Topic 8: Human populations and urban systems

Births and immigration add people to a population; a crude input rate makes the count comparable across populations of different sizes.

Use events ÷ total population × 1,000 when the question asks for a rate per 1,000 per year. Births happen inside the population; immigration crosses its boundary from outside.

25,000 births in 500,000 people gives (25,000 ÷ 500,000) × 1,000 = 50 births per 1,000 people.

50 per 1,000: (2,000 ÷ 40,000) × 1,000.

2,000 is an event count, not a rate. A rate needs the population denominator and its units.

Deaths and emigration remove people from a population; their crude rates express those annual outputs per 1,000 residents.

Use the same events ÷ population × 1,000 calculation at town, country or global scale. A crude death rate describes the population’s current output, not an individual’s lifetime risk.

15,000 deaths among 750,000 people gives 20 deaths per 1,000 people for that year.

The smaller population: 50 versus 10 per 1,000.

Do not compare raw deaths without population size; that confuses a count with a rate.

Each population metric answers a different question: TFR describes births per woman, life expectancy describes expected years, natural increase compares birth and death rates, and doubling time estimates growth speed.

Use doubling time ≈ 70 ÷ growth rate (%) only after converting the growth rate to a percentage. Use natural increase = birth rate − death rate; if rates are per 1,000, divide the result by 10 to express a percent.

At 2% annual growth, doubling time is about 35 years; birth rate 30 and death rate 10 per 1,000 gives natural increase 20 per 1,000, or 2%.

Doubling time, not life expectancy or TFR; use 70 ÷ percentage growth.

Do not put a per-1,000 rate directly into 70 ÷ r; the formula expects percent units.

The global human population has followed a rapid growth curve, but future projections are conditional scenarios rather than fixed predictions.

UN high, medium and low scenarios diverge mainly because different fertility assumptions compound across generations; mortality and migration assumptions also affect the path.

If fertility falls faster than assumed, the population path can move below an earlier medium scenario even though current population is unchanged.

Read each curve's fertility, mortality, migration and time assumptions, and treat the spread as evidence of uncertainty.

The middle scenario is not guaranteed, and uncertainty does not make every scenario equally plausible.

Direct population policies explicitly target births or migration: anti-natalist policies seek fewer births, pro-natalist policies seek more births, and migration policies alter immigration or emigration.

Evaluate intended rate, cultural and religious context, economic and social measures, rights, uptake, time lag and measured outcome.

Anti- or pro-natalist describes intent, not automatic success or ethical acceptability.

Indirect policies change economic, social, health or development conditions; these alter births, deaths or migration through intermediate choices and survival pathways rather than a population quota.

Gender equality, education, sanitation, public health, pensions and welfare can act through different pathways; effects depend on access, culture, employment and time.

An indirect influence is probabilistic: improved education or welfare does not produce the same demographic result in every society.

An age-sex pyramid is a snapshot of population composition: age groups run vertically, the two sexes sit on opposite sides, and width shows number or percentage.

Start with shape before causes. A broad base signals many young people; a wider upper section signals more older people. The diagram describes structure, not automatically a DTM stage or forecast.

A pyramid that narrows sharply above age 60 has fewer older people relative to the base; a column-like shape has more similar-sized age groups.

The population is relatively youthful; you cannot yet infer the cause or exact DTM stage.

Do not explain a pyramid’s cause before describing its axes and shape.

DTM stages describe how birth and death rates change; the gap between those rates, not the stage label, determines whether population grows, stabilizes or declines.

Stage 1 has high births and deaths; in stage 2 deaths fall first, stage 3 births fall, stage 4 both are low, and stage 5 may have births below deaths. Use the rate sequence to explain the curve.

When deaths drop from 30 to 10 per 1,000 while births stay at 35, the gap widens and growth accelerates; later, falling births narrow the gap.

Stage 2 pattern: the death rate has fallen first, so the positive gap is large.

DTM stages are a generalized pattern, not a universal timetable every country follows identically.

An urban ecosystem combines living components—plants, animals, microbes and people—with non-living conditions such as soil, water, air, climate, topography and built surfaces.

Classify by whether the feature is alive, then explain the interaction. Humans are biotic; roads and buildings are abiotic stores even though people design them.

