AP Biology

## Learning Outcomes

- Define ecological succession and explain why communities change after disturbance.
- Distinguish primary succession from secondary succession using soil presence, starting conditions, and examples.
- Describe how pioneer species modify abiotic conditions and affect later species.
- Explain how disturbance, biodiversity, and resilience are related.
- Predict changes in species abundance, biomass, and diversity during succession.
- Interpret succession data from graphs, tables, and field studies in AP Biology-style questions.
- Connect ecological succession to natural selection, community interactions, and human impacts.

## AP Biology Syllabus

Ecological succession belongs in Unit 8: Ecology, especially the study of disruptions to ecosystems, community structure, biodiversity, and recovery after environmental change. AP Biology questions usually do not ask you to memorize a fixed sequence of species. Instead, you should be able to explain mechanisms, interpret data, and justify predictions about how a community changes over time after events such as fire, glacial retreat, storms, volcanic activity, agriculture, pollution, or habitat clearing.

- Explain how disturbances change abiotic and biotic conditions in a community.
- Compare primary and secondary succession.
- Describe pioneer species and their role in changing the environment.
- Predict short-term and long-term changes in species composition after disturbance.
- Connect succession to competition, mutualism, predation, decomposition, and nutrient cycling.
- Explain how biodiversity can increase resistance to, or recovery from, disturbance.
- Analyze graphs showing species abundance, percent cover, biomass, or diversity over time.
- Use evidence from data to support or reject claims about succession.

## 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. How does primary succession differ from secondary succession?
2. Why are lichens, mosses, grasses, or fast-growing plants often described as pioneer species?
3. After a forest fire, why is recovery usually faster than after a volcanic lava flow?
4. How might species diversity change during succession, and why might it not increase forever?
5. In a graph of plant species abundance after disturbance, what evidence would show that one species is replacing another?

## Introduction

Ecological succession is the pattern of community change that occurs after a new habitat forms or after an existing community is disturbed. In Unit 8, succession helps explain how disruptions to ecosystems alter community structure and how living systems may recover over time. A disturbance can remove organisms, change soil nutrients, alter light levels, shift water availability, or create open space for new colonists. After that change, species do not simply “return” all at once. Different species arrive, survive, reproduce, compete, and modify the environment in different ways.

AP Biology often tests succession through applied scenarios. You may see an abandoned farm, a burned forest, a new volcanic island, a retreating glacier, a hurricane-damaged coastal forest, or a human-restored prairie. Your job is to identify the starting conditions, explain the biological processes, and support claims using evidence from graphs or tables.

> **Key Term: Ecological succession**
> The gradual change in species composition and community structure over time, especially after a new habitat forms or after disturbance.

### Primary succession begins without developed soil

Primary succession occurs when organisms colonize an area where no developed soil is present. This can happen on newly cooled lava, exposed rock after a glacier retreats, bare sand dunes, or a newly formed volcanic island. Because soil is absent or poorly developed, early colonists must tolerate harsh abiotic conditions such as limited nutrients, low water retention, high exposure, and temperature swings.

> **Key Term: Primary succession**
> Succession that begins in an area without developed soil, such as bare rock, newly cooled lava, or land exposed by glacial retreat.

In primary succession, pioneer species are the first organisms able to establish themselves. Classic examples include lichens and mosses on bare rock. Lichens are associations between fungi and photosynthetic partners such as algae or cyanobacteria. They can attach to rock surfaces, tolerate drying, and contribute to the slow breakdown of rock. When pioneer organisms die, decomposers help add organic matter. Over time, tiny pockets of soil form. This allows small plants to grow, which add more organic material and help retain water. Later, grasses, shrubs, and trees may establish if climate and dispersal conditions permit.

> **Key Term: Pioneer species**
> Early colonizing species that can survive in newly opened or harsh conditions and often change the environment in ways that affect later species.

Primary succession is usually slow because soil formation takes time. The sequence is not identical everywhere. Local climate, rock type, wind patterns, animal movement, water availability, and chance colonization all affect what happens. On a volcanic island, wind and birds may bring seeds. Near a retreating glacier, nearby plant communities may supply seeds and spores. In both cases, the key exam feature is that the process starts with little or no soil.

> **Test Tip:**
> When choosing between primary and secondary succession, first ask: “Is developed soil already present?” If the answer is no, the scenario is probably primary succession. If soil remains, it is probably secondary succession.

Primary succession also links to AP Biology themes beyond ecology. Early colonists must have traits that allow survival under stressful conditions. Those traits are produced by preexisting genetic variation and mutation over generations; organisms do not mutate because they “need” to colonize bare rock. Environmental conditions select among existing variants. If a lichen type survives better in dry, exposed conditions, it may leave more descendants than less tolerant types.

