AP Environmental Science

## Learning Outcomes

- Define **biodiversity**, **species richness**, **species evenness**, and species diversity accurately
- Explain why species richness is only one measure of biodiversity
- Understand how disturbance affects species richness and habitat recovery
- Compare **primary succession** and **secondary succession** and explain why recovery rates differ
- Describe how pioneer species help natural areass recover after disturbance
- Evaluate why returning species counts do not always mean full ecological balance recovery
- Apply these ideas to AP Environmental Science multiple-choice and free-response questions

## AP Environmental Science Syllabus

For the AP Environmental Science, you are expected to understand species richness and recovery after disturbance, with a focus on the following syllabus points:

- Know that **species richness** means the number of different species in an area
- Identify the difference between species richness, species evenness, and overall biodiversity
- Explain how ecological disturbances such as fire, flood, storms, farming, or logging affect communities
- Understand how **ecological succession** changes species composition over time
- Explain the difference between **primary succession** and **secondary succession**
- Understand why secondary succession usually leads to faster recovery than primary succession
- Describe the role of **pioneer species** in early succession
- Recognise that ecological recovery recovery involves species composition and resilience, not just species count

## 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 biodiversity and species richness?
2. How can two communities have the same species richness but different biodiversity?
3. Why does secondary succession usually happen faster than primary succession?
4. How does a disturbance such as wildfire affect species richness over time?
5. Why does a return to the original number of species not always mean full recovery?

## Introduction

Species richness and recovery are linked ideas in AP Environmental Science. Species richness tells you how many different species are present in a community. Recovery describes how an community responds after disruption, usually through succession and the rebuilding of populations, interactions, and energy flow.

These ideas matter because AP questions often ask you to interpret disturbed habitatss over time. You may need to explain why one area recovers faster, why species numbers change during succession, or why a graph showing more species does not automatically prove that the environment is “back to normal.”

> **Key Term: biodiversity**
> The variety of life in an area, including variation within species, between species, and among habitatss.

### Species richness, evenness, and biodiversity

Students often lose marks here by using these terms as if they mean the same thing. They do not.

**Species richness** is simply the number of different species present in a given area. If a forest has oak trees, pines, squirrels, hawks, fungi, and beetles, each different kind adds to the richness. AP Environmental Science treats richness as a species count.

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

But richness does not tell you how individuals are distributed among those species. That is where **species evenness** matters. If one species makes up almost all individuals and the others are rare, evenness is low. If individuals are spread more evenly across species, evenness is high.

> **Key Term: species evenness**
> The relative abundance of individuals among the species present in a community.

This leads to a bigger point: **species diversity** is broader than richness because it includes both richness and evenness. Two communities can each contain 10 species, but one may still be more diverse if none of its species dominates strongly.

A common exam setup is this:

- Forest 1 has 10 species, but 90% of all individuals belong to one species.
- Forest 2 has 10 species, and individuals are spread fairly evenly.

Both forests have the same **species richness**. Forest 2 has higher **evenness**, so it has higher overall diversity.

This distinction matters because natural systemss with more balanced communities often have more alternative pathways in food webs. If one species declines, another may partly fill its role. That tends to improve resilience.

> **Test Tip:** If an AP question says “number of species,” think **species richness**. If it gives counts of individuals in each species, check whether the real issue is **evenness** or full diversity, not just richness.

### Why species richness often supports recovery

In general, natural systemss with greater species richness tend to recover more effectively from disruption. The reason is not magic; it is redundancy and variation.

Imagine a grassland with many plant species. Some tolerate drought better. Some regrow quickly after grazing. Some survive fire through underground roots. If one species declines after a disturbance, others may continue photosynthesis, hold soil in place, or support herbivores. The grassland system still functions.

By contrast, if a community has very few species, losing one species has a much larger effect. Fewer species means fewer feeding links, fewer nutrient pathways, and fewer ways to respond to changing conditions.

Recovery is also connected to genetic variation. A population with greater genetic diversity is more likely to include individuals able to tolerate a changed environment. This helps populations rebound after disease, drought, or temperature shifts.

Still, be careful. Higher richness does not guarantee perfect recovery. It increases the chance that ecological processes continue, but recovery also depends on which species survive, how severe the disturbance was, and whether habitat conditions remain suitable.

> **Key Term: environmental system resilience**
> The ability of an system to recover after disturbance and return toward its previous structure or function.

A useful chain of reasoning for exam answers is:

1. Higher species richness creates more ecological roles and food-web links.
2. This gives the system more than one pathway for energy flow and nutrient cycling.
3. If one species declines, another may partly replace its function.
4. Therefore the environmental system is often more resilient after disturbance.

That is analysis, not just description.

### Disturbance, succession, and recovery

A **disturbance** is an event that disrupts community structure structure by removing organisms or changing conditions. Examples include wildfire, flood, hurricane, drought, logging, farming, and volcanic activity.

> **Key Term: disturbance**
> An event that alters community structure by removing organisms or changing environmental conditions.

Disturbance usually lowers biomass immediately and may reduce species richness at first. But it can also create opportunities. Open ground, more sunlight, reduced competition, and newly available nutrients allow some species to establish. That is why recovery is best understood through **ecological succession**.

> **Key Term: ecological succession**
> The gradual change in a community’s species composition over time, often after disturbance or on newly exposed land.

Succession is not random replacement. It tends to follow a sequence because early species change the environment in ways that affect later species. In early stages, species that tolerate harsh conditions arrive first. Later, soil deepens, shade increases, nutrients cycle more efficiently, and larger or slower-growing species can establish.

**Pioneer species** begin this process. In primary succession, common textbook examples are lichens and mosses. In secondary succession, pioneer species may be grasses, weeds, or other fast-growing plants.

