AP Environmental Science

## Learning Outcomes

- Understand the main human-driven causes of species endangerment using **HIPPCO**
- Explain why habitat loss and habitat fragmentation are not the same thing
- Identify how overexploitation, including **bycatch**, pushes populations toward extinction
- Describe how invasive species, pollution, and climate change reduce biodiversity
- Distinguish broad APES biodiversity drivers from legal listing factors under the **Endangered Species Act**
- Apply these ideas to common AP Environmental Science examples and free-response questions

## AP Environmental Science Syllabus

For the AP Environmental Science, you are expected to understand causes of species endangerment, with a focus on the following syllabus points:

- Know the **HIPPCO** framework: habitat destruction, invasive species, population growth, pollution, climate change, and overexploitation
- Identify habitat loss and explain that habitat fragmentation is usually treated as part of habitat destruction
- Explain how overexploitation removes organisms faster than populations can recover
- Understand how invasive species harm natives through competition, predation, disease spread, and food-web disruption
- Know that climate change can shift temperature and precipitation patterns, alter habitat ranges, and act as a threat multiplier
- Recognise pollution as a major driver of biodiversity loss, including pesticides, plastics, oil spills, and nutrient loading
- Understand that human population growth in HIPPCO refers to human pressure on land, water, and resources, not growth of the endangered species itself
- Link causes of endangerment to laws and responses such as the **Endangered Species Act (1973)** and **CITES (1975)**

## 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 does **HIPPCO** stand for?
2. How does habitat fragmentation reduce biodiversity even when some habitat still remains?
3. Why is **bycatch** considered a form of overexploitation?
4. Name two ways invasive species can endanger native species.
5. Why does climate change often make existing threats to species worse?

## Introduction

Species do not become endangered for one simple reason. Most declines happen because several pressures act together: habitats shrink, populations become isolated, food webs shift, and reproduction can no longer keep pace with losses. For AP Environmental Science, you need to know both the broad patterns and the specific mechanisms.

The main exam framework is **HIPPCO**, which groups the most important human causes of biodiversity loss and species endangerment. If you can explain each letter clearly, use examples, and link them to falling population size or reduced reproductive success, you will be in a strong position for multiple-choice and FRQ answers.

> **Key Term: endangered species**
> A species in danger of extinction throughout all or a significant portion of its range.

### HIPPCO: the main framework for causes of endangerment

**HIPPCO** is the standard APES way to organise the major causes of species decline:

- **H**abitat destruction and fragmentation
- **I**nvasive species
- **P**opulation growth
- **P**ollution
- **C**limate change
- **O**verexploitation

This framework matters because it turns a long list of threats into six broad drivers. On the exam, a question may ask for the acronym directly, or it may describe a scenario and expect you to recognise which driver is operating.

> **Key Term: HIPPCO**
> An AP Environmental Science acronym for the major human causes of biodiversity loss: habitat destruction, invasive species, population growth, pollution, climate change, and overexploitation.

Human population growth sits inside HIPPCO because more people usually means more land conversion, more water use, more roads, more resource extraction, and more waste. It is not referring to the endangered population growing. Students often lose marks by confusing those two ideas.

A useful second framework comes from conservation science more generally. Major biodiversity drivers identified globally are land- and sea-use change, direct exploitation of organisms, climate change, pollution, and invasive species. That is very close to HIPPCO, which is why HIPPCO is so useful: it matches the bigger scientific picture while staying easy to memorise.

> **Test Tip:** If an FRQ asks for “major causes of endangerment,” start with HIPPCO. That gives your answer structure and helps you avoid missing a major category.

### Habitat loss and fragmentation: the biggest driver

Habitat loss is often the clearest and most important cause of endangerment. If the place a species needs for feeding, breeding, shelter, or migration is removed or degraded, the population declines even if the animals themselves are not directly hunted.

> **Key Term: habitat loss**
> The destruction, degradation, or reduction of habitat area so that species can no longer survive there.

Common causes include:

- deforestation
- wetland drainage
- urbanisation
- agriculture
- mining
- road building
- dam construction

The key point is not just “less space.” Habitat loss reduces access to food, nesting sites, cover from predators, and breeding locations. It can also push organisms into smaller areas, where competition rises and disease spreads more easily.

Habitat fragmentation is a specific problem within habitat loss. A large continuous habitat is broken into smaller isolated patches. Even if the total area left seems substantial, fragmentation can still be severe because isolated patches function like islands.

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

Why fragmentation matters:

- populations become separated and may not find mates
- gene flow declines, reducing genetic diversity
- movement routes are blocked
- edge effects increase
- small isolated populations face higher risk of local extinction
  > **Key Term: edge effects**
  > Environmental changes at habitat boundaries that often make conditions less suitable for species adapted to the habitat interior.

A forest split by roads shows this well. Some generalist species may still survive near edges, but forest-interior birds or mammals may decline because edges are hotter, drier, brighter, and easier for predators to access. A habitat can therefore remain present on a map but become much less usable biologically.

Examples worth knowing include habitat fragmentation threatening large mammals such as the Florida panther, and dams disrupting migration routes for salmon. In marine and freshwater systems, habitat destruction includes wetland loss, coral reef damage, dredging, and altered river flow.

### Overexploitation, invasive species, and pollution

Overexploitation means removing individuals from a population faster than reproduction can replace them. This is a direct route to endangerment because the population decline is built into the harvest rate.

> **Key Term: overexploitation**
> The harvesting or removal of organisms faster than the population can replenish itself through reproduction.

