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F3.5 · Explain plants’ role in ecological succession and biodiversity

Learn to explain plants’ role in ecological succession and biodiversity through clear examples and targeted practice.

Ontario Grade 11 Biology

Plants: Anatomy, Growth, and Function

How plant communities change and shape the living variety of an ecosystem

After a disturbance, the plants in a place may change over time. A bare surface might first support a few hardy plants. Later, shrubs or trees may grow, and the kinds of animals and other organisms present may also change. This lesson explains that pattern and how plants help shape it. In Grade 10 science, you learned that organisms interact with one another and with their environment. An ecosystem is a community of organisms and the non-living surroundings in which they interact. Succession is a change in the kinds of organisms living in an area over time. Plants are important in succession because they are living parts of ecosystems and can also alter the conditions other organisms experience.

What you will learn

Describe ecological succession and distinguish primary from secondary succession.
Explain how plants can change conditions and resources in an ecosystem as succession proceeds.
Explain how plant communities can support biodiversity while recognizing that the pattern varies among ecosystems.

1. From an SNC2D bridge to a biological question

A useful starting point is to think of an ecosystem as a system. Living things interact with non-living factors such as light, water, and soil. A change to one part of the system can affect other parts. For example, a disturbance can remove plants and expose the ground. The place has not stopped being an ecosystem, but its conditions and community have changed.

Ecologists study these patterns by observing how communities differ over time or between places. A community is the different populations of living organisms in an area. Biodiversity means the variety of life in an area. It can refer to the variety of species and the variety of habitats that support them. More kinds of plants do not automatically mean that every other group of organisms also becomes more diverse.

The biological question is: How do plants contribute to the changes in an ecosystem after a disturbance, and how can those changes affect biodiversity? To answer it, separate what is observed from the model used to explain it. An observed pattern might be that different plant communities occur at different times after a disturbance. Succession is a model for understanding such change; it does not mean that every ecosystem follows one fixed path.

An ecosystem includes living organisms and their non-living surroundings.
A community is the living organisms in an area.
Biodiversity means variety of life; it is not simply a count of plant species.

2. A labelled process: two starting conditions

Primary succession begins where there is no developed soil, such as a newly exposed rock surface. Pioneer species are among the first organisms able to establish there. Some pioneer plants can grow in harsh conditions. As they live and die, organic material from their remains can contribute to soil development. Soil is a mixture of mineral particles, organic material, water, and air. As soil develops, it can hold more water and provide conditions for additional plants.

Secondary succession begins after a disturbance where soil remains. A fire or an abandoned field can leave soil in place, although the disturbance may remove many plants. Seeds, roots, or other living parts may also remain. Since soil is already present, plants can return without the area first needing to develop soil from bare rock.

At either starting point, the plant community may change over time. Different plants can establish, grow, and reproduce under the conditions available. Plants may affect light, shelter, and the availability of resources. These changes can make a place more or less suitable for other plants and animals. In turn, organisms can affect plants. Succession is therefore a changing set of interactions, not a simple ladder that always ends in the same community.

Primary succession starts without developed soil.
Secondary succession starts where soil remains after disturbance.
Pioneer species are early organisms in succession; they do not guarantee a particular final community.

3. How plants link succession and biodiversity

Plants can affect succession by changing the physical conditions of a place. Plant cover can shade the ground, and stems and leaves can provide shelter. Plant growth and remains can add organic material to soil. These effects can influence which organisms are able to live there next. The strength and direction of an effect depend on the place and the plants involved.

Plants also support other organisms as sources of food and as places to live. Different plant forms can create different conditions: low ground cover, shrubs, and taller trees may provide different amounts of shade and shelter. If a changing plant community offers a wider range of resources or habitats, it may support a wider variety of organisms. This is one way plants can contribute to biodiversity.

However, succession does not always increase biodiversity at every stage. A disturbance can reduce the organisms present. A later plant community may provide different habitats, but some organisms may lose suitable conditions. The relationship depends on the ecosystem, the disturbance, and the organisms present. A careful explanation links a change in plants to a possible change in resources or habitat, rather than claiming that more plants always mean more biodiversity.

Plants can alter conditions and provide food, shelter, and habitat.
Changes in plant communities can change opportunities for other organisms.
The effect on biodiversity can differ among stages and ecosystems.

4. Reading the evidence and using the model carefully

Imagine observations from several sites after a disturbance. Early sites have mostly small plants, while sites observed later have more shrubs and trees. This pattern is consistent with succession. It does not, by itself, prove that each site changed in exactly that order. The sites may differ in soil, weather, or disturbance history. Repeated observations of the same place over time give stronger evidence about change at that place.

A simple way to explain a succession pattern is to connect three parts: the starting condition, the plant change, and the possible effect on organisms. For example, if soil remains after a disturbance, plants may re-establish. As plant cover changes, so may shade and shelter. Those conditions can affect which organisms can use the area. This is an explanatory model: it organizes possible links, but observations are needed to assess what actually happens.

