AP Environmental Science

## Learning Outcomes

- Explain how climate change alters habitats on land and in oceans
- Identify the main pathways linking greenhouse gases to biodiversity loss
- Distinguish between habitat loss, habitat fragmentation, and range shifts
- Describe how sea-level rise harms coastal wetlands, mangroves, and estuaries
- Explain how ocean warming and acidification damage coral reef habitats
- Understand why polar, alpine, specialist, and reef species are especially at risk
- Apply cause-and-effect chains in AP Environmental Science exam responses

## AP Environmental Science Syllabus

For the AP Environmental Science, you are expected to understand how climate change causes habitat loss, with a focus on the following syllabus points:

- Know that climate change is one of the major human drivers of biodiversity loss
- Identify the chain from greenhouse gas increase to warming to habitat change to biodiversity decline
- Explain how sea-level rise results from **thermal expansion** and melting **land-based ice**
- Understand that melting floating sea ice does not significantly raise sea level
- Describe how climate change causes species **range shifts** poleward and upslope
- Explain how ocean warming causes **coral bleaching** and how ocean acidification weakens reef-building
- Distinguish habitat loss from habitat fragmentation and explain how both can interact
- Use examples such as coastal wetlands, coral reefs, tundra, and mountain habitats

## 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. How does an increase in greenhouse gases lead to habitat loss?
2. What is the difference between habitat loss and habitat fragmentation?
3. Why does sea-level rise threaten coastal wetlands and mangroves?
4. How do ocean warming and ocean acidification harm coral reefs in different ways?
5. Why are alpine and polar species especially vulnerable to climate-driven habitat change?

## Introduction

Climate change causes habitat loss by changing the abiotic conditions that organisms depend on. If temperature, precipitation, salinity, water depth, oxygen availability, or pH shift beyond a species’ tolerance range, the habitat may still exist physically, but no longer function as suitable living space. That is why AP Environmental Science treats climate change as a biodiversity issue, not just a weather issue.

This topic often appears in exam questions as a chain of linked steps. You may be asked to explain how fossil fuel use affects wetlands, coral reefs, fish populations, or species distributions. Strong answers do not stop at “warming is bad.” They trace the mechanism clearly: greenhouse gas increase, warming, physical habitat change, biological response, then biodiversity decline.

> **Key Term: biodiversity**
> The variety of life at the genetic, species, and habitat level levels.

### From greenhouse gases to habitat loss

The starting point is increased greenhouse gases, especially **carbon dioxide** and **methane**, from human activities such as burning fossil fuels. These gases trap more heat in the lower atmosphere, raising average global temperatures. That warming then changes the conditions that define habitats.

A strong APES answer often uses a four-step pattern:

- climate driver
- physical change
- habitat effect
- biodiversity effect

For example:

- more atmospheric carbon dioxide
- warmer climate
- altered rainfall and temperature patterns
- some species lose suitable habitat and decline

This matters because most species are adapted to a specific environmental range. A cold-water fish, a salt-marsh grass, or a tundra mammal can only tolerate so much change before reproduction, feeding, or survival drops.

> **Key Term: habitat loss**
> The destruction, degradation, or reduction of suitable habitat so organisms can no longer survive there successfully.

Climate change causes habitat loss in several major ways:

- increasing air and water temperatures
- shifting precipitation patterns
- increasing drought in some regions and flooding in others
- changing disturbance patterns such as wildfire and storms
- raising sea level
- changing ocean chemistry through acidification

Students sometimes think habitat loss only means bulldozers or chainsaws. In AP Environmental Science, habitat can also be lost when the abiotic conditions change too much. A mountain meadow that becomes too warm for a cold-adapted species is still “there,” but not as usable habitat for that organism.

> **Key Term: ecological tolerance**
> The range of environmental conditions within which an organism can survive and reproduce.

This is why specialists are often more vulnerable than generalists. A generalist can often switch food sources, move more easily, or tolerate wider conditions. A specialist may depend on one temperature range, one host plant, or one breeding site.

> **Test Tip:** In FRQs, avoid vague statements like “climate change hurts animals.” Write a chain: “Increased greenhouse gases warm the climate, which shifts rainfall patterns, reducing wetland water levels and causing habitat loss for amphibians.”

### Sea-level rise and the loss of coastal habitats

One of the clearest climate-driven habitat loss examples is sea-level rise. Sea level rises mainly for two reasons:

- warming seawater expands, called **thermal expansion**
- land-based ice such as glaciers and ice sheets melts

A very common exam trap is confusing land ice with sea ice.

> **Key Term: thermal expansion**
> The increase in water volume as seawater warms, which contributes to sea-level rise.

Melting floating sea ice does not significantly raise sea level. Melting land ice does. That distinction matters.

As sea level rises, coastal habitats such as salt marshes, mangroves, estuaries, and wetlands face inundation, erosion, and saltwater intrusion. These habitats are biologically rich and provide nursery grounds, feeding areas, nesting sites, and storm buffering.

The cause-and-effect chain looks like this:

- greenhouse gas increase
- warmer climate
- thermal expansion and melting land ice
- sea-level rise
- coastal flooding and erosion
- habitat loss for fish, birds, amphibians, and invertebrates

Coastal wetlands can sometimes survive moderate sea-level rise by trapping sediment and building upward. But they cannot always keep pace, especially where sediment supply is limited or the rise is too fast.

A further problem is **coastal squeeze**.

> **Key Term: coastal squeeze**
> The loss of coastal habitats when sea-level rise pushes them inland but development or seawalls block their movement.

If a wetland could migrate inland naturally, some habitat might be preserved. But roads, cities, seawalls, farms, and other development often block that movement. The wetland is trapped between rising water and fixed human infrastructure, so it narrows or disappears.

