AP Environmental Science

## Learning Outcomes

- Explain how natural upheaval changes habitats by altering soil, water, vegetation, and species composition
- Distinguish clearly between **primary succession** and **secondary succession**
- Identify how volcanic eruptions, glacial retreat, wildfires, floods, hurricanes, and landslides affect natural systemss
- Understand the role of **pioneer species** in early habitat recovery
- Explain how **disturbance regime**, **resistance**, and **resilience** affect 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 habitat change caused by natural disruption, with a focus on the following syllabus points:

- Know that natural disturbances can alter ecological communities structure, habitat availability, and biodiversity
- Identify the difference between **primary succession** and **secondary succession**, especially by whether soil remains
- Explain how volcanic eruptions and glacial retreat often create conditions for primary succession
- Understand how wildfires, floods, hurricanes, and many landslides usually lead to secondary succession if soil remains
- Know the role of **pioneer species** such as lichens, mosses, grasses, and herbs in habitat recovery
- Explain how disturbance can increase erosion, sedimentation, nutrient movement, and species displacement
- Understand the meaning of **resistance**, **resilience**, and **disturbance regime**
- Recognise that some natural communitiess are adapted to periodic fire or flooding and may depend on them

## 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 most important difference between primary and secondary succession?
2. How can a volcanic eruption destroy habitat and also create new habitat?
3. Name two examples of pioneer species and explain what they do.
4. Why do many wildfires lead to secondary succession rather than primary succession?
5. What is the difference between ecological stability resistance and resilience?

## Introduction

Natural disruptions are not just disasters from a human point of view. In ecology, they are events that change the physical environment enough to alter which organisms can live there. Habitat change from upheaval can happen when fire removes vegetation, floods drown roots and move sediment, hurricanes strip coastlines and forests, or lava and glaciers expose bare rock.

This matters for AP Environmental Science because exam questions often ask you to link disturbance to succession, biodiversity, erosion, nutrient cycling, and recovery. The key is to track both the **immediate damage** and the **longer-term ecological response**.

> **Key Term: disturbance**
> An event or force that causes mortality, alters habitat conditions, and changes the structure of an biological community.

### How upheaval changes habitat

A natural disruption changes habitat by changing abiotic conditions first. That is the core idea students sometimes miss. Species composition changes because the physical environment changes.

For example, after a wildfire, the first obvious effect is loss of vegetation. But the deeper ecological effect is that shade decreases, soil temperature changes, more light reaches the ground, and ash may add mineral nutrients to surface soil. That new set of conditions favors different species from the ones that dominated before the fire.

Floods show the same pattern. Fast-moving water can erode topsoil, uproot plants, destroy nests and burrows, and carry nutrients away. At the same time, flooding on floodplains can deposit fresh, nutrient-rich sediment. So a flood may reduce biodiversity in the short term but improve soil fertility in some places afterward.

Volcanic eruptions can be even more dramatic. Lava flows and ash can kill organisms and bury existing communities. But once rock and ash begin to weather, they can eventually support soil formation. That means a severe disruption can erase one habitat and begin the formation of another.

Hurricanes change habitats in both terrestrial and coastal systems. Strong winds remove branches, uproot trees, and open the canopy. Storm surge can push seawater inland, causing **saltwater intrusion** into freshwater or low-salinity habitats. That changes which plants can survive, especially in marshes, estuaries, and coastal forests.

> **Key Term: habitat change**
> A shift in environmental conditions that alters which species can survive, reproduce, and compete in an area.

A useful APES way to think about upheaval is this chain:

- disturbance occurs
- abiotic conditions change
- some species die or leave
- new species colonize or recover
- community structure shifts over time

That is why habitat change and succession are tied together.

> **Test Tip:** In FRQs, do not stop at “the disturbance killed organisms.” Add the next step: explain how soil, light, water, nutrients, or temperature changed, and then link that to species change.

### Primary and secondary succession after disturbance

The single most testable rule in this topic is simple: **primary succession means no soil; secondary succession means soil remains**.

> **Key Term: primary succession**
> Ecological succession that begins on bare substrate where no soil is present.
>
> **Key Term: secondary succession**
> Ecological succession that occurs after a disturbance when soil remains.

