AP Environmental Science

## Learning Outcomes

- Define **species richness** precisely as the number of different species in an area or community
- Distinguish species richness from **species evenness**, **species diversity**, and **biodiversity**
- Explain why species richness is only one measure of biodiversity, not the whole idea
- Compare communities that have the same richness but different overall diversity
- Apply species richness to island biogeography examples involving island size and distance
- Avoid common AP Environmental Science errors, especially confusing richness with abundance

## AP Environmental Science Syllabus

For the AP Environmental Science, you are expected to understand species richness as a measure of biodiversity, with a focus on the following syllabus points:

- Know that **species richness** means the number of different species present in a given area or community
- Identify that species richness counts species, not the number of individual organisms
- Explain the difference between **species richness** and **species evenness**
- Understand that **species diversity** is broader than species richness and often includes both richness and evenness
- Compare two communities that have the same species richness but different relative abundances
- Apply species richness to ecological systems resilience and recovery from disturbance
- Understand that island biogeography uses species richness as the outcome affected by immigration and extinction
- Know that larger, less isolated islands generally have higher species richness than small, remote islands

## 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 species richness in one sentence?
2. Why can two habitatss have the same species richness but different overall diversity?
3. Does species richness include the number of individuals of each species?
4. How is species richness connected to island biogeography?
5. Why are natural communitiess with higher species richness often better able to recover from disturbance?

## Introduction

## Introduction

Biodiversity is one of the most tested ideas in AP Environmental Science because it connects ecology, resilience, conservation, and human impacts. Within that topic, **species richness** is one of the most basic but most often confused terms. Students often remember that it has something to do with “more species,” but miss the exact definition and mix it up with evenness, abundance, or biodiversity as a whole.

This subtopic matters because AP questions often test fine distinctions. A question may show two communities with the same number of species but different numbers of individuals in each species. If you do not separate richness from evenness, you can pick the wrong answer even if you understand the general idea of biodiversity.

> **Key Term: species richness**
> The number of different species present in a given area, habitat, or community.

### What species richness means and what it does not mean

Species richness is a **count of species**, not a count of organisms. If a forest contains oak trees, pine trees, mosses, ferns, squirrels, hawks, and fungi species, richness asks only one question: how many different species are there?

If there are 12 distinct species in a pond, the species richness is **12**. It does not matter whether one of those species has 5 individuals or 5,000 individuals. As long as the species is present, it adds one to the richness count.

This is why species richness is simple but limited. It is useful because it gives a quick way to compare how many kinds of organisms are present in different places. But it leaves out a lot of information about the structure of the community.

For example:

- Community A has 5 species, each with 20 individuals
- Community B has 5 species, but one species has 96 individuals and the other 4 species have 1 individual each

Both communities have the same species richness: **5 species**. But they do not have the same ecological balance.

> **Key Term: community**
> Populations of different species living in the same area.

A common mistake is to confuse “many organisms” with “many species.” A grass field with 20,000 individual plants of one species has very low species richness if only one species is present. By contrast, a small woodland with fewer total organisms but 18 different species has higher richness.

Another common mistake is to use species richness as if it means the same thing as biodiversity in every case. On the AP exam, that is too vague. Species richness is **one measure** of biodiversity, but biodiversity is broader.

> **Test Tip:** If an AP question asks for species richness, ignore the total number of individuals at first. Count only the number of different species present.

### Species richness, species evenness, and species diversity

Students lose marks here more than anywhere else in this subtopic. You need a clean distinction.

Species richness asks:

- **How many species are present?**

Species evenness asks:

- **How evenly are individuals distributed among those species?**

Species diversity usually combines both ideas:

- **How many species are there, and how balanced are their population sizes?**
  > **Key Term: species evenness**
  > A measure of how evenly individuals are distributed among the species present in a community.
  >
  > **Key Term: species diversity**
  > A broader measure of biodiversity that includes both the number of species present and how evenly individuals are distributed among them.

Here is the key reasoning.

Imagine two forests.

Forest 1:

- 10 species of trees
- 60 total trees
- 6 trees of each species

Forest 2:

- 10 species of trees
- 60 total trees
- 40 trees of one species, and the remaining 20 trees spread across the other 9 species

Both forests have the same **species richness** because both have 10 species.

But Forest 1 has higher **species evenness** because no single species dominates.

Because Forest 1 has the same richness and greater evenness, it has higher **species diversity** overall.

This is a favorite AP style distinction. The exam may show a table, graph, or diagram and ask which community is more diverse. Students who look only at richness may say the communities are equal. But if the evenness differs, the more balanced community is more diverse even when richness is identical.

You can think of the terms this way:

- **Richness** = number of species
- **Evenness** = balance among species
- **Diversity** = richness plus balance

Biodiversity is broader still, because AP Environmental Science also includes:

- **genetic diversity**
- **species diversity**
- **environmental diversity**

So species richness sits inside species diversity, and species diversity sits inside biodiversity.

> **Key Term: biodiversity**
> The variety of life in an area, including variation within species, between species, and among habitatss.
>
> **Test Tip:** When two communities have the same number of species, do not stop there. Check whether one species dominates. If it does, richness may be equal but diversity is not.

### Why species richness matters for ecological system stability and resilience

Species richness matters because communitys with more species often have more ways to keep functioning when conditions change. This does not mean every species-rich community is automatically safe, but it does mean higher richness often increases resilience.

