AP Environmental Science

## Learning Outcomes

- Explain why competition happens in natural habitatss and why it becomes stronger when resources are limited
- Distinguish clearly between **interspecific competition** and **intraspecific competition**
- Understand what an ecological niche includes and why it is more than just a habitat
- Compare a **fundamental niche** with a **realized niche**
- Explain the **competitive exclusion principle** and predict likely outcomes of niche overlap
- Describe how **resource partitioning** reduces direct competition and allows coexistence
- Apply these ideas to AP-style examples involving birds, plants, predators, and aquatic organisms

## AP Environmental Science Syllabus

For the AP Environmental Science, you are expected to understand competition and resource partitioning in natural communitiess, with a focus on the following syllabus points:

- Know that competition occurs when organisms seek the same **limited resource**
- Identify the difference between **intraspecific competition** and **interspecific competition**
- Explain what an **ecological niche** includes, including resource use and environmental conditions
- Understand the difference between a **fundamental niche** and a **realized niche**
- Explain the **competitive exclusion principle** or **Gause’s principle**
- Describe how **resource partitioning** reduces direct competition and can allow species to coexist
- Apply these ideas to examples involving food, light, space, water, nesting sites, or habitat use
- Predict likely outcomes when niche overlap is high and a resource becomes limiting

## 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 intraspecific and interspecific competition?
2. Why is a niche not the same thing as a habitat?
3. How does competition affect the size of a species’ realized niche?
4. What does the competitive exclusion principle predict?
5. Name two ways species can reduce competition through resource partitioning.

## Introduction

Competition and resource partitioning are core ecology ideas because they explain why some species can live together while others cannot. In any habitat, resources such as food, water, light, space, and nesting sites are limited. Because they are limited, organisms do not simply exist side by side without affecting one another. They interact, and one of the most common interactions is competition.

For AP Environmental Science, this topic matters because it links several ideas that are often tested together: species interactions, ecological niches, limiting resources, coexistence, and biodiversity. You need to understand not only what each term means, but also the cause-and-effect chain: limited resources lead to competition, competition can narrow niches, and species may either be excluded or reduce overlap through resource partitioning.

> **Key Term: competition**
> The interaction that occurs when organisms seek the same limited resource, reducing access for one or both.

### Why competition happens and why limited resources matter

Competition begins with scarcity. If resources were unlimited, organisms would not need to compete for them. But habitatss always contain limits. A forest has only so much sunlight reaching the ground. A lake has only so much dissolved oxygen and food at each depth. A nesting area has only so many safe sites. Because of this, not every organism can use every resource whenever it wants.

This is why the phrase **limiting resource** is so important. A resource becomes limiting when its supply is low enough to restrict growth, survival, or reproduction. The key AP idea is that competition is strongest when two conditions happen together:

- the resource is limited
- the organisms use the resource in similar ways

If both happen, overlap increases and conflict becomes more intense.

> **Key Term: limiting resource**
> A resource in short enough supply that it restricts the growth, survival, or reproduction of organisms.

Competition can involve animals, plants, fungi, and microorganisms. Students often picture two predators fighting over prey, but plant competition is equally important. Plants compete for sunlight, root space, water, and mineral nutrients. A tall tree canopy may prevent smaller plants from photosynthesizing enough to survive. Some plants also release chemicals that suppress nearby species, which reduces competition for soil resources.

There are two major forms of competition you must keep separate.

> **Key Term: intraspecific competition**
> Competition among individuals of the same species.
>
> **Key Term: interspecific competition**
> Competition between individuals of different species.

**Intraspecific competition** is often intense because members of the same species usually need almost exactly the same resources. Two squirrels of the same species need similar food, shelter, and breeding space. Two oak seedlings need similar light and soil conditions. Because their needs overlap so strongly, members of the same species often compete very directly.

**Interspecific competition** happens between different species. This may be weaker if the species use different resources, but it can become very strong when their niches overlap. For example, two bird species that both eat the same insects in the same shrubs at the same time of day are likely to compete strongly.