> **Key Term: pioneer species**
> The first species to colonize a barren or disturbed area during succession.

Why do pioneer species arrive first? Because they can handle stressful conditions. They often grow quickly, reproduce fast, and disperse widely. They do not need rich soil or stable conditions. In APES terms, they are usually more generalist-like than later species.

As succession continues, species richness often increases. Early stages may have few species because conditions are harsh. Mid and later stages often support more species because there are more niches, more plant structure, and more food sources. However, the rate of increase usually slows over time.

A classic sequence after disturbance might be:

- bare or disturbed ground
- grasses and herbaceous plants
- shrubs
- young trees
- larger, later-successional trees

At each step, the community changes the abiotic environment. Grasses reduce erosion and add organic matter. Shrubs create shade and habitat. Trees alter moisture, light, and nutrient storage. So recovery is not just “things growing back.” It is a chain of ecological change.

### Primary vs secondary succession

This is one of the highest-yield distinctions in the topic.

**Primary succession** begins where no soil exists. Examples include bare rock exposed by glacier retreat or fresh lava surfaces after volcanic activity. Recovery is slow because soil must first form.

**Secondary succession** begins after disturbance when soil remains. Examples include abandoned farmland, logged forest, or land after wildfire if the soil is still intact.

> **Key Term: primary succession**
> Ecological succession that begins on a surface without soil, such as bare rock or fresh lava.
>
> **Key Term: secondary succession**
> Ecological succession that occurs after disturbance when soil remains in place.

Why is secondary succession usually faster?

Because the disturbance did not erase everything. Secondary succession often begins with:

- existing soil
- stored nutrients
- microbes and decomposers
- seed bank in the soil
- roots, bulbs, or underground stems
- nearby surviving plants and animal populations

That means recolonization does not start from zero. In primary succession, by contrast, weathering and organic matter accumulation must first create soil, and that can take a very long time.

Here is the comparison students need to be able to state clearly:

- **Primary succession:** no soil, slower, starts with pioneer species able to colonize bare substrate
- **Secondary succession:** soil remains, faster, starts with surviving seeds, roots, and soil organisms

A common mistake is calling any severe disturbance primary succession. Severity alone is not the key test. The key question is: **Is soil still present?**

> **Test Tip:** On the exam, classify succession by checking the soil first. Soil present = **secondary succession**. No soil = **primary succession**.

Consider two examples:

1. A glacier retreats and leaves bare rock. Lichens colonize first. This is primary succession because no soil exists.
2. A forest burns, but the soil remains. Grasses return the next spring. This is secondary succession because recolonization begins from existing soil.

Even if a fire kills many organisms, it is still usually secondary succession if the soil survives.

### Recovery is more than getting the same number of species back

This is where stronger AP answers stand out. A recovering environment may regain its former species richness but still not be fully recovered.

Why? Because richness counts species, but does not identify them.

Suppose a forest had 20 species before logging:

- 8 tree species
- 4 bird species
- 4 insect species
- 4 fungi species

Years later, it again contains 20 species. That sounds like full recovery. But what if many original specialists are gone and have been replaced by generalist weeds, edge species, and invasive plants? The number of species is the same, yet the community is different.

This is called a change in **species composition**. Recovery should consider not just how many species are present, but which species are present and what ecological roles they perform.

Specialist species may be slow to return because they need particular conditions. Generalists often come back first because they tolerate a wider range of environments. That means species richness can rebound while the original community structure remains altered.

A student might say:

“Species richness has returned to pre-disturbance levels, so the environment has fully recovered.”

That is too simplistic.

A better AP-style judgement is:

“Species richness has increased, which suggests recovery is underway. However, full recovery cannot be assumed without evidence that the original species composition and ecological functions have also returned.”

That is balanced and evidence-based.

### Intermediate disturbance and exam judgement

A broader ecological idea worth knowing is that species richness may be highest at intermediate levels of disturbance. Very little disturbance allows dominant species to exclude weaker competitors. Too much disturbance wipes many species out. Intermediate disturbance can allow both early and later successional species to coexist.

This is often called the **intermediate disturbance hypothesis**.

> **Key Term: intermediate disturbance hypothesis**
> The idea that species richness may be highest at intermediate levels of disturbance, where neither competitive exclusion nor severe destruction dominates.

Use this carefully. It is a helpful concept, but not a rule for every case. If you mention it in an answer, present it as a possible explanation, not as an automatic law.

For example:

- low disturbance: dominant species may outcompete others, lowering richness
- high disturbance: few species can survive, lowering richness
- intermediate disturbance: both colonizers and later species may coexist, increasing richness

This can help explain graph questions where richness rises after some disturbance but falls when disturbance becomes extreme.

## Key Point Checklist

_This article has covered the following key knowledge points:_

- Biodiversity includes variation within species, between species, and among habitatss
- Species richness means the number of different species in a community
- Species evenness describes how evenly individuals are distributed among species
- Two communities can have the same species richness but different diversity because evenness differs
- Communitiess with greater species richness often recover more effectively from disturbance
- Disturbance can lower biomass and species richness at first but also create conditions for succession
- Ecological succession is the gradual change in species composition over time
- Pioneer species are the first colonizers in disturbed or barren areas
- Primary succession begins without soil and is usually slow
- Secondary succession begins with soil still present and is usually faster
- Secondary succession is faster because soil, seeds, roots, microbes, and nearby colonists remain
- Full ecological recovery recovery cannot be judged by species richness alone because species composition and function also matter

## Key Terms and Concepts

- biodiversity
- species richness
- species evenness
- environmental system resilience
- disturbance
- ecological succession
- pioneer species
- primary succession
- secondary succession
- intermediate disturbance hypothesis


Introduction to Biodiversity - Species richness and recovery