Unit 2 Population and natural resources
Key unit competence: Analyse the factors that influence population size and the importance of natural resources.
Key ideas
- A population is a group of organisms of the same species living in the same area at the same time and able to interbreed. Characteristics: size (number of individuals), density (number per unit area or volume), distribution/dispersion (clumped — most common, around resources or in social groups; uniform — e.g. territorial birds, planted crops; random — rare, e.g. dandelions), age and sex structure, birth rate (natality), death rate (mortality), immigration and emigration.
- Age–sex pyramids: developing countries (e.g. many in sub-Saharan Africa) have a wide base (high birth rate), narrow top (short life expectancy) — rapid growth; developed countries have a narrow base and wide middle/top (low birth rate, long life) — stable or declining populations with an ageing workforce.
- Density-dependent factors have a greater effect as density rises: competition for food and space, predation, disease and parasites, accumulation of toxic waste. They regulate populations by negative feedback. Density-independent factors affect populations whatever the density: weather extremes, floods, drought, fire, volcanic eruptions, earthquakes, pollution.
- Estimating populations: quadrats (random placement using random numbers; count individuals or estimate percentage cover; density = mean number per quadrat ÷ quadrat area); species frequency = quadrats containing the species ÷ total quadrats × 100%; line and belt transects (record species at intervals along a line to show changes across an environmental gradient, e.g. from a lake shore); capture–mark–recapture for mobile animals: Lincoln index N = (n₁ × n₂) ÷ m₂, where n₁ = first sample marked, n₂ = second sample, m₂ = marked animals recaptured. Assumptions: marks not lost or harmful, marked animals mix randomly, no births, deaths or migration between samples.
- Growth patterns: exponential (J-shaped) growth when resources are unlimited (bacteria in new medium, populations after a crash), ending in a crash ("boom and bust"). Logistic (S-shaped / sigmoid) growth: lag phase (few individuals, adapting), log/exponential phase (rapid growth), deceleration, stationary phase at the carrying capacity (K) — the maximum population the environment can support. Then numbers fluctuate around K.
- Environmental resistance is the sum of the factors that limit population growth: biotic (predators, parasites, disease, competition) and abiotic (food, water, space, climate, shelter). It keeps populations near carrying capacity and maintains the balance of nature; predator–prey cycles (e.g. lynx and snowshoe hare) show prey peaks followed by predator peaks.
- The human population grew slowly for thousands of years, then rapidly after about 1800 (agricultural and industrial revolutions, better medicine, vaccines, sanitation and food supply lowered death rates). World population passed 8 billion in 2022. Rwanda has one of the highest population densities in Africa (over 500 people per km²).
- Effects of rapid population growth: pressure on land (fragmentation, overcultivation), deforestation, soil erosion, loss of biodiversity, water shortage and pollution, waste and slums in towns, unemployment, pressure on schools and health services. Family planning helps families decide the number and spacing of children (contraception, education of girls, health services), improving maternal and child health and reducing pressure on resources.
- Natural resources are materials from nature used by humans. Renewable: can be replaced naturally within a human lifetime if used sustainably — solar energy, wind, water, forests, fish, soil, wildlife. Non-renewable: limited and not replaced on a human time scale — fossil fuels (coal, oil, natural gas, Lake Kivu methane is used but finite), minerals (tin, coltan, gold, wolfram). Conservation: reduce, reuse, recycle; afforestation; protected areas (Volcanoes, Nyungwe, Akagera, Gishwati-Mukura national parks); renewable energy; laws such as the ban on plastic bags (2008).
Common misconceptions
- "Population density and population size mean the same." No: size is the total number; density is the number per unit area.
- "Carrying capacity is fixed forever." No: it changes as resources, climate and technology change.
- "Forests are renewable so they cannot run out." No: they are renewable only if harvesting does not exceed regrowth.
- "Disease is a density-independent factor." No: disease spreads more easily in dense populations, so it is density-dependent.
Worked examples
Example 1. 40 beetles were caught, marked and released. Later 50 were caught, of which 8 were marked. Estimate the population.
Solution
N = (n₁ × n₂) ÷ m₂ = (40 × 50) ÷ 8 = 2 000 ÷ 8 = 250 beetles. This assumes marked beetles mixed fully and no births, deaths or migration occurred.
