Dependent vs Density-Independent Factors

Density-dependent factors (competition, predation, disease, parasitism) act more strongly as N rises. Density-independent factors (floods, bushfire, drought, volcanic eruption) kill the same proportion regardless of how crowded the population is.

WAEC examiners frequently test whether candidates recognise that deforestation and bush burning in West Africa are density-independent pressures on forest wildlife.

Life-History Strategies

r-strategists (houseflies, weeds, tilapia) reproduce quickly, mature early, and die young. K-strategists (elephants, humans, mahogany trees) produce few offspring and invest heavily in each. Neither is “better”; they suit unstable versus stable environments respectively.

Biogeography Basics

Endemic species live in one restricted region (e.g. drill in the Cross–Sanaga forests). Cosmopolitan species occur across continents (e.g. Rattus rattus). Isolation — islands, mountain peaks, forest refugia — drives endemism.

🔴 Extended — Deep Study (3mo+)

Comprehensive coverage for students on a longer study timeline.

Interpreting Age Pyramids

The base of the pyramid shows the proportion of pre-reproductive individuals, the middle the reproductive group, and the top the post-reproductive. A wide base = growing population (Nigeria today); a narrow base = declining population (Italy, Japan). Pyramids are the fastest WAEC Paper 2 essay opener when combined with a growth-rate calculation.

Worked Density Calculation

A farmer counts 1,200 maize plants in a 4-hectare plot. D = 1,200 / 4 = 300 plants per hectare. Trap: students divide by N instead of by A, or forget to convert m² to hectares.

Succession and Climax Community

Primary succession begins on bare rock or lava, where pioneer lichens and mosses break down substrate over centuries. Secondary succession follows fire, farming abandonment or flood on soil that already retains seed banks — recovery takes decades, not centuries. The climax community (e.g. tropical rainforest in high-rainfall southern Nigeria; Guinea savanna in the middle-belt) is the relatively stable endpoint, but is not permanent: shifting climate or human action can reset succession to an earlier stage.

Common Mistakes and Traps

  • Forgetting to convert area units (m² → hectares) before dividing.
  • Mixing up the J- and S-curves on graphs when K is not drawn.
  • Treating competition and predation as density-independent factors.
  • Claiming all climax communities in West Africa are rainforest — climate determines whether the climax is forest, savanna or swamp forest.

Practice Prompts

  1. A population of 500 antelopes gains 120 births and 40 immigrants, and loses 80 deaths and 20 emigrants in one year. Calculate r and state its meaning.
  2. Outline three density-dependent and three density-independent factors operating in the Nigerian savanna, and explain why conservation efforts must address both groups.

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