Ecological Pyramids and Succession

Ecological Pyramids and Succession

Ecological pyramids are graphical representations of the relationship between different trophic levels of an ecosystem. They help us understand the distribution of organisms, biomass and energy at successive trophic levels.

Ecological succession is the gradual and orderly process of change in the species composition of an ecological community over time, ultimately leading to the formation of a relatively stable community.

1. Ecological Pyramids

An ecological pyramid represents the quantitative relationship between organisms belonging to different trophic levels.

The three major types are:

• Pyramid of Number
• Pyramid of Biomass
• Pyramid of Energy

2. Pyramid of Number

The pyramid of number represents the number of organisms present at each trophic level.

The shape of the pyramid depends on the type of ecosystem.

Upright Pyramid of Number

In a grassland ecosystem, the number of producers is usually greater than the number of herbivores, and the number of herbivores is greater than that of carnivores.

Grass → Grasshoppers → Frogs → Snakes

Therefore, the pyramid of number is generally upright.

Inverted Pyramid of Number

In a tree ecosystem, one large tree may support many herbivorous insects, and these insects may support several predators.

Therefore, the pyramid may become inverted.

3. Pyramid of Biomass

The pyramid of biomass represents the total mass of living organisms present at each trophic level.

Biomass may be expressed as dry weight or other appropriate measures of living material.

Upright Pyramid of Biomass

In most terrestrial ecosystems, producers have a greater biomass than herbivores, which in turn have greater biomass than higher-level consumers.

Inverted Pyramid of Biomass

In aquatic ecosystems, the pyramid of biomass may be inverted.

For example, phytoplankton have a relatively small standing biomass but reproduce rapidly and can support a larger standing biomass of zooplankton.

4. Pyramid of Energy

The pyramid of energy represents the amount of energy available at different trophic levels.

Energy decreases progressively from one trophic level to the next because much of the energy is lost as heat during metabolic activities.

Therefore, the pyramid of energy is always upright.

Producers > Primary Consumers > Secondary Consumers > Tertiary Consumers

5. Comparison of Ecological Pyramids

Type	Represents	Shape
Pyramid of Number	Number of organisms	Upright or inverted
Pyramid of Biomass	Biomass at each trophic level	Upright or inverted
Pyramid of Energy	Energy available at each trophic level	Always upright

6. Limitations of Ecological Pyramids

Ecological pyramids are useful representations, but they have certain limitations.

• They generally do not represent food webs; they show trophic relationships in a simplified manner.
• An organism may occupy more than one trophic level because it may feed on organisms from different trophic levels.
• Decomposers are generally not included in ecological pyramids, although they play an essential role in ecosystem functioning.
• Pyramids do not always provide a complete picture of the complexity of an ecosystem.

Ecological Succession

7. What is Ecological Succession?

Ecological succession is the gradual and predictable change in the species composition and structure of a community over time.

एक community अचानक दूसरी community में change नहीं होती। Different species धीरे-धीरे establish होती हैं, existing species को replace करती हैं और environmental conditions को भी modify करती हैं।

The sequence of communities that replace one another during succession is called a successional sequence or sere.

8. Basic Sequence of Succession

The major stages can be represented as:

Bare Area → Pioneer Community → Intermediate Communities → Climax Community

9. Pioneer Community

The first community of organisms to colonise a previously bare or disturbed area is called the pioneer community.

The organisms that establish first are called pioneer species.

Pioneer species are generally capable of surviving under harsh environmental conditions.

10. Sere

A sere is the entire sequence of communities that develop during ecological succession from the pioneer community to the climax community.

Each stage in this sequence is called a seral stage.

Term	Meaning
Pioneer species	First organisms to colonise an area
Seral stage	Intermediate stage of succession
Sere	Complete sequence of communities
Climax community	Relatively stable final community

11. Climax Community

The climax community is the relatively stable and mature community that develops at the end of ecological succession under a given set of environmental conditions.

इस stage में community comparatively stable होती है और उसके organisms तथा environment के बीच एक relatively balanced relationship develop हो जाता है।

12. Primary Succession

Primary succession begins in an area where there was previously no established soil or vegetation.

Examples include:

• Bare rock surfaces
• Newly exposed rock areas
• Newly formed land surfaces

Primary succession is generally a very slow process because soil has to develop gradually.

13. Primary Succession on Bare Rock

On a bare rock surface, lichens can act as pioneer organisms.

Lichens help in weathering of rocks and contribute to the formation of soil. As soil develops, other organisms such as mosses and herbaceous plants can establish.

A simplified sequence can be represented as:

Bare Rock → Lichens → Mosses → Herbs → Shrubs → Forest

This sequence is a simplified representation; the exact stages may vary according to environmental conditions.

