AP Environmental Science

## Learning Outcomes

- Define **urban sprawl** and recognise its main features
- Explain how sprawl causes **habitat fragmentation** and biodiversity loss
- Understand how **impervious surfaces** change the water cycle and increase flooding
- Describe why sprawl increases **car dependence**, air pollution, and greenhouse gas emissions
- Distinguish between habitat fragmentation, edge effects, runoff, and urban heat island effects
- Apply these ideas to AP Environmental Science multiple-choice and free-response questions

## AP Environmental Science Syllabus

For the AP Environmental Science, you are expected to understand urban sprawl problems, with a focus on the following syllabus points:

- Know that urban sprawl is outward, low-density development that spreads cities into surrounding rural or undeveloped land
- Identify common features of sprawl, including single-use development, low-density housing, and heavy automobile use
- Explain how urban sprawl causes **habitat fragmentation** and increases **edge effects**
- Understand how impervious surfaces increase **stormwater runoff**, reduce **infiltration**, and decrease **groundwater recharge**
- Explain why sprawling development increases **vehicle miles traveled**, air pollution, and greenhouse gas emissions
- Recognise additional impacts such as farmland loss, wetland loss, urban heat island effects, and higher infrastructure costs
- Distinguish environmental problems caused by sprawl from mitigation strategies such as **smart growth**, greenbelts, and infill development

## Test Your Knowledge

_Attempt these questions before reading this article. If you find some difficult or cannot remember the answers, look more closely at that area during your revision._

1. What is urban sprawl, and how is it different from compact urban development?
2. How does urban sprawl lead to habitat fragmentation?
3. Why do impervious surfaces increase flooding and reduce groundwater recharge?
4. How does sprawl increase air pollution and greenhouse gas emissions?
5. Name two environmental problems and one economic problem caused by urban sprawl.

## Introduction

## Introduction

Urbanization is not always environmentally harmful in the same way. Dense cities can reduce land use per person and make mass transit practical. **Urban sprawl**, by contrast, spreads homes, roads, parking lots, and shopping areas over a much larger area. That pattern creates a distinct set of environmental problems that AP Environmental Science expects you to know well.

Sprawl matters because it links several units of the course. It changes biodiversity through habitat loss and fragmentation, alters the hydrologic cycle by increasing runoff, and raises fossil fuel use through car dependence. On the exam, these links are often tested as cause-and-effect chains rather than as isolated facts.

> **Key Term: urban sprawl**
> Outward expansion of cities into surrounding rural or undeveloped land, usually with low-density development and heavy automobile use.

### What urban sprawl looks like in practice

Urban sprawl is not just “a city getting bigger.” The exam expects you to recognise a particular pattern of growth. Sprawl usually means **single-family homes on large lots**, separate residential and commercial zones, long commuting distances, and limited practical public transit. People live in one area, work in another, shop in another, and often need a car for all of it.

This pattern develops for several reasons. Land at the edge of a city is often cheaper to build on than central urban land. Families may want larger homes and yards. Businesses may move outward where land is more available. Roads then connect these new developments, which encourages even more outward growth. This is a feedback pattern: more roads support more sprawl, and more sprawl creates demand for more roads.

A key exam distinction is between **compact development** and sprawling development. Compact development concentrates more people in a smaller area. Sprawl spreads the same population across a larger area. That difference is why sprawl usually consumes more farmland, forest, and habitat per person.

> **Key Term: low-density development**
> Development in which homes, businesses, and roads are spread out over a large area, with fewer people per unit of land.

One common form is **single-use development**. Residential, commercial, industrial, and institutional land uses are separated. This matters environmentally because separated land uses increase travel distances. If homes, schools, shops, and jobs are far apart, walking and cycling become less practical, and bus or rail systems serve fewer people efficiently.

Sprawl can also include **leapfrog development**, where developed areas are separated by undeveloped land. Students sometimes think this undeveloped land cancels out the harm. It does not. Leapfrog growth often fragments habitat even more because roads, utility corridors, and scattered construction divide the area into smaller pieces.

> **Test Tip:** If a multiple-choice question describes large lots, separated land uses, and long car commutes, the answer is likely urban sprawl rather than urbanization in general.

### Ecological impacts: habitat fragmentation, edge effects, and biodiversity loss

The strongest biodiversity consequence of sprawl is habitat loss combined with **habitat fragmentation**. When forests, grasslands, or wetlands are replaced by subdivisions, roads, and shopping areas, continuous habitat is broken into smaller isolated patches.

> **Key Term: habitat fragmentation**
> The breaking of a large, continuous habitat into smaller, isolated patches.