When comparing biodiversity, ask what variety is being described. A list of plant species describes plant variety, not necessarily the variety of all organisms. Evidence about animals or other groups would be needed to make a claim about those groups. Also note the time and place of observations. A single snapshot may show a difference between communities without showing how either community developed.

A pattern can support a model without proving every step of the model.
Observations over time can help distinguish change from a difference between places.
Be precise about which organisms the evidence covers.

Comparing succession starting conditions

Feature	Primary succession	Secondary succession
Starting condition	No developed soil	Soil remains
Plant role	Early plants may contribute organic material as soil develops	Plants may re-establish in existing soil
Biodiversity connection	Changing conditions may support different organisms over time	Changing plant cover may alter food, shelter, and habitat

Worked example

Example 1: Bare rock after a disturbance

A newly exposed rock surface has no developed soil. Years later, observers find small plants and some soil. Explain the likely type of succession and one way plants may contribute to further change.

Identify the starting condition
The key detail is that developed soil was absent at the start. The type of succession depends on this starting condition.
Name the succession
This is primary succession because it begins without developed soil. Early plants may establish in the exposed area.
Connect plants to change
Plant remains can add organic material as soil develops. The developing soil can provide conditions for other plants, although the exact sequence depends on the site.

Answer: The pattern is consistent with primary succession. Plants may contribute organic material as soil develops, making conditions more suitable for some later plants.

Check: The explanation uses the starting condition and describes a possible plant effect without claiming that all sites follow the same sequence.

Worked example

Example 2: A field after farming stops

A field is no longer farmed. Soil remains, and plants begin to grow. Explain why this is secondary succession and give one possible link between plant change and biodiversity.

Check whether soil remains
The field still has soil. This separates the situation from primary succession, which begins without developed soil.
Classify the change
This is secondary succession because plants are returning after a change while soil remains.
Explain a possible biodiversity link
As the plant community changes, it may offer different food or shelter. That can affect which organisms use the field. This is a possible effect, not proof that biodiversity must increase.

Answer: It is secondary succession because soil remains. Changing plants may provide different food and shelter, affecting the variety of organisms able to use the area.

Check: The explanation avoids treating an increase in biodiversity as certain.

Worked example

Example 3: Interpreting a comparison

A class compares two sites. One has mostly low plants; the other has shrubs and trees. The second site also has more kinds of birds in the class's observations. What can the class reasonably conclude, and what can it not conclude?

State the observed pattern
The class observed different plant communities and different numbers of bird kinds at the two sites. This is a comparison, not a record of change through time.
Offer a possible explanation
Shrubs and trees may provide kinds of shelter or habitat not present in the low-plant site. That could help explain the bird difference.
Set a limit on the conclusion
The comparison alone cannot show that succession caused the difference. The sites could differ in other ways, and the observations may not include every bird kind.

Answer: The observations show an association between plant community and observed bird variety at these sites. They suggest a possible habitat link, but do not prove that succession caused the difference.

Check: The conclusion separates the observed pattern from the explanatory model.

Common mistakes and how to avoid them

Saying that primary and secondary succession are distinguished by whether plants are present.

Correction: Focus on soil. Primary succession begins without developed soil; secondary succession begins where soil remains.

Claiming that succession always increases biodiversity.

Correction: Explain how plant changes may affect resources and habitat. The effect can vary, and biodiversity can change in different ways.

Treating a difference between two sites as proof of succession.

Correction: A comparison shows a pattern between places. Evidence over time is needed to show change at a place.

Using the number of plant kinds as proof of the variety of all organisms.

Correction: State which group was observed. Evidence about plants alone does not establish variety among animals or other organisms.

Lesson summary

Succession is change in the organisms living in an area over time.
Primary succession starts without developed soil; secondary succession starts where soil remains.
Plants can contribute to soil development and affect shade, shelter, food, and habitat.
Plant changes can influence biodiversity, but the outcome depends on the ecosystem and evidence.

Check your understanding

Question 1

A disturbed area still has soil. Which type of succession best fits this starting condition?

Primary succession
Secondary succession
Neither, because succession only happens on bare rock
correctIndex: 1

Show answer and explanation

Secondary succession

Secondary succession begins after a disturbance where soil remains.

Question 2

Which statement is the most careful conclusion when a site with more shrubs has more observed bird kinds?

The shrubs prove that succession caused more bird kinds.
The observation is consistent with a possible habitat link, but does not prove the cause.
More shrubs always cause greater biodiversity.
correctIndex: 1

Show answer and explanation

The observation is consistent with a possible habitat link, but does not prove the cause.

The comparison is a pattern. Other differences between sites could also help explain it.

Key terms

Biodiversity: The variety of life in an area, including variety among species and habitats.
Community: The different populations of living organisms in an area.
Ecological succession: Change in the kinds of organisms living in an area over time.
Pioneer species: Organisms among the first able to establish in an area during succession.
Primary succession: Succession that begins where there is no developed soil.
Secondary succession: Succession after a disturbance where soil remains.

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Published by DoAssignment. This reviewed lesson follows Ontario Grade 11 Biology (SBI3U), expectation F3.5. It is a study resource, not an official curriculum publication.

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