Why?

First, more species usually means more ecological roles and more possible responses to stress. If drought, fire, disease, or pollution harms one species, another species may still perform a similar role in the food web or nutrient cycle.

Second, species-rich systems tend to have more complex food webs. In a simple food chain, the loss of one species can break the whole transfer of energy. In a more complex food web, organisms may have alternative food sources or ecological pathways.

Third, richness often reflects broader environmental suitability. Places with many species often also have greater habitat variety, more stable conditions, and more available niches.

A useful AP example is recovery after disturbance. communitiess with higher species richness often recover more rapidly from natural disruption because there are more species available to recolonize space, recycle nutrients, or rebuild food-web links.

This does not mean richness is the only reason recovery happens. Other factors matter too:

- intensity of the disturbance
- frequency of disturbance
- soil condition
- climate
- presence of surviving organisms
- degree of habitat fragmentation

But richness is still a major factor.

Suppose a grassland has only a few plant species. If drought wipes out one dominant producer, herbivores may lose most of their food supply. If the same drought hits a grassland with many producer species, some drought-tolerant species may remain, so the whole system is less likely to collapse.

This is one reason tropical those regionss, which often have high species richness, can support very complex food webs. It is also why communitiess closer to the poles, where richness is often lower, may be less resilient to some disruptions.

Species richness also links to succession. As succession proceeds after disturbance, the number of species present often increases over time, especially from early stages toward later stages. This does not happen forever without limit, but richness often rises as the community becomes more established.

### Species richness in comparisons, data questions, and worked examples

On the AP exam, species richness is often tested through comparison.

The method is simple:

1. Define the area or community being measured
2. Count the number of different species present
3. Do not count repeated individuals more than once
4. Compare totals across communities

Worked example:

Community X contains:

- 14 robins
- 9 sparrows
- 6 frogs
- 2 snakes
- 20 beetles

Community Y contains:

- 45 robins
- 1 sparrow
- 1 frog
- 1 snake
- 1 beetle

Step 1: Count species in Community X

- robin
- sparrow
- frog
- snake
- beetle

Species richness of Community X = **5**

Step 2: Count species in Community Y

- robin
- sparrow
- frog
- snake
- beetle

Species richness of Community Y = **5**

So the two communities have the same richness.

But they do not have the same evenness. Community X is much more balanced. Therefore:

- same richness
- Community X has higher evenness
- Community X has higher overall diversity

Now consider another example.

Community M contains:

- oak
- pine
- fern
- moss
- lichen
- squirrel
- owl

Species richness = **7**

Community N contains:

- oak
- pine
- fern
- moss

Species richness = **4**

Here, Community M has greater species richness.

Notice how easy it is to score the richness part correctly if you stay disciplined and count only distinct species.

Common student errors:

- counting individuals instead of species
- confusing richness with abundance
- assuming equal richness means equal biodiversity
- using “biodiversity” when the question specifically asks for richness

### Species richness and island biogeography

Species richness is also central to **island biogeography**, because the theory tries to explain how many species an island or isolated habitat can support.

> **Key Term: island biogeography**
> The study of species richness on islands or other isolated habitats, based mainly on immigration and extinction rates.

In this context, species richness is the outcome being explained. The central idea is that the number of species on an island depends on a balance between:

- **immigration rate**: new species arriving
- **extinction rate**: species disappearing from the island

A nearby island usually has higher immigration because organisms can reach it more easily. A distant island usually has lower immigration.

A large island usually has lower extinction because it contains:

- more space
- more resources
- larger populations
- more habitat variety

A small island usually has higher extinction because populations are smaller and more vulnerable to chance events.

So the classic AP conclusion is:

- **highest species richness** = large island close to mainland
- **lowest species richness** = small island far from mainland

This also applies beyond literal islands. In AP Environmental Science, an “island” can mean any isolated habitat, such as:

- a forest fragment surrounded by roads
- a mountaintop
- an isolated lake
- a patch of grassland surrounded by urban development

This matters because habitat fragmentation creates island-like conditions on land. When habitats are broken apart, immigration drops and extinction risk rises, especially for specialist species and species requiring large territories. As a result, species richness often falls.

You can see the chain clearly:

habitat fragmentation → greater isolation and smaller habitat patches → lower immigration and higher extinction → lower species richness

This is one of the most useful applications of the concept.

> **Test Tip:** In island biogeography questions, identify what affects immigration and what affects extinction. Nearness raises immigration; larger size lowers extinction.

## Key Point Checklist

_This article has covered the following key knowledge points:_

- Species richness means the number of different species in a given area or community
- Species richness does not include the number of individuals of each species
- A community with many organisms can still have low species richness
- Species richness is one measure of biodiversity, not the whole concept
- Species evenness describes how evenly individuals are distributed among species
- Two communities can have the same species richness but different species diversity if their evenness differs
- A more balanced community has higher evenness and usually higher overall diversity
- Species-rich communitiess are often more resilient because they have more ecological roles and alternative pathways
- Species richness is often greater closer to the equator than near the poles
- In island biogeography, species richness depends on immigration and extinction
- Large islands close to a mainland usually have higher species richness than small, remote islands
- Habitat fragmentation can reduce species richness by creating smaller, more isolated habitat patches

## Key Terms and Concepts

- species richness
- community
- species evenness
- species diversity
- biodiversity
- island biogeography


Introduction to Biodiversity - Species richness definition