AP Environmental Science

## Learning Outcomes

- Understand how **competition** can increase extinction risk in natural populations
- Explain the role of **interspecific competition**, **niche overlap**, and the **competitive exclusion principle**
- Distinguish competition from related threats such as predation and habitat loss
- Identify why specialist species and small populations are more vulnerable to competition-driven decline
- Apply examples such as zebra mussels and invasive plants to AP-style questions
- Explain how laws and conservation strategies can reduce extinction risk

## AP Environmental Science Syllabus

For the AP Environmental Science, you are expected to understand competition and extinction risk, with a focus on the following syllabus points:

- Know that **competition** occurs when organisms rely on the same limiting resource
- Identify **interspecific competition** as the form most relevant to native and invasive species interactions
- Explain the **competitive exclusion principle**: two species cannot occupy the same niche indefinitely when resources are limiting
- Understand that greater **niche overlap** usually increases competition intensity
- Know that invasive species can **outcompete** native species for food, space, light, water, or habitat
- Distinguish **local extinction** from global extinction
- Explain why specialist species, low-reproductive-rate species, and small populations face higher extinction risk
- Recognise that competition is one threat among several, alongside habitat loss, climate change, pollution, and overexploitation

## 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 the difference between **interspecific competition** and **predation**?
2. Why does greater **niche overlap** increase extinction risk?
3. How can an invasive species make a native species endangered without directly eating it?
4. Why are specialist species usually more vulnerable to competitive pressure than generalists?
5. What is the difference between **local extinction** and global extinction?

## Introduction

Competition is a normal part of natures. Species compete for food, water, light, shelter, nesting sites, and territory. On its own, competition is not automatically a sign that an habitat is failing. In many habitats, competition helps shape niches and allows natural selection to act.

For AP Environmental Science, the key issue is not just that competition happens, but that competition can push already vulnerable species toward **population decline**, **range contraction**, or extinction. This is especially true when humans introduce invasive species into habitats where native species already have narrow ecological limits.

> **Key Term: competition**
> The interaction that occurs when organisms rely on the same limited resource, such as food, water, light, space, or habitat.

## Introduction to competition as an extinction pressure

### How competition works in natures

Competition matters because resources are limited. If two species use the same food, the same breeding space, or the same patch of habitat, they reduce what is available to each other. That lowers survival, growth, or reproduction.

There are two main kinds of competition:

- **Intraspecific competition**: competition within the same species
- **Interspecific competition**: competition between different species

For endangered-species questions, interspecific competition is usually the more relevant one because APES often focuses on conflicts between native and invasive species.

> **Key Term: interspecific competition**
> Competition between members of different species for the same limiting resources.

A common mistake is to treat competition as if it only means “fighting over food.” On the exam, competition can also be for:

- space on a rock or lake bottom
- sunlight in plant communities
- burrow or nest sites
- breeding habitat
- oxygen-rich water
- shelter from predators

That wider definition is important because many invasive species harm natives without directly consuming them.

> **Test Tip:** If a question asks how an invasive species threatens a native species, do not jump straight to predation. First ask: is it competing for food, space, light, or habitat instead?

### Why competition becomes dangerous

A species does not become endangered just because another species is present nearby. Risk rises when three things happen together:

1. the species share a limiting resource
2. one species is the stronger competitor
3. the weaker species cannot adapt or move

This produces a cause-and-effect chain that APES examiners like to see clearly explained:

invasive species introduction -> competition for limiting resources -> reduced native survival or reproduction -> native population decline -> lower biodiversity -> higher extinction risk

Notice that the decline often starts with reduced reproduction, not immediate death. If a species gets less food, fewer nesting sites, or less light, it may produce fewer offspring. Over time, that slow pressure can be enough to drive a population below a viable level.

> **Key Term: extinction risk**
> The likelihood that a species will become extinct, especially when small populations face environmental stress or competition.

## Niche overlap, competitive exclusion, and displacement

### Why niche overlap matters

A **niche** includes both where a species lives and how it uses resources. If two species have high niche overlap, they use very similar resources in similar ways. That raises competition because each species reduces what the other needs.