### Secondary succession follows disturbance where soil remains

Secondary succession occurs after a disturbance removes or alters part of a community but leaves soil in place. Examples include recovery after forest fire, abandoned agricultural fields, hurricanes, floods, logging, or some forms of human land clearing. Because soil, seeds, roots, spores, microorganisms, and nutrients may remain, secondary succession is usually faster than primary succession.

> **Key Term: Secondary succession**
> Succession that occurs after disturbance in an area where soil remains, such as after fire, farming, flooding, or forest clearing.

A common example is an abandoned farm field. At first, fast-growing annual plants may dominate because open soil and high light create favorable conditions for species that reproduce quickly. Over time, perennial grasses and herbaceous plants may become more common. Shrubs may then establish, followed by early-successional trees that grow quickly in bright light. If conditions allow, later-successional tree species may increase as the canopy closes and shade levels rise.

After a forest fire, grasses and herbs may appear rapidly from surviving roots or seeds in the soil. Fire can also release nutrients from burned organic matter, temporarily increasing soil nutrient availability. Some plants have adaptations linked to fire-prone environments, such as seeds that germinate after heat exposure or cones that open after fire. Animals may return as vegetation structure and food resources recover.

> **Key Term: Disturbance**
> An event or process that changes abiotic or biotic conditions in a community, such as fire, drought, flood, storm damage, disease, invasive species, or human land-use change.

Secondary succession does not always mean the original community will return. Repeated disturbance, invasive species, soil erosion, pollution, climate change, or loss of native seed sources can shift the path of recovery. For example, if overgrazing removes native grasses and exposes soil to erosion, the later community may be dominated by shrubs or invasive plants rather than the original grassland species. If a severe fire burns organic soil layers, the site may recover much more slowly than after a low-intensity fire.

> **Exam Warning:**
> Do not write that succession always moves toward a single permanent “perfect” community. AP Biology rewards explanations based on conditions, interactions, and evidence, not memorized progress toward one fixed endpoint.

A “climax community” is sometimes used to describe a relatively stable later community under a given set of conditions. For example, a mature temperate forest may persist for long periods if climate and disturbance patterns remain similar. However, stability is relative. Storms, droughts, pests, fires, and human impacts can restart or redirect succession.

> **Key Term: Climax community**
> A relatively stable later-stage community that may persist under particular climate, soil, and disturbance conditions.

### Species interactions shape the path of succession

Succession is not just a list of species arriving in order. It is driven by interactions among organisms and between organisms and abiotic factors. Three useful mechanisms are facilitation, inhibition, and tolerance.

> **Key Term: Facilitation**
> A process in which early species make conditions more suitable for later species.

Facilitation is common in primary succession. Lichens and mosses contribute to soil formation. Early plants add organic matter, stabilize substrate, and increase water retention. Nitrogen-fixing species may increase nitrogen availability, allowing other plants to grow. In coastal dunes, pioneer grasses can trap sand and reduce erosion, making the habitat more suitable for shrubs.

Inhibition occurs when early species make it harder for other species to establish. Fast-growing grasses may monopolize light, water, or nutrients. Dense shrubs may shade seedlings. Some plants release chemicals that reduce germination or growth of competitors. In this case, later species may only establish after the early species are damaged, die, or are disturbed.

> **Key Term: Inhibition**
> A process in which established species reduce the ability of other species to colonize or grow.

Tolerance occurs when later species are not strongly helped or harmed by early species, but they can survive under the conditions created as succession proceeds. For example, shade-tolerant tree seedlings may grow slowly beneath a developing canopy. As early trees die, shade-tolerant species may replace them because they can survive low-light conditions better than shade-intolerant species.

> **Key Term: Tolerance**
> A process in which species persist because they can survive the abiotic and biotic conditions present during succession.

Competition is especially common in succession questions. Early-successional species often have high dispersal ability, rapid growth, and many offspring. These traits help them colonize open spaces. Later-successional species may grow more slowly but compete well under crowded or shaded conditions. This connects to population ecology: species with rapid reproduction can increase quickly when resources are abundant, while species that compete well may dominate later when resources become limiting.

Predation and herbivory can also affect succession. Deer browsing may prevent tree seedlings from establishing, keeping a community in a shrub or grass-dominated state. Insects or pathogens may reduce a dominant plant species and create space for others. Mutualisms also matter. Mycorrhizal fungi can help plants obtain nutrients, and pollinators or seed-dispersing animals can affect which plants spread.

> **Test Tip:**
> In free-response answers, link the interaction to the result. For example: “Shrubs reduce light at the soil surface, so shade-intolerant grasses decrease while shade-tolerant tree seedlings have higher survival.”