This is more damaging than many students first think. Imagine a continuous forest. Before development, animals, pollen, and seeds can move through a large connected area. After roads and housing divide it, populations may become isolated. Small isolated populations are more vulnerable to local extinction because they have fewer individuals, reduced gene flow, and less access to mates and resources.

Fragmentation also changes habitat quality through **edge effects**.

> **Key Term: edge effects**
> Changes in environmental conditions and species interactions at the boundary between two habitats.

Edges usually have more light, more wind, higher temperatures, and greater human disturbance than interior habitat. Predators, pets, invasive species, and noise may also be more common near edges. As fragmentation increases, the amount of edge habitat increases and the amount of interior habitat decreases. That is especially harmful for **interior species**, which need large undisturbed core habitat.

This creates an important exam chain:

Urban sprawl → roads and scattered development → habitat fragmentation → more edge habitat and less core habitat → reduced biodiversity

Not all species respond the same way. Generalist species, like raccoons or some invasive plants, may do fine or even increase near developed edges. That does not mean biodiversity is healthy. A rise in a few adaptable species often happens at the same time as a decline in specialist or interior species. AP questions often reward that distinction.

Roads make the problem worse. They do not just separate habitat. They also create direct mortality from vehicle collisions and can block migration routes. For some animals, a road is not simply a line on a map; it is a barrier they cannot safely cross.

Sprawl also converts **farmland**, forests, and wetlands into built land. Farmland loss matters because land near cities is often among the most productive agricultural land. Wetland loss matters because wetlands provide flood control, water filtration, and habitat.

> **Test Tip:** If an FRQ asks how sprawl reduces biodiversity, do not stop at “habitat loss.” Add fragmentation, reduced gene flow, more edge effects, and decline of interior species.

### Hydrologic impacts: impervious surfaces, runoff, and groundwater recharge

Sprawl changes the water cycle by replacing vegetation and soil with **impervious surfaces** such as roads, driveways, rooftops, and parking lots.

> **Key Term: impervious surfaces**
> Built or paved surfaces that prevent water from infiltrating into the soil.

Under natural conditions, precipitation is split among runoff, infiltration, and evapotranspiration. In a forest or grassland, much water infiltrates into soil, replenishes groundwater, and is later released slowly to streams. In a sprawled urbanized area, more precipitation runs off the surface instead.

A useful APES-style relationship is:

Impervious surface increases → infiltration decreases → runoff increases → groundwater recharge decreases

> **Key Term: groundwater recharge**
> The process by which water infiltrates into the ground and replenishes aquifers.

Why does this matter? First, higher runoff means more **surface flooding**. Water that once soaked into soil now moves rapidly into storm drains, ditches, and streams. This can create flashier streamflow, meaning water levels rise faster after rainfall. Higher, faster peak flows increase flood risk and erode stream channels.

Second, less infiltration means less groundwater recharge. Aquifers refill more slowly, and streams may receive less groundwater input during dry periods. That can reduce baseflow in streams and stress aquatic organisms.

Third, runoff from urban and suburban land carries pollutants. This is a classic **nonpoint source pollution** problem.

> **Key Term: stormwater runoff**
> Precipitation that flows over land rather than soaking into the ground.

As water flows across parking lots, roads, and lawns, it picks up gasoline, motor oil, heavy metals, fertilizers, pesticides, sediment, road salt, and trash. These pollutants are then carried into streams, ponds, wetlands, and estuaries. The result may include poorer water quality, algal growth from nutrient runoff, and degraded aquatic habitat.

Students sometimes confuse sprawl with nonpoint source pollution itself. Better wording is: urban sprawl increases impervious cover, which increases stormwater runoff, which often increases nonpoint source pollution.

The loss of vegetation matters too. Trees and plants intercept rainfall, reduce runoff speed, and help water move into the soil. When lawns replace native plant communitiesss, infiltration may still be lower than in the original habitat, and chemical inputs often rise.

One more subtle effect is stream degradation. More intense runoff can scour stream banks, widen channels, and bury habitat with sediment. Aquatic organisms adapted to steadier flows may decline even if the stream still contains water year-round.

### Transportation, energy, and climate impacts

Sprawl creates **car dependence** because destinations are spread far apart. In a dense city, many people can walk, cycle, or use buses and trains efficiently. In a low-density suburb with separated land uses, that becomes harder. Destinations are too far apart, and transit routes serve too few riders per stop to be as effective.

> **Key Term: car dependence**
> A pattern in which people rely heavily on private vehicles because homes, jobs, shops, and services are spread out.

