AP Environmental Science

## Learning Outcomes

- Explain how ecological succession changes communities over time
- Distinguish clearly between primary succession and secondary succession
- Describe how biodiversity, biomass, soil quality, and nutrient cycling change through succession
- Explain why net primary productivity changes differently from biomass
- Apply succession trends to diagrams, examples, and AP Environmental Science questions
- Predict habitat recovery after disturbance using ecological evidence

## AP Environmental Science Syllabus

For the AP Environmental Science, you are expected to understand succession effects over time, with a focus on the following syllabus points:

- Know that **ecological succession** is the predictable change in a community’s species composition over time
- Identify the difference between **primary succession** and **secondary succession**, especially the presence or absence of soil
- Explain how **pioneer species** modify abiotic conditions and make later stages possible
- Understand the usual trend of increasing **biomass**, increasing **soil depth and organic matter**, and increasing **biodiversity** over time
- Explain why **secondary succession** is usually faster than primary succession
- Understand that **net primary productivity (NPP)** is often low at first, rises during early to mid-succession, and may level off or decline in mature systems
- Recognise that a **climax community** is relatively stable, not permanent
- Apply succession concepts to examples such as glacial retreat, lava flow, wildfire, abandoned farmland, and storms

## 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 main difference between primary succession and secondary succession?
2. Why does biomass usually increase as succession proceeds?
3. How do pioneer species help later species establish?
4. Why does net primary productivity not simply increase forever?
5. Name one reason secondary succession is usually faster than primary succession.

## Introduction

Ecological succession is one of the clearest examples of ecological community change over time. AP Environmental Science often tests it through diagrams, disturbance scenarios, and trend questions about biodiversity, biomass, soil, and productivity. To answer these questions well, you need more than the definitions. You need to see the pattern of change and understand why each stage leads to the next.

Succession matters because natural systemss are not fixed. Fire, storms, glacial retreat, lava flows, and human land use can all reset communities. Once that happens, species return in a sequence, and the physical environment changes with them. Over time, the system usually becomes more complex, more stable, and richer in stored organic matter.

> **Key Term: ecological succession**
> The gradual, predictable change in a community’s species composition over time, especially after new land forms or a disturbance alters an habitat.

### Primary and secondary succession: what starts the process

The first distinction you must know is whether succession begins with soil already present.

> **Key Term: primary succession**
> Succession that begins on a surface with no soil, such as bare rock exposed by glacial retreat or fresh lava.
>
> **Key Term: secondary succession**
> Succession that begins after a disturbance in an area where soil remains, such as after wildfire, storm damage, or abandoned farmland.

This distinction matters because it controls the speed of recovery. In **primary succession**, organisms must first create or accumulate soil before most plants can grow. In **secondary succession**, the area does not start from zero. Soil, nutrients, microbes, buried seeds, and root systems may already be present.

That is why a post-fire forest can begin with grasses and herbs within a short time, while land exposed by retreating ice may take far longer to develop a mature plant community.

A common AP mistake is to classify any dramatic disturbance as primary succession. That is wrong. A wildfire may destroy vegetation, but if soil remains, it is **secondary succession**. A lava flow or newly exposed glacial rock is **primary succession** because there is no developed soil at the start.

A second mistake is to assume primary succession always means “nothing at all can live there.” In reality, some hardy organisms can colonize very harsh surfaces first.

> **Key Term: pioneer species**
> The first organisms to colonize a newly exposed or disturbed area, usually hardy species able to tolerate harsh conditions.

In primary succession, pioneer species often include lichens and mosses. In secondary succession, pioneer species are more often grasses, herbs, weeds, and plants regrowing from surviving roots.

Why are pioneer species so important? Because they do not just survive the environment; they change it. Lichens can chemically weather rock. Mosses trap dust and moisture. When pioneer organisms die, decomposers break them down and add organic matter. This begins soil formation. Once a thin soil layer exists, other plants can establish more easily.

This is a cause-and-effect chain AP questions like to test:

- bare rock or disturbed land appears
- pioneer species colonize
- abiotic conditions become less harsh
- soil depth and organic matter increase
- more plant species establish
- food and habitat for animals increase
- the community becomes more complex
  > **Test Tip:** If a diagram shows bare rock with lichens or mosses appearing first, the answer is almost always **primary succession**. If it shows grasses regrowing after a fire with soil already present, it is **secondary succession**.

### How succession changes biodiversity, biomass, and soil over time

The easiest trend to remember is that most communitiess become more complex over time during succession. But AP questions often separate that big idea into specific measurable trends.

#### Biodiversity over time

In early succession, biodiversity is usually low. Few species can tolerate exposed rock, intense sun, little soil, low water retention, or unstable conditions. As soil develops and vegetation structure becomes more varied, more niches appear.

For example, a patch of bare rock may initially support only lichens and a few microbes. Later, grasses can root there. Then shrubs create shade, trap leaf litter, and provide nesting sites. Trees create canopy, understory, bark surfaces, and litter layers. Each new habitat feature allows more species to live there.

So biodiversity generally increases from early succession to mid and later stages. In many AP descriptions, it levels off in a mature community. Some sources note it may dip slightly at the latest stage if a few dominant species outcompete others, but for AP purposes the key pattern is that biodiversity rises overall as succession proceeds.

#### Biomass over time

> **Key Term: biomass**
> The total mass of living organic matter in a given area.

Biomass usually increases over time. Early succession has small plants, sparse cover, and little stored organic material. Later succession includes shrubs, trees, deeper roots, more stems, more leaves, and more total living tissue.

This is different from biodiversity. A young field might gain species quickly but still have less biomass than a mature forest. Students sometimes confuse “more species” with “more total mass.” They are related, but not the same.