Forms of overexploitation include:

- overfishing
- overhunting
- poaching
- wildlife trade
- overharvesting plants
- logging when species depend on those trees or forest conditions

Some species are especially vulnerable because they reproduce slowly, mature late, or require large territories. Many large mammals fit this pattern. If a species has low reproductive rates, even moderate extra mortality can be enough to push it toward endangerment.

**Bycatch** is an especially important marine example.

> **Key Term: bycatch**
> The unintentional capture of non-target species during fishing.

Sea turtles, seabirds, dolphins, and sharks may be caught in nets or on longlines intended for other species. This matters because the mortality is real even when the species is not being targeted. On an APES question, bycatch is usually best classified under overexploitation or direct human-caused mortality.

Invasive species are another major cause of endangerment. These are nonnative species that spread in a new area and harm native populations.

> **Key Term: invasive species**
> A nonnative species that spreads and harms native species or natural habitatss.

They can harm native species in four main ways:

- **competition** for food, water, or habitat
- **predation** on native species
- **disease spread**
- **food-web disruption**

A native specialist species often loses against an invasive generalist because the invasive can tolerate wider conditions and reproduce faster. A classic example is the zebra mussel in the Great Lakes, which outcompetes native species and alters ecological balances. On islands and isolated habitats, introduced predators can be especially destructive because native species may lack defences.

Pollution can also endanger species directly or indirectly.

> **Key Term: pollution**
> The contamination of air, water, or soil in ways that harm organisms and environments.

Examples include:

- pesticides harming non-target organisms
- nutrient runoff causing eutrophication
- oil spills coating organisms and damaging habitats
- plastics causing ingestion or entanglement
- heavy metals accumulating in food webs
- acidification damaging aquatic systems

Pollution reduces biodiversity by lowering survival, reproduction, or habitat quality. Amphibians are often very vulnerable because of their permeable skin and reliance on both aquatic and terrestrial habitats. Birds of prey were famously harmed by **DDT**, which caused eggshell thinning and lowered breeding success.

> **Test Tip:** In FRQs, do not just name a threat. Add the mechanism: “Pollution lowers reproductive success,” “fragmentation reduces gene flow,” or “bycatch increases mortality of non-target marine species.” That extra link often earns the point.

### Climate change, species vulnerability, and legal factors

Climate change is a major cause of endangerment because it alters the abiotic conditions species are adapted to. Temperature, precipitation, seasonality, sea level, and ocean chemistry can all shift.

> **Key Term: climate change**
> Long-term shifts in temperature, precipitation, sea level, and related environmental conditions that alter habitats and species ranges.

Climate change harms species by:

- shifting suitable habitat ranges
- changing timing of breeding or migration
- increasing drought, heat waves, or storms
- melting sea ice
- warming oceans
- increasing ocean acidification
- worsening existing habitat stress

One of the most useful ways to explain climate change in APES is as a **threat multiplier**. It often does not act alone. Instead, it makes other pressures worse. A fragmented population may be unable to move as climate zones shift. A polluted wetland may become even less suitable under drought. Coral reefs stressed by warming may also be weakened by acidification and pollution.

Species are not equally vulnerable. Common risk factors include:

- limited geographic range
- specialised diet
- low reproductive rate
- large territory requirement
- fixed migration route
- small population size
- narrow tolerance range

These are not separate HIPPCO drivers. They are traits that make a species more likely to become endangered when the drivers act on it. A panda’s restricted diet is not itself a human cause, but it makes habitat loss much more dangerous. A polar bear’s difficulty adapting quickly makes climate change especially serious.

For the exam, it also helps to know that legal systems use related but slightly different categories. Under U.S. law, the **Endangered Species Act (1973)** uses listing factors such as habitat destruction, overutilization, disease or predation, inadequate regulation, and other natural or human-caused factors.

> **Key Term: Endangered Species Act (1973)**
> U.S. law designed to protect endangered and threatened species and the habitats they depend on.

This overlaps with HIPPCO but is not identical. Disease and predation may appear explicitly in legal listing factors, while APES usually keeps its broad revision structure under HIPPCO.

Internationally, **CITES (1975)** addresses trade.

> **Key Term: CITES (1975)**
> The Convention on International Trade in Endangered Species, a treaty that regulates or bans trade in endangered and threatened species.

That matters because if endangerment is driven by poaching or wildlife trade, CITES is a major policy response. Habitat corridors, protected areas, invasive-species control, bycatch reduction, restoration, and captive breeding can all also reduce endangerment, but the response should match the cause.

## Key Point Checklist

_This article has covered the following key knowledge points:_

- The main APES framework for causes of endangerment is **HIPPCO**
- **Habitat destruction** includes habitat loss, degradation, and fragmentation
- Habitat fragmentation isolates populations, reduces gene flow, and increases edge effects
- Overexploitation means populations are harvested faster than they can recover
- **Bycatch** is an important marine form of overexploitation affecting non-target species
- Invasive species can endanger natives through competition, predation, disease spread, and food-web disruption
- Pollution can lower survival and reproductive success or damage habitats directly
- Climate change alters habitat conditions and often acts as a threat multiplier
- Species with specialised diets, limited ranges, or low reproductive rates are often more vulnerable
- Human population growth in HIPPCO refers to pressure on land, water, and resources
- The **Endangered Species Act (1973)** and **CITES (1975)** are major policy responses
- On the exam, strong answers link each cause to declining population size, lower reproductive success, or shrinking range

## Key Terms and Concepts

- endangered species
- HIPPCO
- habitat loss
- habitat fragmentation
- edge effects
- overexploitation
- bycatch
- invasive species
- pollution
- climate change
- Endangered Species Act (1973)
- CITES (1975)


Endangered Species - Causes of endangerment