A distinction students miss: interspecific competition is not automatically stronger just because it is between different species. In many cases, intraspecific competition is stronger because the overlap in needs is even greater.

> **Test Tip:** If an AP question asks which type of competition is likely to be strongest, look for the greatest resource overlap, not just the words “same species” or “different species.”

### Niche, habitat, and the difference between fundamental and realized niches

A **habitat** is where an organism lives. A **niche** is broader. It includes not just place, but role. A niche includes what the species eats, when it is active, what conditions it tolerates, how it gets resources, and how it interacts with other species.

> **Key Term: ecological niche**
> The role of a species in its environment, including the resources it uses, the conditions it tolerates, and its interactions with biotic and abiotic factors.

This matters because competition is really about niche overlap, not just habitat overlap. Two species can live in the same habitat and still avoid strong competition if they use different foods, different parts of the space, or different activity times.

For example, several species may live in one forest, but one feeds in the canopy, one on the trunk, and one on the ground. Same habitat, different niches.

Ecologists distinguish between the **fundamental niche** and the **realized niche**.

> **Key Term: fundamental niche**
> The full range of conditions and resources a species could use in the absence of limiting biotic interactions such as competition or predation.
>
> **Key Term: realized niche**
> The actual range of conditions and resources a species uses after biotic interactions such as competition and predation restrict it.

The easiest way to think about this is:

- **fundamental niche** = possible
- **realized niche** = actual

A species might be physically capable of living across a wide range of temperatures, feeding across a broad area, or using many food sources. That is its fundamental niche. But in the real world, competitors, predators, and parasites reduce what it actually uses. That smaller, restricted role is its realized niche.

Competition often pushes a species from its fundamental niche into a narrower realized niche. This is a favorite AP distinction.

A classic way to understand this is vertical zonation in coastal organisms. A species may be able to survive across a broad tidal range when alone, but in the presence of a stronger competitor, it may be restricted to a smaller band. It is not just “choosing” that area. Its actual distribution reflects biotic pressure.

Students often make this mistake: they define realized niche as simply “where the species lives.” That is incomplete. The realized niche is where the species lives **after** competition and other biotic limits have shaped its actual range.

### Competitive exclusion and why exact niche overlap is unstable

If two species depend on exactly the same limiting resource in exactly the same place under the same conditions, stable coexistence is unlikely. This is the **competitive exclusion principle**, also called **Gause’s principle**.

> **Key Term: competitive exclusion principle**
> The principle that two species competing for the exact same limiting resource in the same place at the same time cannot stably coexist.
>
> **Key Term: Gause’s principle**
> Another name for the competitive exclusion principle.

The logic is straightforward. If species A and species B use the same resource in the same way, even a small advantage can matter over time. One species may use food slightly more efficiently, tolerate stress a little better, or reproduce a little faster. Over many generations, that small edge can lead to the weaker competitor being pushed out, reduced in number, forced into a smaller niche, or locally eliminated.

The outcome does not have to be immediate extinction. Competitive exclusion can produce several results:

- local disappearance of the weaker species
- movement to a different habitat
- a shift in feeding behavior or activity time
- restriction to a smaller realized niche

The key AP idea is that complete niche overlap tends to be unstable.

Imagine two fish species in a lake. If both feed on the same prey at the same depth, during the same time of day, and under the same temperature conditions, competition will be intense. If one species can capture prey more effectively, the other may decline unless it shifts its niche.

This is why competition and niche theory are linked. Competitive exclusion is really about excessive overlap plus limitation.

A common exam trap is to assume that any two similar species must exclude one another. Not necessarily. They exclude one another only if niche overlap remains too great and no mechanism reduces direct competition.

> **Test Tip:** In FRQs, use a causal chain: “Because the resource is limiting and niche overlap is high, interspecific competition increases, which may lead to competitive exclusion unless the species partition the resource.”

### How resource partitioning allows coexistence

If competition becomes strong, natural selection may favor ways of reducing overlap. This is **resource partitioning**.