By late succession, especially in forests, much of the community biomass’s biomass is locked in trunks, branches, roots, and accumulated organic matter. This helps explain why later successional systems often appear more stable: they have more stored material and more complex food webs.

#### Soil development over time

Soil changes may be the most dramatic trend in primary succession. At the start, there is little or no soil. Over time:

- rock weathers into smaller particles
- dead organisms add organic matter
- humus accumulates
- water-holding capacity improves
- nutrient retention improves
- decomposer communities become more developed

As a result, the soil becomes deeper and more fertile. In secondary succession, soil starts at a much better point, so this change is less dramatic but still important. Disturbed soil can still gain organic matter, improve structure, and support more efficient nutrient cycling as vegetation recovers.

This matters because improving soil changes everything else. Better soil means more water retention, better root anchorage, more nutrient availability, and more microbial activity. That supports larger plants, which then support more consumers.

A good AP way to think about succession is that living organisms and abiotic conditions build on each other. Plants improve the soil, and improved soil allows more plants.

### Why NPP changes differently from biomass

This is the part students most often oversimplify.

> **Key Term: net primary productivity (NPP)**
> The rate at which producers store energy as biomass after subtracting the energy they use in respiration.

The formula is:

NPP = GPP − R

where **GPP** is gross primary productivity and **R** is respiration by producers.

Many students assume that if biomass rises, NPP must also keep rising. That is not always true.

In the earliest stage of succession, there are few producers, so total productivity is limited. NPP is often low because very little plant matter exists yet. As plants establish and spread, productivity rises quickly. Early and mid-succession often show rapid growth because plants are adding new tissue fast.

Later, however, mature communitiess contain a lot of biomass that must be maintained. Large trees and other long-lived plants use more energy for respiration and maintenance. Even if gross productivity stays high, respiration also rises. That means NPP may level off or decline somewhat.

So the typical AP trend is:

- **early stage:** low NPP at first because producer biomass is small
- **early to mid-succession:** NPP rises as plant growth is rapid
- **late succession:** NPP may level off or decrease because respiration and maintenance costs are high

Here is a worked example using simple numbers.

Suppose an early successional community has:

- GPP = 500 units
- R = 150 units

Then:

NPP = 500 − 150 = 350 units

Now suppose a mature forest has:

- GPP = 900 units
- R = 820 units

Then:

NPP = 900 − 820 = 80 units

The mature forest has much more biomass, but it is adding new biomass more slowly because so much energy is used just to maintain what already exists.

This is why AP questions may ask which stage has the highest biomass and which has the highest NPP, and the answers may be different.

> **Test Tip:** Do not confuse **biomass** with **NPP**. Biomass is the amount of living matter present. NPP is the rate at which new biomass is added.

### Later succession, climax communities, and stability after disturbance

As succession continues, species composition shifts from fast-growing, stress-tolerant species toward larger, longer-lived species. In many terrestrial examples, the pattern is:

- pioneer species
- grasses and herbs
- shrubs
- young trees
- mature forest species

This does not mean every habitat area ends as a forest. The final community depends on climate, soil, water, and local conditions. But the broad AP pattern is that later stages usually have:

- higher biomass
- greater structural complexity
- more developed soils
- more complex nutrient cycling
- higher biodiversity than the earliest stages
- greater resistance to stress
  > **Key Term: climax community**
  > A relatively stable later-stage community whose species composition remains fairly constant until another disturbance or major environmental change occurs.

The word “stable” needs care. A climax community is not permanent. Disturbance can restart succession. Fire, logging, storms, drought, invasive species, or climate change can all push the system back to earlier stages.

This is especially important for APES because natural systemss are dynamic. Stability in this context means relative balance, not no change.

Later successional communities are often more resistant because they have more species, more niches, and more pathways for energy flow. If one species declines, another may partly fill its role. Early successional systems are usually less buffered. If only a few species are present, losing one matters more.

However, recovery rate can be slower in late successional communities. A mature forest may be stable under ordinary conditions, but once seriously disturbed it may take a long time to rebuild.

This is one place where AP questions may ask you to compare **stability** and **recovery speed**:

- early succession: less stable, but often faster recovery
- late succession: more stable, but slower recovery if badly disturbed

A wildfire example helps. After fire in a secondary succession system:

- soil remains
- buried seeds may survive
- nutrients in ash may be available
- grasses and herbaceous plants return quickly
- shrubs follow
- young trees establish
- mature forest species return more slowly

By contrast, after glacial retreat:

- no developed soil exists
- lichens and mosses arrive first
- soil formation takes much longer
- succession proceeds much more slowly overall

## Key Point Checklist

_This article has covered the following key knowledge points:_

- Ecological succession is the predictable change in community composition over time
- Primary succession begins where no soil is present, such as after glacial retreat or lava flow
- Secondary succession begins after disturbance where soil remains, such as after wildfire or abandoned farming
- Pioneer species are the first colonizers and help change abiotic conditions
- In primary succession, lichens and mosses often help weather rock and begin soil formation
- Biodiversity generally increases through succession as more niches and habitat complexity develop
- Biomass generally increases over time as vegetation becomes larger and more structurally complex
- Soil depth, organic matter, nutrient retention, and water-holding capacity generally increase over time
- Secondary succession is usually faster because soil, seeds, roots, microbes, and nutrients may remain
- Net primary productivity often rises during early to mid-succession, then may level off or decline in mature communities
- Biomass and NPP are not the same; biomass is an amount, while NPP is a rate
- A climax community is relatively stable, but disturbance can restart succession

## Key Terms and Concepts

- ecological succession
- primary succession
- secondary succession
- pioneer species
- biomass
- net primary productivity (NPP)
- climax community


Ecological Succession - Succession effects over time