A street tree is biotic; compacted soil and hot pavement are abiotic conditions that limit its roots and water supply.

Both: plants are biotic, while roof material, water and heat are abiotic conditions interacting with them.

Do not classify by who built or manages something; classify living versus non-living first.

An urban area is a built-up settlement where people, buildings and infrastructure are concentrated for residential, cultural, productive, trade or social functions.

Use settlement pattern and function rather than a single universal population cut-off. Cities, towns and suburbs can all be urban; rural settlements are generally more dispersed and less dense.

A dense suburb with shops, roads, schools and apartment blocks is urban even though it lies outside the city centre.

Urban: density, built infrastructure and function matter more than height or fame.

‘Urban’ does not mean only a capital or megacity.

An urban area is an open system: resources enter, buildings and services transform them, and products, waste and pollution leave or feed back into management.

Draw inputs, processes, outputs and feedback rather than a disconnected list. Water, energy, materials, transport, people, plants and animals are linked stores and flows.

A city imports water, treats and distributes it, then outputs wastewater; greywater reuse changes the next water input.

As feedback that changes an output into a later input, not as a detached label.

Naming departments without arrows is not a systems-flow model.

Urbanization is a rising share of people living in urban systems together with land becoming more built-up, industrialized and continuously settled.

Check both the population share and the land-use change. A city can have a high urbanization level but slow current growth, while another has rapid construction from a lower base.

When fields become housing and roads while the urban share rises, the demographic and land-use parts of urbanization occur together.

Not necessarily; growth rate and urbanization level measure different things.

Do not define urbanization as simply a taller skyline or a larger city population.

Rural–urban migration moves people from rural to urban systems; push factors weaken the origin, pull factors attract the destination, and movement may be voluntary, forced or mixed.

Most rural–urban migration is internal. Compare a country's trend over time with deurbanization, in which people or activities move away from large urban centres toward smaller settlements or rural areas.

Drought may push a farming household while employment and services pull it to a city; later high housing costs or remote work may support movement away from the city.

Separate origin push, destination pull, internal/international boundary, degree of choice and the evidence for net direction.

A perceived urban advantage may not be real or equally accessible, and migration is not always wholly voluntary or wholly forced.

Suburbanization moves people from dense central areas to lower-density peripheries; urban sprawl is the spread-out land-use pattern that can follow.

Trace the spatial shift to land demand, longer trips, car dependence and extended roads or utilities. Suburbanization is descriptive; calling it sprawl adds a planning judgement about uncontrolled spread.

A new housing belt beyond the old boundary needs roads and water pipes across former farmland, lengthening commutes to the centre.

No. Look for the spread-out, land-intensive pattern and its controls, not movement alone.

Suburbanization is the movement; sprawl is the low-density expansion pattern.

Urban expansion changes environmental systems through specific pathways: land replacement, runoff, water diversion, traffic or industrial emissions.

Choose one pressure and trace it to one receptor. Expansion can remove farms, forests or wetlands; construction can alter water quality; drainage can change river flow; traffic can add air pollution.

Paving a wetland for housing removes habitat and speeds runoff, so biodiversity and local water flow both change through identifiable mechanisms.

Link construction/runoff to pollutants and compare water quality or flow before and after; the boundary alone is not proof.

Do not claim every expansion causes every listed impact; state the mechanism and evidence.

Urban planning decides how land and buildings are used while balancing physical, domestic, environmental, commercial, industrial, financial and health needs.

Treat a plan as a coordination decision: identify who benefits, who bears costs and which evidence supports the choice. Technical efficiency alone cannot settle stakeholder conflict.

A transport corridor may improve jobs and access while imposing noise or displacement costs on nearby residents.

Ask efficient for whom, at whose cost, and whether health and environmental needs are protected.

Planning is not only engineering; distribution and participation are part of the decision.

Sustainable urban planning combines housing, mobility, green space, security, services, employment, energy, waste and community participation rather than relying on one green technology.

Named example—Copenhagen reduces car dependence through connected cycling and walking routes and public transport. This can lower traffic emissions and energy use while improving access, but affordability, safety, winter usability and unequal access still require evaluation.

Evaluate a planning package by housing quality and affordability, transport integration, green-space access, renewable resources, reuse, energy efficiency, jobs, education, security and community voice.