Mangroves face a similar problem. In some places they may shift inland or poleward, but only if suitable land is available. Where the shoreline is armored or heavily developed, their habitat is cut off.

The biodiversity effects are large because these habitats support many food webs. Juvenile fish use wetlands and estuaries as nurseries. Birds rely on nesting and feeding grounds. Amphibians and reptiles may lose breeding areas. If the habitat shrinks, species populations often shrink with it.

### Range shifts, fragmentation, and species with nowhere to go

On land and in freshwater systems, warming often does not erase habitat instantly. Instead, it shifts where suitable habitat is located. As temperatures rise, many species move poleward or upslope to stay within their preferred climate conditions.

> **Key Term: range shift**
> A change in the geographic area where a species can survive, often poleward or to higher elevations as temperatures rise.

Recent observations show many species moving northward and upward in elevation. This is not always good news. A shift in range can produce new competition, mismatches with food sources, and shrinking total habitat area.

Mountain species show this problem clearly. As a species moves upslope, the available area often becomes smaller. Near the mountain top, habitat may run out completely. This is called mountaintop habitat contraction in some questions and examples.

Polar species have a similar issue. They are already living near the coldest available environments. If warming continues, there may be no cooler place left to move. That is why Arctic and alpine organisms are often identified as especially vulnerable.

Students must also distinguish range shift from fragmentation.

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

Climate change can interact with fragmentation in harmful ways. A species may need to move north or uphill, but roads, cities, farms, pipelines, and cleared land can block movement. So even if suitable climate exists somewhere else, the species may not be able to reach it.

Fragmentation also reduces gene flow between populations. Small isolated populations are less resilient, more vulnerable to local extinction, and less able to recover after disturbance.

Freshwater examples are especially useful. As streams and rivers warm, warm-water fish may expand into areas once dominated by cold-water fish. Species such as trout and salmon can lose suitable habitat because water temperature is a major limiting factor for them. If stream temperatures rise too high, oxygen levels also tend to fall, which adds further stress.

### Oceans, coral reefs, and climate-driven habitat degradation

Marine habitat loss often appears on the AP exam through coral reefs. Reefs are extremely biodiverse and also provide physical structure that serves as shelter, feeding space, and nursery habitat.

Ocean warming and ocean acidification are both climate-related, but they are different mechanisms.

Ocean warming raises water temperature and can trigger coral bleaching.

> **Key Term: coral bleaching**
> A stress response in which corals expel their symbiotic algae, usually because of elevated water temperature.

The algae provide much of the coral’s energy. When they are expelled, the coral turns pale and becomes stressed. Bleaching does not always mean immediate death, but prolonged stress often leads to mortality and reef degradation.

Ocean acidification works chemically. As atmospheric carbon dioxide rises, more dissolves into seawater:

CO2 + H2O ⇌ H2CO3 ⇌ H+ + HCO3-

The increase in hydrogen ions lowers pH and reduces carbonate ion availability. That matters because corals and many shell-building organisms need carbonate to build calcium carbonate structures.

> **Key Term: ocean acidification**
> The decrease in ocean pH as seawater absorbs atmospheric carbon dioxide, reducing carbonate availability for shell and reef formation.

So the reef is hit from two directions:

- warming damages corals through bleaching
- acidification weakens calcification and reef building

That means even where corals survive, the reef structure may become weaker or grow more slowly. Once reef complexity declines, many organisms lose shelter and nursery areas. Biodiversity then falls because the habitat itself becomes less suitable.

This is a strong APES marine chain:

- more atmospheric carbon dioxide
- warmer oceans and more dissolved carbon dioxide
- coral bleaching plus reduced calcification
- reef degradation
- loss of shelter and nursery habitat
- lower marine biodiversity
  > **Test Tip:** Do not say ocean warming and ocean acidification are the same thing. Warming mainly drives bleaching. Acidification mainly reduces calcification.

A related concept is **phenology**, the timing of seasonal biological events such as flowering, breeding, and migration. Climate change can shift this timing and create mismatch. For example, a breeding habitat may still exist physically, but if food peaks earlier than the species’ breeding season, the habitat becomes less useful.

> **Key Term: phenology**
> The timing of recurring seasonal biological events such as migration, flowering, or breeding.

This matters because habitat quality is not only about place. It is also about time. A nesting area without food at the right season is effectively degraded habitat.

## Key Point Checklist

_This article has covered the following key knowledge points:_

- Climate change causes habitat loss by changing temperature, precipitation, sea level, salinity, disturbance patterns, and ocean chemistry
- A useful exam chain is greenhouse gas increase to warming to habitat change to biodiversity decline
- Habitat loss is broader than habitat fragmentation
- Habitat fragmentation means a large habitat is broken into smaller isolated patches
- Sea-level rise is caused mainly by thermal expansion and melting land-based ice
- Melting floating sea ice does not significantly raise sea level
- Coastal wetlands, mangroves, estuaries, and salt marshes are threatened by flooding, erosion, and saltwater intrusion
- Coastal squeeze happens when coastal habitats cannot migrate inland because development blocks them
- Range shifts move species poleward or upslope, but some species face shrinking habitat or no suitable area left
- Coral reefs are damaged by both ocean warming and ocean acidification
- Ocean warming causes coral bleaching, while ocean acidification reduces calcification
- Polar, alpine, specialist, and reef species are especially vulnerable to climate-driven habitat change

## Key Terms and Concepts

- biodiversity
- habitat loss
- ecological tolerance
- thermal expansion
- coastal squeeze
- range shift
- habitat fragmentation
- coral bleaching
- ocean acidification
- phenology


Human Impacts on Biodiversity - Climate change causes habitat loss