Example 2. Ten 0.25 m² quadrats contained 3, 5, 0, 2, 4, 6, 1, 3, 0, 6 plants of a species. Calculate density and frequency.
Solution
Total = 30, mean = 3.0 per quadrat. Density = 3.0 ÷ 0.25 = 12 plants per m². The species was present in 8 of 10 quadrats, so frequency = 80%.
Practice questions
State five characteristics of a population.
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Size; density; distribution/dispersion; age structure; sex ratio; birth rate; death rate; growth rate; immigration/emigration.
Common mistake: Listing features of a community.
Distinguish between density-dependent and density-independent factors with examples.
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Density-dependent: effect increases with density, e.g. competition, predation, disease. Density-independent: effect is the same whatever the density, e.g. drought, flood, fire, frost.
Common mistake: Calling food supply density-independent.
Describe how to estimate the population of grasshoppers using capture–mark–recapture.
Show answer
Catch a sample with sweep net; count and mark harmlessly; release; wait for mixing; catch second sample; count marked and unmarked; N = n₁ × n₂ ÷ m₂.
Common mistake: Using quadrats for mobile animals.
State three assumptions of the Lincoln index.
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Marked animals mix randomly; marks not lost or harmful; no births, deaths, immigration or emigration between samples; equal chance of capture.
Common mistake: Ignoring migration.
Describe the phases of a sigmoid growth curve.
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Lag: slow growth, few individuals adapting; log: rapid exponential growth, abundant resources; deceleration: resources limit; stationary: births equal deaths at carrying capacity.
Common mistake: Missing the carrying capacity.
Explain four effects of rapid human population growth on the environment.
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Deforestation for land and fuel; soil erosion and loss of fertility; loss of biodiversity; water pollution and shortages; waste and pollution; overgrazing; urban slums.
Common mistake: Giving only social effects.
Distinguish renewable from non-renewable resources with examples.
Show answer
Renewable: replaced naturally if used sustainably, e.g. sun, wind, forests, fish. Non-renewable: finite, not replaced in human time, e.g. oil, coal, gas, minerals.
Common mistake: Calling forests non-renewable.
Suggest five ways to conserve natural resources in Rwanda.
Show answer
Afforestation and agroforestry; terracing; protected areas; recycling and plastic ban; renewable energy (solar, hydro); efficient cookstoves; water harvesting; family planning; education.
Common mistake: Giving vague answers like "protect nature".
Matching and drag-and-drop
Matching
Match each item in column A with the correct answer in column B. Write the letter next to each number.
Column A Column B 1. Which is a renewable resource? a. natural gas 2. Which is a non-renewable resource? b. wind energy 3. Movement of individuals into a population is: c. disease 4. Which is a density-dependent factor? d. immigration Show answer
1 → b (wind energy) · 2 → a (natural gas) · 3 → d (immigration) · 4 → c (disease)
- Replaced naturally.
- Finite.
- It increases size.
- Spreads faster in crowds.
Drag and drop
Choose the correct word or phrase from the box for each gap. One word in the box is not needed.
density · a stationary population · predator–prey cycles · mobile animals- Prey peaks followed by predator peaks show .
- Capture–mark–recapture is used for .
- The number of individuals per unit area is population .
Show answer
1: predator–prey cycles · 2: mobile animals · 3: density
- Predators depend on prey.
- Animals move around.
- E.g. per m².
Matching
Match each item in column A with the correct answer in column B. Write the letter next to each number.
Column A Column B 1. Which is a density-independent factor? a. a severe frost 2. An age pyramid with a wide base shows: b. lag phase 3. In an S-shaped curve, the first slow phase is the: c. transect 4. The method used to show changes across a gradient is a: d. a high birth rate Show answer
1 → a (a severe frost) · 2 → d (a high birth rate) · 3 → b (lag phase) · 4 → c (transect)
- Affects all regardless of density.
- Many young people.
- Few breeding adults.
- Line or belt.
Drag and drop
Choose the correct word or phrase from the box for each gap. One word in the box is not needed.
960 · 400 · population · volcanoes National Park- Organisms of the same species living in an area form a .
- A Rwandan national park in the north-west volcanoes is .
- 60 fish were marked; later 80 caught, 12 marked. The estimated population is .
Show answer
1: population · 2: volcanoes National Park · 3: 400
- They can interbreed.
- Home of mountain gorillas.
- 60 × 80 ÷ 12.