Students often confuse “severe disturbance” with primary succession. Severity is not the deciding factor. Soil presence is.

If a glacier retreats and leaves bare rock, there is no soil. That starts primary succession. If lava hardens into fresh rock, there is no soil. That also starts primary succession. These pathways are slow because soil has to form before larger plants can establish.

If a wildfire burns a forest but leaves soil, buried seeds, roots, fungi, and microbes, then succession is secondary. The same is true for many hurricanes and floods. Even though the disturbance may be major, recovery is faster because the basic biological foundation remains.

Landslides are a good mixed example. If the slide removes the soil and exposes bare rock, primary succession may follow. If some soil remains, or nearby soil and organic matter quickly settle on the slope, secondary succession is more likely. That is why AP questions may ask you to judge the outcome based on conditions, not just the event name.

Why is secondary succession faster? Because soil already contains:

- nutrients
- decomposers
- microorganisms
- seed banks
- root systems
- organic matter

That means colonizing species do not need to start from bare rock.

Primary succession starts much closer to ecological zero. The first species must tolerate harsh conditions such as strong temperature variation, little organic matter, and no developed soil.

> **Test Tip:** If a question asks whether a disturbance causes primary or secondary succession, first ask: “Did the soil remain?” That is the fastest route to the correct answer.

### Pioneer species and the stages of recovery

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

Pioneer species matter because they make later stages possible. In primary succession, lichens and mosses are classic pioneers. Lichens can grow directly on rock, help break it down physically and chemically, and trap dust and organic matter. Over time, that contributes to soil formation. Mosses can then grow in the thin developing soil and hold moisture, allowing small plants to follow.

A common sequence in primary succession is:

- bare rock
- lichens and mosses
- grasses or herbaceous plants
- shrubs
- trees
- mature community

The exact species vary by biome, but the pattern of increasing complexity is consistent.

In secondary succession, the pioneer species are usually fast-growing plants such as grasses, herbs, or weeds rather than lichens. That is because soil is already present. These early species are often **r-selected** and generalist. They grow quickly, disperse widely, and tolerate variable conditions. Later, shrubs and trees establish, and eventually a more complex community develops.

This change over time affects more than just plants. As vegetation structure becomes more complex, more niches appear for insects, birds, mammals, and decomposers. Species richness usually rises as succession proceeds, though the rate of increase slows later.

Early succession and late succession differ in ways APES likes to test:

- early stages have lower biomass
- early stages often have lower biodiversity
- early stages recover quickly from stress
- later stages have more complex nutrient cycling
- later stages are usually more stable

That does not mean late stages are always “better.” In some systems, repeated disturbance is normal. A mature forest is not the endpoint for every place at every time.

### Disturbance regime, resistance, and resilience

Natural disruptions are not random in every habitat. Many habitatss experience a typical pattern of disturbance called a **disturbance regime**.

> **Key Term: disturbance regime**
> The characteristic frequency, intensity, size, duration, and type of disturbances in an environment.

A grassland with regular fire and grazing has a different disturbance regime from a coastal marsh shaped by periodic flooding. A conifer forest may experience less frequent but more intense fire. The species present are often adapted to that usual pattern.

This leads to two more key ideas.

> **Key Term: resistance**
> The ability of an community to remain largely unchanged during a disturbance.
>
> **Key Term: resilience**
> The ability of an natural system to recover after disturbance.

An biological community with high resistance changes little when disturbed. An community with low resistance may change a lot, but if it has high resilience, it can recover relatively well.

A fire-adapted grassland may have low resistance to fire in the sense that aboveground biomass burns easily, but high resilience because grasses regrow quickly from roots and seeds. A mature forest with thin-barked trees may have lower resilience after severe fire because recovery is slower.

Biodiversity often supports resilience. If one species is lost or reduced, others may still perform similar ecological roles. More pathways for nutrient cycling and more feeding links usually make the system less fragile.