> **Key Term: niche overlap**
> The degree to which two species use the same resources or ecological role.

For example:

- two mussel species filtering the same plankton from the same water column
- an invasive vine and native plants competing for sunlight
- two bird species relying on the same nesting cavities

The greater the overlap, the stronger the pressure. If overlap is small, coexistence is easier because each species is using a somewhat different part of the environment.

### The competitive exclusion principle

If two species occupy essentially the same niche and resources are limited, long-term stable coexistence is unlikely. One species tends to outcompete the other.

> **Key Term: competitive exclusion principle**
> The idea that two species cannot occupy the exact same niche in the same habitat indefinitely when resources are limiting.

This does not mean the weaker species always goes globally extinct. More often, it is displaced from that habitat, pushed into a smaller range, or suffers a severe drop in numbers.

That is why APES questions often reward the phrase **local extinction**. It is a more precise result than saying “the species dies out everywhere.”

> **Key Term: local extinction**
> The disappearance of a species from a particular area, even though it may still exist elsewhere.

### Why competitive exclusion does not always happen

Students often overstate this principle. Two species in the same habitat do not automatically exclude each other. They may avoid direct competition through **resource partitioning**.

> **Key Term: resource partitioning**
> The use of the same broad resource in different ways, places, or times so that competition is reduced.

Resource partitioning can happen by:

- feeding at different times
- using different microhabitats
- eating different sizes of prey
- rooting or growing at different depths or heights

If partitioning is possible, coexistence may continue. If it is not, exclusion or displacement becomes more likely.

> **Test Tip:** In FRQs, stronger answers often say “competitive exclusion may occur unless the species can partition resources.” That shows precise reasoning.

## Why some species are much more vulnerable than others

### Specialists versus generalists

Species are not equally threatened by competition. Broadly, **generalist** species handle change better, while **specialist** species are more fragile.

A generalist can use many food types or habitats. A specialist depends on a narrow set of conditions. If competition reduces that one food source or habitat, the specialist has fewer alternatives.

> **Key Term: specialist species**
> A species with narrow habitat or resource requirements, making it less adaptable to environmental change.

In contrast, invasive species are often successful because they behave more like generalists. They tend to reproduce quickly, disperse widely, and tolerate many conditions. That gives them a major advantage over native specialists.

This is one reason competition from invasive species is such a serious biodiversity threat. It is not just species versus species; it is often adaptable generalist versus vulnerable specialist.

### Small populations and low reproductive rates

Competition is even more dangerous when the native population is already small. A modest loss of food or habitat can have a big effect because:

- fewer individuals survive to breed
- mates become harder to find
- genetic diversity may fall further
- chance events become more damaging

Species with low reproductive rates are also at greater risk. If an elephant, whale, or large predator loses breeding success for several years, recovery is slow. A fast-breeding species may rebound. A slow-breeding one may not.

This links competition to wider extinction risk factors. A species is especially vulnerable when it has:

- a limited geographic range
- specialised diet
- low population size
- large territory requirements
- slow reproduction
- low genetic diversity

Competition may not be the only pressure, but it can be the final stress that pushes a species into endangered status.

## Invasive species as a major driver of competition-led decline

### How invasive species outcompete natives

Invasive species are among the clearest APES examples of competition increasing extinction risk. They may:

- consume the same food more efficiently
- grow faster and block light
- occupy physical space needed by natives
- reproduce quickly and dominate habitat
- alter food webs in ways that disadvantage native species

This can reduce biodiversity even without direct killing.

> **Key Term: invasive species**
> A non-native species that spreads successfully and causes ecological or economic harm in its introduced environment.

A useful exam phrase is: invasive species can **outcompete and displace native species**, reducing native abundance and increasing extinction risk.

### Example: zebra mussels

Zebra mussels are a strong APES case study. Introduced from Eurasia into the Great Lakes through ballast water, they spread rapidly after **1988**. They are filter feeders, so they consume plankton and bacteria that native mussels and other organisms also need.

This creates competition for **food**. They also attach to hard surfaces, including native mussels, which creates competition for **space** and interferes with movement and feeding.

The result is not just inconvenience to humans through clogged pipes. Ecologically, zebra mussels can reduce native mussel populations and contribute to local extirpation.