### Diversity, resilience, and data interpretation in succession

Succession often changes biodiversity, but the pattern depends on the system and the time scale. Early after a severe disturbance, species richness may be low because only a few tolerant colonizers can survive. As more habitats and resources become available, richness may increase. In some later stages, a few strong competitors may dominate, so diversity may level off or even decrease.

> **Key Term: Biodiversity**
> The variety of life in a biological community, including genetic diversity, species diversity, and the range of community types.

Biodiversity can help ecosystems withstand disruptions. A community with many species may include some that tolerate drought, disease, fire, or temperature change. If one species declines, others may continue similar roles in food webs or nutrient cycling. Genetic variation within populations also matters because it provides raw material for natural selection after conditions change.

One measure used in AP Biology is Simpson’s Diversity Index:

\text{Simpson's Diversity Index}=1-\sum\left(\frac{n}{N}\right)^2
In this equation, $$n$$ is the number of individuals of one species, and $$N$$ is the total number of individuals of all species in the community being sampled. A higher value generally indicates greater diversity because individuals are spread more evenly among multiple species.

> **Key Term: Resilience**
> The ability of a community to recover structure or function after disturbance.

Resilience is not the same as resistance. Resistance means a community changes little when disturbed. Resilience means it recovers after being changed. A fire-adapted grassland may not resist fire because aboveground biomass burns, but it may be highly resilient if roots survive and regrowth is rapid. A tropical forest may contain high biodiversity but still recover slowly after severe clearing if soil nutrients are lost or seed sources are far away.

AP Biology questions often show succession with graphs. Common variables include time since disturbance on the x-axis and percent cover, species abundance, biomass, soil nitrogen, or diversity on the y-axis. Read axes and units carefully. A line for one species increasing while another decreases may show replacement, competition, or changing abiotic conditions. If diversity rises after disturbance and then plateaus, explain that new niches or resources became available until later competition or stable conditions limited further increases.

> **Exam Warning:**
> Do not claim that a graph proves causation unless the study design supports it. Observational succession data can show patterns, but experimental evidence is needed to test a specific cause.

Experimental design may appear in free-response questions. For example, researchers might test whether nitrogen-fixing plants speed succession on bare volcanic substrate. The independent variable could be the presence or absence of nitrogen-fixing plants. The dependent variable might be soil nitrogen concentration, plant biomass, or species richness after a set time. Controls should be similar plots without the added factor. Constants could include plot size, starting substrate, watering, light exposure, and sampling time. A null hypothesis might state that nitrogen-fixing plants have no effect on soil nitrogen or later plant colonization.

Succession can also be affected by human activities. Logging, mining, agriculture, urban development, pollution, nutrient runoff, and invasive species can change the starting point or direction of community recovery. Habitat fragmentation may limit seed dispersal and animal movement, slowing recovery. Invasive plants may dominate disturbed areas because they disperse rapidly, grow quickly, or lack local herbivores. Restoration ecology uses succession principles to help damaged communities recover, such as planting native pioneer species, restoring soil, removing invasive species, or reintroducing natural fire cycles.

When writing AP Biology explanations, use cause-and-effect chains. A strong answer might say: “The fire removed aboveground vegetation but left soil and some seeds intact, so secondary succession would occur. Increased light at ground level would favor fast-growing herbaceous plants at first. Over time, shrubs and trees may increase as they disperse into the area and compete for light and nutrients.” This is more exam-ready than simply listing “grass, shrubs, trees.”

## Key Point Checklist

This article has covered the following key knowledge points:

- Ecological succession is community change over time after new habitat formation or disturbance.
- Primary succession begins without developed soil and is often slow because soil must form.
- Secondary succession begins where soil remains and is usually faster than primary succession.
- Pioneer species are early colonizers that tolerate harsh or open conditions.
- Lichens, mosses, grasses, herbs, and fast-growing plants are common pioneer examples, depending on the habitat.
- Disturbances include fire, floods, storms, drought, disease, volcanic activity, glacial retreat, agriculture, and habitat clearing.
- Succession is shaped by abiotic conditions, dispersal, competition, herbivory, mutualism, and chance events.
- Facilitation, inhibition, and tolerance are mechanisms that help explain species replacement.
- Succession does not always lead to one fixed endpoint or the original community.
- Biodiversity may increase during succession, but patterns depend on time scale and local conditions.
- Resilience describes recovery after disturbance; resistance describes limited change during disturbance.
- AP Biology questions often require graph interpretation, experimental design, and evidence-based justification.

## Key Terms and Concepts

- Ecological succession
- Primary succession
- Pioneer species
- Secondary succession
- Disturbance
- Climax community
- Facilitation
- Inhibition
- Tolerance
- Biodiversity
- Resilience


Disruptions in ecosystems - Ecological succession