AP® Human Geography models and theories

A population pyramid is a pair of bar graphs back to back: males on the left, females on the right, with the youngest age group at the bottom. Its shape tells you how fast a place is growing, how old its people are and what it will need next, from schools to nursing homes.

Expansive

A wide base that narrows quickly. Lots of births, a young population and fast growth, typical of DTM stage 2 or early stage 3.

Stationary

Bars of similar width up into old age. Low birth and death rates and slow growth, typical of DTM stage 4.

Constrictive

A base narrower than the middle. Births have fallen, the population is aging and may shrink, typical of late stage 4 or stage 5.

Bulges and notches

A bulge is a baby boom moving up the pyramid over time. A notch is a gap from war, famine or a policy like China's one-child policy. Extra working-age men often mean male migrant workers.

Example: Niger's pyramid is strongly expansive, the United States' is close to stationary, and Japan's is constrictive.

The DTM shows how birth and death rates change as a country develops. Death rates fall first, so the population booms. Then birth rates fall too, and growth slows to near zero. The gap between the two lines is natural increase.

Stage 1: high and stable

High birth and death rates, so very slow growth. No country is in stage 1 today.

Stage 2: death rates fall

Better food, clean water, sanitation and medicine cut death rates, while birth rates stay high. Growth is fastest here.

Stage 3: birth rates fall

People move to cities, children cost more to raise, women get more education and jobs, and contraception spreads. Growth slows.

Stage 4: low and stable

Low birth and death rates, so growth is close to zero.

Stage 5 (added later): decline

Not in the original model. Birth rates drop below death rates, so the population ages and shrinks.

Example: England and other Western European countries moved through stages 1 to 4 between the 1700s and the late 1900s. Japan and Italy, with more deaths than births each year, are the usual examples of a possible stage 5.

Malthus argued that population grows geometrically (1, 2, 4, 8) while food supply grows only arithmetically (1, 2, 3, 4). Sooner or later people outrun their food. He expected famine, disease and war to check growth unless people chose to have fewer children.

Positive checks

Things that raise the death rate: famine, disease and war.

Preventive checks

Things that lower the birth rate. The kind Malthus favored, which he called moral restraint in a later edition (1803), was marrying later and having fewer children.

Neo-Malthusians

Modern thinkers who say the warning still matters, but widen it from food to water, energy, pollution and climate. Paul Ehrlich's The Population Bomb (1968) is the best-known example.

Example: Since 1798 the world has grown from about 1 billion people to more than 8 billion, yet food production grew even faster, thanks to the Second Agricultural Revolution and the Green Revolution.

Land closest to a market or the center of a city is the easiest to reach, so it costs the most. Only uses that earn a lot from each acre can afford it. Each use is willing to pay less the farther out it goes, and whichever use bids the most for a spot gets it.

In cities

Shops and offices outbid everyone for the central business district (CBD), which is why it has the tallest buildings. Industry and then housing take land farther out.

On farmland

Intensive farming that earns a lot per acre, like vegetables and dairy, sits near cities. Extensive farming, like grazing, sits where land is cheap and far away. Von Thünen's rings are a bid-rent pattern.

Example: In Midtown Manhattan, land is so expensive that only offices and stores in tall towers can pay for it, while warehouses, houses and eventually farms spread out where land is cheaper.

Von Thünen asked why farmers grow different things at different distances from a market town. His answer was transportation cost. Goods that are perishable or heavy to haul are produced close to the market, and goods that are cheap to move or need lots of land are produced farther out. The result is a set of rings.

Market town

The one city where all farm goods are sold.

Ring 1: market gardening and dairy

Vegetables, fruit and milk spoil fast, so they're grown closest, on the most expensive land.

Ring 2: forest

Wood was heavy and needed for fuel and building, so it had to be close to town.

Ring 3: grain and field crops

Grain keeps well and is lighter to ship for its value, so it can be grown farther out.

Ring 4: ranching

Animals can walk themselves to market and need lots of cheap land, so they're farthest out. Beyond is wilderness.

Assumptions

One isolated market town, flat land with the same soil and climate everywhere, no roads or rivers, and farmers who choose the most profitable crop.

Example: The pattern still shows up loosely: market gardens and dairy farms cluster around big cities, while ranching uses cheap land far away. But Kenya flies fresh-cut flowers to Europe overnight, which the model can't explain.

Central place theory explains the size and spacing of towns that sell goods and services to the area around them. Many small places sell everyday goods, and fewer, bigger places sell specialized ones. Each place's market area is drawn as a hexagon so the areas cover the land without gaps or overlaps.

Range

The farthest people will travel to buy a good or service.

Threshold

The smallest number of customers needed to keep a business open.