As a result, people drive longer distances and more often. More driving means more fossil fuel use, more air pollution, and more greenhouse gas emissions. Transportation is a major source of emissions, and sprawl pushes that sector upward by increasing vehicle miles traveled.

The pollutants from heavier automobile use include nitrogen oxides, volatile organic compounds, carbon monoxide, and particulate matter. Nitrogen oxides and volatile organic compounds also contribute to ground-level ozone formation. On an FRQ, a strong answer links sprawl not just to “pollution” but to specific consequences: more driving, more combustion, more emissions.

Sprawl also raises energy demand in indirect ways. Spread-out infrastructure requires more roads, more lighting, more water and sewer lines, and more utility expansion. It often costs more to provide public services across a large low-density area than in a compact one.

Another related issue is the **urban heat island effect**. While often associated with dense urban cores, sprawl can intensify warming locally by increasing dark surfaces and reducing vegetation.

> **Key Term: urban heat island effect**
> The tendency for developed areas to be warmer than nearby rural areas because built surfaces absorb and retain heat and vegetation is reduced.

Students must avoid a common mistake here: the urban heat island effect is a real consequence of development, but it is not the same as habitat fragmentation or runoff. Treat it as a separate impact.

### Economic and planning problems linked to sprawl

APES focuses mainly on environmental science, but some economic and planning consequences are worth knowing because they often appear in stimulus questions or FRQs.

Sprawl raises infrastructure costs. Roads, sewer lines, water pipes, power lines, schools, and emergency services must cover a larger area. A compact community can often provide the same services more efficiently. With sprawl, governments may spend more per person to maintain the built environment.

Traffic congestion can also worsen, even though sprawl is often chosen partly to escape crowded cities. As more people rely on cars and commute longer distances, roads fill up. So sprawl can create the very transportation problem it seems to solve.

There is also a land-use opportunity cost. When farmland or wetlands are developed, the community loses future agricultural land, wildlife habitat, flood buffering, and open space. Those are real losses even if the financial transaction seems profitable in the short term.

A good APES answer often recognises that these are **externalities**: costs imposed on natural systemss or the wider public that are not fully included in the price of sprawling development.

### Distinguishing problems from solutions

The AP exam often tests whether you can separate the problems of sprawl from the strategies used to reduce them. If the question asks for a problem, do not give a solution.

The major solutions you should recognise include:

- **smart growth**
- mixed-use development
- transit-oriented development
- greenbelts
- urban growth boundaries
- infill development
- brownfield redevelopment
- permeable pavement and other runoff-reduction methods
- wildlife corridors
  > **Key Term: smart growth**
  > A planning approach that encourages compact, mixed-use, transit-friendly development to reduce the impacts of sprawl.

For example, if asked how to reduce flooding caused by sprawl, a strong answer could name permeable pavement or rain gardens and explain the mechanism: they increase infiltration, reduce runoff, and improve groundwater recharge. If asked how to reduce fragmentation, a strong answer could mention habitat corridors and protected greenbelts.

But do not confuse these with the problem itself. “Urban sprawl causes smart growth” is wrong. Smart growth is a response to sprawl.

For FRQs, structure your answer as a chain:
policy or strategy → environmental mechanism → environmental result

Example:
Urban growth boundary → limits outward low-density expansion → reduces farmland conversion and habitat fragmentation

## Key Point Checklist

_This article has covered the following key knowledge points:_

- Urban sprawl is outward, low-density, car-dependent expansion into rural or undeveloped land
- Sprawl commonly includes large lots, separated land uses, and long commuting distances
- Habitat fragmentation breaks continuous habitat into smaller isolated patches
- Fragmentation increases edge effects and reduces interior habitat
- Interior and specialist species often decline when fragmentation increases
- Impervious surfaces reduce infiltration and groundwater recharge
- More impervious cover increases stormwater runoff and flood risk
- Urban runoff often carries nonpoint source pollutants such as oil, metals, fertilizers, and pesticides
- Sprawl increases car dependence, fossil fuel use, and air pollution
- Greater vehicle use also increases greenhouse gas emissions
- Sprawl can cause farmland, forest, and wetland loss
- Smart growth, mixed-use development, transit-oriented development, and greenbelts are strategies used to reduce sprawl impacts

## Key Terms and Concepts

- urban sprawl
- low-density development
- habitat fragmentation
- edge effects
- impervious surfaces
- groundwater recharge
- stormwater runoff
- car dependence
- urban heat island effect
- smart growth


Impacts of Urbanization - Urban sprawl problems