A cycle lane or green building alone does not prove that the whole city is sustainable or socially inclusive.

Ecological urban planning treats the city as an ecosystem and matches an approach—habitat, farming, biophilic, resilience or regenerative design—to a specific urban problem.

Urban ecology connects habitats; farming produces food; biophilic design reconnects people and nature; resilience prepares for shocks; regenerative architecture aims to improve functions. Always name problem → mechanism → outcome.

For repeated flash flooding, permeable surfaces and rain capture are resilience responses; a decorative green wall is not the main drainage mechanism.

Urban farming; the action is food production, not simply adding a plant aesthetic.

A green roof or smart building is a tactic; it is not automatically the whole ecological approach.

Urban air pollution includes NOx, SO₂, CO and particles; PM2.5 and PM10 are size fractions, so a monitor label tells you which particles it counts.

PM2.5 is no larger than 2.5 µm and PM10 no larger than 10 µm. The finer fraction can penetrate more deeply, but the syllabus labels describe aerodynamic size, not a complete toxicity ranking.

If PM10 is 40 µg/m³ and PM2.5 is 18 µg/m³, the finer fraction is nested inside the PM10 reading; the remainder is larger than 2.5 µm but no larger than 10 µm.

No. PM2.5 is nested within PM10, but each fraction can change by a different amount.

PM10 does not mean ‘ten times worse’; it is a particle-size threshold, not a gas or toxicity score.

A primary pollutant is directly active at emission; its source may be natural or anthropogenic.

Dust lifted by a storm is natural; dust from road building is anthropogenic. Both can be primary PM because they enter the atmosphere directly.

Natural does not mean harmless, and anthropogenic does not mean every later pollutant is emitted directly.

Combustion can emit PM2.5, PM10, CO and SO₂ directly, while tropospheric ozone is secondary and forms in the atmosphere from precursors.

Label a pollutant by where it appears in the pathway. NOx from a tailpipe can react in sunlight later; ozone is therefore not simply an exhaust gas even when traffic supplies its precursor.

A car emits NOx directly at 8 a.m.; sunlight can help form ozone downwind later in the morning.

No. It may show secondary ozone formed from traffic precursors and sunlight.

‘From fossil fuels’ describes the source pathway; it does not make every pollutant primary.

Air-pollution management can reduce the activity, control emissions at release or reduce exposure; choose the level that interrupts the causal pathway.

A bus and cycle network can reduce vehicle kilometres while converters reduce emissions from vehicles that remain; a tree screen alone does not remove the source.

Set a pollutant target, identify the causal level, combine complementary measures and monitor emissions and exposure.

A visible green feature is not automatically source control or sufficient management.

NOx and SO₂ react with oxygen and water in the atmosphere to form nitric and sulfuric acids that return by wet or dry deposition.

Use the chain precursor → atmospheric reaction → acid → deposition. Natural rain is already slightly acidic, so acid rain means additional acidification rather than ‘dirty water’ alone.

SO₂ from fuel burning dissolves in cloud water and is oxidized, adding sulfuric acid to precipitation.

Water and oxygen; naming only the emission source skips the formation mechanism.

Acid rain is not simply rain with visible dirt; explain the gas reactions that create acids.

Acid deposition harms different receptors through different mechanisms: nutrient leaching and aluminium toxicity in ecosystems, corrosion in materials, and particle-related lung inflammation.

Choose receptor → mechanism → effect. Acidified soil can lose calcium; mobilized aluminium can damage fish gills; acids corrode carbonate stone; associated particles can enter lungs.

Acidified soil releases aluminium into a stream, where fish gills are damaged and survival falls.

Nutrient leaching and root/foliage damage reduce uptake; name that pathway instead of saying ‘acid kills trees’.

Acid rain does not usually burn skin directly; respiratory harm follows particle exposure pathways.

Acid deposition can be managed by changing the activity, controlling release, or restoring damage; those levels prevent different parts of the causal chain.

Reduce fossil-fuel use at source, use scrubbers or converters at release, and use healthcare or lake liming for existing damage. Prevention limits new loading; restoration cannot remove upstream emissions.

A scrubber cuts sulfur emissions from a plant, while liming an acidified lake neutralizes stored acidity after deposition.

Restoration such as liming; pair it with source reduction to prevent recurrence.

Restoration is not source control.