The effect of a disturbance also depends on its details:

- **intensity**: how strong it is
- **frequency**: how often it happens
- **season**: when it happens in the life cycle of organisms
- **spatial pattern**: whether it affects small patches or huge areas
- **topography**: slope, drainage, and terrain form shape

This is why the same type of disturbance does not always have the same result. A low-intensity fire in a fire-adapted habitat may maintain biodiversity. An unusually intense fire after fuel buildup may kill seeds, damage soil, and lengthen recovery.

### Comparing major types of upheaval

Wildfire often leads to secondary succession because soil usually remains. Immediate effects include plant death, wildlife mortality, smoke production, and erosion risk if roots no longer hold soil. Longer-term effects may include nutrient return through ash, more light for new growth, and recovery of fire-adapted plants. Some pines even require fire to open cones or release seeds.

Flooding usually leads to secondary succession as well, though effects vary by region. Short-term impacts include plant root suffocation in saturated soil, erosion, nest destruction, and displacement of animals. But floods can also replenish floodplain soils with fresh sediment and nutrients. In some systems, periodic flooding is part of normal ecological balance function.

Volcanic eruption commonly creates conditions for primary succession if new lava or ash leaves bare substrate without soil. Immediate effects are highly destructive: burial, heat, gas release, and habitat loss. Over longer timescales, weathering of volcanic material can help create fertile soils.

Glacial retreat is a classic cause of primary succession. Newly exposed rock or till has little or no developed soil. Colonization begins with hardy pioneers. This process is slow, but it provides one of the clearest examples of succession from bare substrate to developed community.

Hurricanes usually trigger secondary succession where soil remains. Inland effects include canopy loss and treefall. Coastal effects may include dune erosion, marsh reshaping, and saltwater intrusion. These changes can shift habitats for years even if vegetation eventually returns.

Landslides are conditional. If a slope loses only vegetation and some surface soil remains, secondary succession follows. If the slide strips the slope to rock, primary succession may begin.

### Fire-adapted habitatss and prescribed burns

Some students assume all fire is ecologically harmful. That is not accurate. In many habitatss, periodic fire is normal and useful.

> **Key Term: fire-adapted habitat**
> An fire-adapted system in which species and ecological processes are adapted to recurring fire.

In grasslands, savannas, chaparral, and some pine forests, periodic fire reduces fuel buildup, returns nutrients, prevents woody plants from taking over, and maintains habitat structure. Without fire, these habitatss may shift toward a different community type.

That is why land managers sometimes use a **prescribed burn**.

> **Key Term: prescribed burn**
> A carefully planned, controlled fire used to mimic natural fire regimes and reduce the risk of severe wildfire.

A prescribed burn is not a natural disruption itself, but it is directly relevant because it imitates natural fire processes. It can lower the chance of catastrophic fire by removing excess dry vegetation. It can also help maintain biodiversity in fire-dependent systems.

Still, prescribed burns are management tools, not universal solutions. Weather conditions, smoke impacts, nearby development, and fuel conditions all matter.

## Key Point Checklist

_This article has covered the following key knowledge points:_

- Natural upheaval changes habitat by altering abiotic conditions such as soil, water, light, temperature, and nutrient availability
- The most important distinction between primary and secondary succession is whether soil is present
- Primary succession begins on bare substrate, such as fresh lava or land exposed by glacial retreat
- Secondary succession begins after disturbance when soil remains, such as after many wildfires, floods, and hurricanes
- Pioneer species in primary succession often include lichens and mosses, which help form soil
- Pioneer species in secondary succession are often grasses, herbs, and other fast-growing plants
- Wildfires can damage habitat immediately but may also recycle nutrients and support fire-adapted natural communitiess
- Floods can cause erosion and root death but may also deposit nutrient-rich sediment on floodplains
- Volcanic eruptions and glacial retreat often create conditions for primary succession
- Resistance means staying unchanged during disturbance, while resilience means recovering after disturbance
- A disturbance regime is the typical pattern of disturbance in an natural community, including frequency and intensity
- Prescribed burns are controlled fires used to imitate natural fire regimes and reduce severe wildfire risk

## Key Terms and Concepts

- disturbance
- habitat change
- primary succession
- secondary succession
- pioneer species
- disturbance regime
- resistance
- resilience
- fire-adapted habitat
- prescribed burn


Natural Disruptions to Ecosystems - Habitat change from upheaval