What is the primary difference between primary and secondary succession?

The presence of soil. Primary succession begins on bare rock or surfaces with no organic matter, while secondary succession begins in areas where soil already exists following a disturbance.

How does the role of a pioneer species differ from a climax species?

Pioneer species are adapted to harsh, high-light conditions and focus on rapid colonization and soil building. Climax species are typically shade-tolerant, slow-growing, and better competitors in a stable, high-biomass environment.

Compare the Net Primary Productivity (NPP) of a pioneer community versus a climax community.

Pioneer communities have high NPP relative to their biomass because most energy goes into growth. Climax communities have low or near-zero NPP because almost all energy produced is used for the respiration of the existing large biomass.

What is a common misconception about the 'climax community'?

A common error is assuming the climax community is a static, unchanging end-point. In reality, it is a state of dynamic equilibrium that can still experience small-scale disturbances and internal species turnover.

Why is it incorrect to say that primary succession is faster than secondary succession?

Primary succession is significantly slower because it requires the long process of soil formation from bare rock. Secondary succession skips this step because the soil and seed bank are already present, allowing for rapid regrowth.

What mistake is made when identifying a keystone species based solely on population size?

Keystone species are defined by their disproportionate impact relative to their abundance. Identifying them by size alone ignores the fact that many keystone species, like top predators, have very small populations but control the entire ecosystem structure.

Define a 'Seral Community'.

A seral community is an intermediate stage found during ecological succession in an ecosystem advancing towards its climax community. Each stage is characterized by specific species that change the environment for the next stage.

What is an 'Indicator Species'?

An indicator species is an organism whose presence, absence, or health reflects the specific environmental conditions or quality of an ecosystem, such as using stonefly nymphs to indicate high dissolved oxygen in water.

State the formula for Net Primary Productivity (NPP).

The formula is $NPP = GPP - R$, where $GPP$ is Gross Primary Productivity (total energy captured) and $R$ is Respiration (energy used by the producers for metabolism).

Why does species richness typically increase during the middle stages of succession?

As soil develops and environmental conditions become less harsh, more niches become available. This allows a wider variety of plants and animals to coexist before a few dominant species potentially reduce diversity in the final climax stage.

Ecological Succession | College Board AP Environmental Science

Revision Notes

College Board

Environmental Science

Unit 2: The Living World: Biodiversity

Ecological Succession

Summary

Ecological succession is the predictable and directional process by which ecosystems change and develop over time. It involves the gradual replacement of species and the modification of physical environments, moving from simple, unstable pioneer communities to complex, stable climax communities.

1. Definition & Core Concepts

Ecological Succession is the dynamic process of change in the species structure of an ecological community over time. It represents the transition from a relatively simple ecosystem to a more complex one as biotic and abiotic factors interact.

The process is characterized by the colonization of a habitat by new species, which then modify the environment, often making it more suitable for subsequent species while making it less suitable for themselves.

A seral community (or sere) refers to any intermediate stage of succession, while the climax community is the final, stable stage where the ecosystem reaches a steady state with its environment.

Diagram showing the stages of succession from bare rock to a climax forest, illustrating the increase in biomass and soil depth over time.

2. Primary vs. Secondary Succession

Primary Succession occurs in environments where no previous ecosystem existed and there is no soil present, such as on cooled lava flows, newly exposed sand dunes, or rock left behind by retreating glaciers.

Secondary Succession occurs in areas where an existing community has been disturbed or destroyed (e.g., by fire or abandoned agriculture) but the soil remains intact, allowing for much faster recovery than primary succession.

The fundamental difference lies in the starting substrate: primary succession must build soil from scratch through the weathering of rock and decomposition of pioneer species, whereas secondary succession begins with a nutrient-rich seed bank already in the soil.

3. Productivity Trends During Succession

4. Key Species Roles

Pioneer Species are the first organisms to colonize a barren area; they are typically hardy, fast-growing, and capable of surviving harsh conditions (e.g., lichens that break down rock into soil).

Keystone Species have a disproportionately large effect on their environment relative to their abundance; their removal can lead to a 'trophic cascade' and the potential collapse of the entire ecosystem structure.

Indicator Species serve as biological monitors for environmental health; their presence, absence, or abundance reflects specific environmental conditions, such as water quality or pollution levels.

5. Key Distinctions

6. Exam Strategy & Tips

Ecological Succession

Summary

1. Definition & Core Concepts

Diagram showing the stages of succession from bare rock to a climax forest, illustrating the increase in biomass and soil depth over time.

2. Primary vs. Secondary Succession

3. Productivity Trends During Succession

In the early stages, Gross Primary Productivity ( $GPP$ ) is low because there are few producers, but Net Primary Productivity ( $NPP$ ) is high as a percentage of $GPP$ because little energy is lost to respiration.

During middle stages, $GPP$ increases rapidly as plant biomass and diversity rise, leading to a peak in $NPP$ as the ecosystem efficiently captures and stores energy in new growth.

In the climax stage, $GPP$ remains high, but $NPP$ approaches zero because the energy produced through photosynthesis is almost entirely consumed by the high respiration ( $R$ ) requirements of the massive biomass ( $NPP = GPP - R$ ).

4. Key Species Roles

5. Key Distinctions

Feature	Primary Succession	Secondary Succession
Starting Point	Bare rock / No soil	Soil already present
Pioneer Species	Lichens, Mosses	Grasses, Weeds
Speed of Process	Very slow (centuries)	Faster (decades)
Common Causes	Volcanic eruption, Glacial retreat	Forest fire, Deforestation

Biomass vs. Diversity: While both increase during succession, biomass tends to stabilize in the climax community, whereas species richness may slightly decline if a few dominant species outcompete others in the final stage.

6. Exam Strategy & Tips

Identify the Substrate: When given a scenario, always check if soil is present. If the prompt mentions 'lava' or 'glacier,' it is almost certainly primary succession; if it mentions 'fire' or 'clearing,' it is secondary.
Energy Balance: Remember the formula $NPP = GPP - R$ . On exams, questions often ask why a mature forest has low $NPP$ ; the answer is always high respiration due to large biomass.
Keystone Logic: If a question asks about the impact of removing a single species and the result is a total change in the landscape, that species is a keystone species, regardless of its population size.