Low-order goods

Things people buy often and won't travel far for, like groceries and gas. Small places can support them.

High-order goods

Things people buy rarely and will travel far for, like a specialist doctor or a stadium. Only big places have a big enough threshold.

Example: Christaller based his theory on southern Germany. Farm towns across the U.S. Midwest are spaced fairly evenly, with bigger regional cities serving many small towns.

Based on Chicago, the concentric zone model says a city grows outward from its center in rings. Each ring has a different use. As new groups arrive and people get richer, they move outward, and newer arrivals take their place.

1. Central business district (CBD)

Offices, stores and the most valuable land.

2. Zone of transition

Factories, warehouses and run-down housing, where new immigrants and the poorest residents often lived.

3. Working-class homes

Modest housing for factory workers who moved out of the zone of transition.

4. Middle-class homes (better residences)

Larger, newer homes for middle-class families.

5. Commuter zone

Suburbs beyond the city, where wealthier people live and travel in to work.

Example: Chicago in the 1920s, which Burgess studied, matched the model fairly well, with immigrant neighborhoods next to the Loop.

Hoyt noticed that cities grow in wedges, or sectors, along transportation routes like rail lines, rivers and main roads. Once a sector gets a certain use, it keeps it as the city spreads outward. Wealthy homes form one wedge, and lower-income homes sit next to the industrial wedge.

1. Central business district (CBD)

The center of business at the hub.

2. Wholesale and light industry

A wedge along a rail line or river, where goods can be shipped.

3. Low-income homes

Next to the industrial wedge, close to the jobs but also to noise and pollution.

4. Middle-income homes

The largest area, between the low- and high-income wedges.

5. High-income homes

A wedge reaching out from the CBD along an attractive route, away from industry.

Example: In Washington, D.C., a wealthy wedge runs northwest from downtown through Georgetown toward the Maryland suburbs, while industry and lower-income areas grew along rail lines to the east.

Harris and Ullman argued that a city doesn't grow around just one center. It grows around several nodes, or nuclei, such as a port, a university, an airport, an industrial district or a suburban business center. Some activities cluster together, and others keep away from each other.

Several centers

The CBD is one center among many, not the only one.

Attract and repel

Related activities cluster, like shops near a university. Others avoid each other, like high-income homes and heavy industry.

Suburbs

Residential and industrial suburbs form at the edges, each with its own center.

Example: Los Angeles fits well: it has many centers, including downtown, Century City, Burbank, Long Beach and the area around the airport.

In the galactic city, the old downtown is no longer the main center. The metro area spreads out like a galaxy: suburbs, office parks, shopping centers and edge cities are scattered around a ring road (beltway) and linked by highways. Much of the business activity has moved to the edges. Chauncy Harris's peripheral model (1997) describes a very similar pattern, so you'll often see the two taught together.

Old central city

The original CBD and older neighborhoods in the middle, now one center among many.

Beltway

A highway ring around the central city that ties the suburbs together.

Edge cities

New downtowns of offices and shopping at highway interchanges.

Suburbs

Low-density housing spread across the area between the highways.

Example: Washington, D.C.'s Capital Beltway has edge cities like Tysons, Virginia, and Atlanta and Houston are other spread-out, beltway-centered metros.

Rostow argued that every country develops by climbing the same five steps, from a farming society to one where people buy lots of goods. A country moves up by investing, building industry and trading. It's a modernization theory: it assumes poorer countries can follow the path wealthy ones took.

1. Traditional society

Most people work in subsistence farming, and technology is limited.

2. Preconditions for take-off

New technology, infrastructure and investment arrive, often from outside, and a few industries begin.

3. Take-off

A few industries grow fast, cities grow and the economy starts to industrialize.

4. Drive to maturity

Industry spreads and diversifies, technology improves and living standards rise.

5. High mass consumption

Most people work in services and can afford many consumer goods.

Example: South Korea fits fairly well: it climbed from a poor farming economy in the 1950s to a wealthy, high-tech consumer society in a few decades. Many other countries have stalled at one stage for decades.

Wallerstein saw the world as one connected, unequal capitalist economy. Countries play different roles in it: core, semi-periphery and periphery. The core gets richer from the periphery's cheap labor and raw materials. Countries can move between tiers, but the system itself keeps inequality going.

Core

Wealthy, high-tech, high-wage countries that control capital and trade, such as the United States, Western Europe and Japan.

Semi-periphery

Industrializing countries that are exploited by the core but exploit the periphery, such as China, India, Brazil and Mexico.

Periphery

Lower-income countries that mostly export raw materials and cheap labor, with low wages and little technology.

Example: China moved from the periphery into the semi-periphery as it industrialized after the 1980s, and South Korea has moved close to the core.