2.7 Ecological Succession

What is ecological succession in AP Environmental Science?

Ecological succession is the gradual change in the species living in an ecosystem over time. Primary succession starts from bare rock with no soil, while secondary succession starts where soil already remains after a disturbance. As succession continues, biomass, species richness, and net productivity usually increase until the community becomes relatively stable.

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Why This Matters for the AP Environmental Science Exam

Succession shows up when you need to explain how ecosystems recover and change after disruptions, a theme that connects to biodiversity, natural disturbances, and adaptation. On the exam you may be asked to describe the difference between primary and secondary succession, explain the role of pioneer species, or interpret how biomass, species richness, and productivity shift over successional time. You may also need to read data in tables or graphs that track these changes and describe the patterns you see. Knowing keystone and indicator species helps you explain why certain organisms have an outsized effect on community structure or signal ecosystem health.

Key Takeaways

• Primary succession begins on bare rock or surfaces with no soil; secondary succession begins where soil is still present after a disturbance.
• Pioneer species such as lichens and mosses help create soil during primary succession, making way for larger plants over time.
• Over successional time, total biomass, species richness, and net primary productivity generally increase.
• A keystone species has an effect on community structure that is large compared to its population size.
• An indicator species signals the health or quality of an ecosystem through its presence, abundance, or scarcity.
• Pioneer members of early successional species can move into open habitat and adapt to its conditions, which may eventually lead to new species.

Succession Defined

Ecological succession is the process by which the species structure of a community changes over time. Eventually an ecosystem may reach a relatively stable state, sometimes called a climax community, where little major change occurs under the current climate conditions. Many diagrams show this stable stage as a mature forest, but stable communities take different forms in different climates.

If conditions change, that stability breaks and succession starts again. For example, if a region that was reliably wet shifted to long term drought, the existing community would no longer fit the new conditions and would change until it reached a new stable state.

Primary Succession

Primary succession happens in an area where there is no soil to start with, such as exposed bedrock after a glacier retreats or after a volcanic lava flow cools. Because soil and nutrients are needed for most plants to grow, the first organisms to move in are pioneer species like lichens and mosses.

With help from weathering and erosion, these pioneer species break down rock and begin building soil. As they die and decompose, they add nutrients and help soil develop. Over a long time, grasses, shrubs, and eventually trees can take root and compete for resources. Because the process has to build soil from scratch, primary succession can take hundreds or even thousands of years.

Secondary Succession

Secondary succession happens after a disturbance that removes living organisms but leaves the soil in place. Events like wildfires or floods can clear out a community without stripping the area down to bare rock.

Because soil already exists, secondary succession moves faster than primary succession. The pioneer species here are often fast growing plants like grasses and weeds. After that, the process looks similar to primary succession: larger and more complex plants move in and compete until the community becomes relatively stable.

How Biomass, Richness, and Productivity Change

As succession moves forward in a disturbed ecosystem, three things generally trend upward over time:

• Total biomass increases as small pioneers give way to larger plants and more organisms.
• Species richness, the number of different species, tends to rise as the habitat becomes more complex.
• Net primary productivity changes as the community develops, generally increasing as more producers establish.

Tracking these patterns is exactly the kind of trend you may be asked to describe from a data table or graph.

Animals Are Part of Succession

Most succession diagrams only show the plants (flora) that get replaced over time, but the animals (fauna) shift too. As the plant community changes, the species that can live there changes with it. Grazing animals suited to open grassland would not thrive in a dense forest, so the animal community turns over alongside the vegetation.

Keystone Species

A keystone species has a particularly significant role in determining community structure. If it were removed, the ecosystem would change dramatically. Keystone species often have a low population density compared to how large their influence is, which can make them more vulnerable to disappearing.

As an application, the reintroduction of wolves to Yellowstone National Park is often used to show keystone effects. Their return changed the behavior and numbers of other species and reshaped parts of the landscape, illustrating how one species can shape an entire community. Treat this as an example, not required content for this topic.

Indicator Species

An indicator species reflects the health or quality of its ecosystem through its presence, abundance, scarcity, or chemical makeup. If an indicator species declines sharply, that can signal that ecosystem conditions are getting worse. Amphibians like frogs are a common example because they live in both water and on land, so their condition can hint at the health of both parts of an ecosystem.

How to Use This on the AP Environmental Science Exam

Free Response

When a prompt asks about succession, be specific about the starting point. State clearly whether soil is present (secondary) or absent (primary), then explain the steps that follow. If you are asked about effects, connect succession to rising biomass, species richness, and net primary productivity over time.

Data Analysis

You may get a table or graph showing how a measurement changes across successional stages. Describe the pattern you see, then explain it using succession concepts. Point to the direction of change rather than just listing numbers.

Common Trap

Do not say primary succession is simply faster or slower than secondary. The key difference is the starting conditions: primary begins with no soil, secondary begins with soil already present. Build your answer from that distinction.

Common Misconceptions

• Primary and secondary succession are only about speed. The real difference is the starting point. Primary starts from bare rock with no soil; secondary starts where soil remains.
• A climax community never changes. It is only stable under current conditions. If climate or other conditions shift, succession begins again.
• Succession is only about plants. Animal communities turn over too as the habitat changes, even though many diagrams only show vegetation.
• Keystone species are the most abundant. They often have low population density; their importance comes from their effect on community structure, not their numbers.
• Keystone and indicator species are the same thing. A keystone species shapes community structure, while an indicator species signals ecosystem health or quality.
• Pioneer species are weak or unimportant. They do the essential early work of building soil and changing conditions so other species can move in.

Related AP Environmental Science Guides

• 2.1 Introduction to Biodiversity
• 2.2 Ecosystem Services
• 2.3 Island Biogeography
• Unit 2 Overview: The Living World: Biodiversity
• 2.4 Ecological Tolerance
• 2.5 Natural Disruptions to Ecosystems