The key chain is:

zebra mussel introduction -> competition for plankton and space -> native mussel decline -> reduced biodiversity -> higher local extinction risk

### Example: invasive plants

Invasive plants often show competition more clearly than invasive predators. A dense invasive plant can crowd out native plants by taking:

- sunlight
- soil water
- soil nutrients
- physical growing space

Kudzu is a classic example of a plant that can overgrow native vegetation. The exact APES point is not “kudzu poisons everything,” but that it can shade native plants and reduce their access to light, leading to displacement.

If native plants decline, the effects move up the food web. Herbivores lose food or habitat. Pollinators may lose nectar sources. Nesting or shelter sites may change. So plant competition can trigger much larger biodiversity loss.

### Distinguishing competition from predation

This distinction matters a lot in exam answers.

- **Competition**: two species use the same limited resource
- **Predation**: one species consumes another

Brown tree snakes on Guam are usually taught in APES, but they are mainly a **predation** example because they consumed native birds. Zebra mussels and invasive plants are more clearly **competition** examples.

If you mislabel predation as competition, you may lose accuracy marks.

## Competition, endangered status, and conservation responses

### Threatened versus endangered

APES expects you to use these terms carefully.

> **Key Term: endangered species**
> A species in danger of extinction throughout all or a significant portion of its range.
>
> **Key Term: threatened species**
> A species likely to become endangered within the foreseeable future.

Competition can help move a species from stable, to threatened, to endangered if it keeps reducing survival and reproduction.

Importantly, competition is not a status category by itself. It is a **threat mechanism**. It may contribute to listing decisions under broader risk factors, especially when invasive species are involved.

### Local versus global consequences

Competition often causes **local extinction** first. A native species may disappear from one lake, island, or forest patch while surviving elsewhere. That still matters greatly because:

- biodiversity in that environment falls
- food webs change
- recovery becomes harder
- isolated populations may lose genetic diversity

Local losses can also spread. If fragmented habitats isolate populations, one local extinction after another may eventually become a regional or global crisis.

### What reduces competition-driven extinction risk

Conservation responses focus on lowering the pressure from stronger competitors and improving survival chances for native species. Examples include:

- preventing introduction of invasive species
- early removal of invasive populations
- habitat restoration
- creation of habitat corridors
- captive breeding and reintroduction where needed
- legal protection under the **Endangered Species Act** and international trade controls under **CITES**

Habitat corridors are especially useful when species need to move to avoid competition or reach better habitat. Restored connectivity can reduce the risk of isolated populations collapsing.

Still, competition is rarely the only threat. If habitat loss, climate change, pollution, and invasive competition all act together, conservation becomes much harder. That “multiple stressors” idea is often what separates an average APES response from a strong one.

> **Test Tip:** For FRQs, avoid saying “competition causes extinction” as if it is instant and absolute. Better wording is “competition can reduce native populations, lower biodiversity, and in severe cases lead to local extinction or raise extinction risk.”

## Key Point Checklist

_This article has covered the following key knowledge points:_

- Competition occurs when organisms depend on the same limiting resource
- Interspecific competition is competition between different species and is especially relevant to native–invasive interactions
- Greater niche overlap usually means stronger competition
- The competitive exclusion principle states that two species cannot occupy the exact same niche indefinitely when resources are limiting
- Competition may cause displacement, range contraction, population decline, or local extinction
- Resource partitioning can reduce competition and allow coexistence
- Invasive species can outcompete native species for food, space, light, water, or habitat
- Zebra mussels are a key example of competition for food and space in aquatic habitatss
- Specialist species are usually more vulnerable to competition than generalist species
- Small populations and low reproductive rates increase extinction risk under competitive pressure
- Competition is different from predation and should not be confused with it in exam answers
- Threatened and endangered are distinct conservation categories, while competition is a threat mechanism rather than a status category

## Key Terms and Concepts

- competition
- interspecific competition
- extinction risk
- niche overlap
- competitive exclusion principle
- local extinction
- resource partitioning
- specialist species
- invasive species
- endangered species
- threatened species


Endangered Species - Competition and extinction risk