> **Key Term: resource partitioning**
> The division of limited resources by species through differences in space, time, or method of use, reducing direct competition.

Resource partitioning does not mean competition disappears. It means competition is reduced enough for species to coexist.

There are several common forms.

#### Spatial partitioning

Species use different parts of the same habitat. This is one of the clearest forms.

A classic example is different warbler species feeding in different parts of the same conifer tree. One may forage near the top, another near the middle, another on outer branches, and another near the trunk. They share the tree habitat and feed on insects, but by separating where they search, they reduce direct overlap.

Another example is fish feeding at different depths in a lake. Same lake, similar food web, but different vertical zones.

#### Temporal partitioning

Species use the same resource at different times. For instance, one predator may hunt by day while another hunts by night. Hawks and owls are a simple AP-style illustration: both may feed on small mammals in the same region, but one is diurnal and the other nocturnal.

Temporal partitioning also occurs in desert rodents, insects, and flowering plant interactions.

#### Morphological or dietary partitioning

Species may differ in body form or feeding structures, which lets them use slightly different versions of the same resource. Different bee species with different proboscis lengths can feed on different flower depths. Darwin’s finches are another helpful example: different beak shapes allow use of different seed sizes.

The ecological benefit is coexistence. Instead of one species eliminating another, species divide resource use enough that both can persist.

This helps explain why diverse communities can exist in limited space. A forest, reef, or wetland can support many species not because competition is absent, but because niches are differentiated.

A subtle but important point: resource partitioning may arise through behavior, evolution, or both. Species can adjust use patterns in the short term, and over longer time scales natural selection can favor traits that reduce overlap.

That is why partitioning connects ecology and evolution.

### Applying the ideas to AP-style scenarios

AP questions rarely ask only for a definition. More often, they give a situation and ask you to infer the interaction.

Suppose two bird species nest in the same woodland. Species 1 feeds on large caterpillars in the canopy, while species 2 feeds on small beetles in the understory. Even though they share habitat, competition is limited because their niches differ in food and space. This supports coexistence through resource partitioning.

Suppose a drought reduces seed availability in a grassland. Two rodent species that previously fed on overlapping seed types now face a stronger limiting resource. Competition would increase because the same food becomes scarcer. If one species begins feeding mostly at night near shrubs while the other feeds by day in open ground, that is temporal plus spatial partitioning.

Suppose an FRQ says a plant species can tolerate a wide range of soil moisture in greenhouse conditions, but in the field it occurs only in wetter microsites because a competitor dominates drier ground. The greenhouse range reflects the **fundamental niche**. The field range reflects the **realized niche**. The restriction is due to interspecific competition.

Suppose a question asks why closely related species are found in different parts of the same habitat. The best explanation is usually not “because they prefer different places” in a vague sense. The stronger answer is that resource partitioning reduces niche overlap and direct competition.

These distinctions are what earn marks.

## Key Point Checklist

_This article has covered the following key knowledge points:_

- Competition occurs when organisms seek the same limited resource
- Resources that may drive competition include food, water, light, space, and nesting sites
- Intraspecific competition occurs within the same species
- Interspecific competition occurs between different species
- An ecological niche includes a species’ role, resource use, tolerated conditions, and interactions
- A fundamental niche is the full potential niche in the absence of limiting biotic interactions
- A realized niche is the actual niche occupied after competition, predation, and other biotic pressures
- Competition often narrows a species’ realized niche compared with its fundamental niche
- The competitive exclusion principle states that two species cannot stably occupy the exact same niche under the same conditions
- Resource partitioning reduces direct competition by dividing resources through space, time, or method of use
- Spatial, temporal, and morphological differences can allow similar species to coexist
- Resource partitioning promotes coexistence, while competitive exclusion predicts local elimination or niche shift when overlap is too high

## Key Terms and Concepts

- competition
- limiting resource
- intraspecific competition
- interspecific competition
- ecological niche
- fundamental niche
- realized niche
- competitive exclusion principle
- Gause’s principle
- resource partitioning


Introduction to Ecosystems - Competition and resource partitioning