14. Primary Succession in Water

Succession occurring in an aquatic environment is called hydrach or hydrosere.

Different aquatic organisms and plants gradually modify the environment, leading towards a terrestrial-type community under suitable conditions.

15. Secondary Succession

Secondary succession occurs in an area where a community existed previously but was disturbed or destroyed, while the soil and some biological components remain.

Examples include:

• Abandoned agricultural fields
• Areas affected by fire
• Areas cleared of vegetation but retaining soil

Because soil is already present, secondary succession is generally faster than primary succession.

16. Primary vs Secondary Succession

Primary Succession	Secondary Succession
Starts in a previously unoccupied area	Starts in an area where a community existed previously
Soil is initially absent or poorly developed	Soil is already present
Very slow	Relatively faster
Begins with pioneer colonisation	Begins after disturbance of an existing community

17. Xerarch Succession

Succession beginning in a dry habitat is called xerarch succession.

The succession that develops on bare rock is a common example of xerarch succession.

The sequence may progress from lichens to mosses, herbs, shrubs and eventually a forest community where climatic conditions permit.

18. Hydrarch Succession

Succession beginning in a water body is called hydrarch succession.

It is also called hydrosere.

A simplified sequence is:

Phytoplankton → Submerged Plants → Floating Plants → Reed Swamp → Marsh Meadow → Shrub → Forest

As succession proceeds, the water body gradually becomes shallower due to accumulation of organic matter and sediments, allowing terrestrial plants to establish.

19. Factors Affecting Succession

Ecological succession is influenced by several factors:

• Climate
• Soil characteristics
• Availability of water
• Light
• Temperature
• Interactions among organisms
• Disturbances such as fire, floods and human activities

20. Role of Pioneer Species

Pioneer species are important because they modify the environment and make it more suitable for subsequent communities.

For example, lichens growing on bare rocks contribute to weathering and soil formation. The developing soil allows mosses and later plants to establish.

21. Succession and Community Development

During succession, several changes occur:

• Species composition changes.
• Biomass generally increases during community development.
• Soil formation and nutrient availability may increase.
• Habitat structure becomes more complex.
• Food webs become more developed.
• The community gradually becomes more stable.

22. Ecological Succession – Complete Flow

Bare Area → Pioneer Species → Seral Stages → Increasing Soil and Biomass → Mature Community → Climax Community

23. Important Terms

Term	Meaning
Ecological succession	Gradual change in community composition over time
Pioneer species	First colonisers of a new or disturbed habitat
Seral stage	An intermediate stage of succession
Sere	Complete sequence of communities during succession
Climax community	Relatively stable mature community
Primary succession	Succession beginning where an established community and soil are absent
Secondary succession	Succession after disturbance where soil remains
Xerarch succession	Succession beginning in a dry habitat
Hydrarch succession	Succession beginning in a water body

24. Important Differences

Primary Succession vs Secondary Succession

Primary Succession	Secondary Succession
Newly exposed or previously unoccupied area	Previously occupied and disturbed area
Soil formation is an important early process	Pre-existing soil is available
Slow	Comparatively fast

Hydrarch vs Xerarch Succession

Hydrarch	Xerarch
Starts in a water body	Starts in a dry habitat
Also called hydrosere	Common example begins on bare rock
Progresses towards terrestrial conditions	Progresses towards a more developed community

25. Very Short Answer Questions

1. What is an ecological pyramid?
2. Name the three major types of ecological pyramids.
3. Which ecological pyramid is always upright?
4. What does a pyramid of number represent?
5. What does a pyramid of biomass represent?
6. What does a pyramid of energy represent?
7. What is ecological succession?
8. What is a pioneer species?
9. What is a seral stage?
10. What is a sere?
11. What is a climax community?
12. Define primary succession.
13. Define secondary succession.
14. What is xerarch succession?
15. What is hydrarch succession?

26. Short Answer Questions

1. Explain the three types of ecological pyramids.
2. Why is the pyramid of energy always upright?
3. Why can the pyramid of biomass be inverted in an aquatic ecosystem?
4. What are the limitations of ecological pyramids?
5. Define ecological succession and explain its major stages.
6. Explain the role of pioneer species in succession.
7. Differentiate between primary and secondary succession.
8. What is a climax community?
9. Explain xerarch succession.
10. Explain hydrarch succession.

27. Long Answer Questions

1. Describe ecological pyramids and explain their different types.
2. Explain the limitations of ecological pyramids.
3. Define ecological succession and explain primary and secondary succession.
4. Describe succession on bare rock with suitable stages.
5. Explain hydrarch succession from an aquatic community towards a terrestrial community.
6. Differentiate between primary succession and secondary succession with suitable examples.

28. Multiple Choice Questions

1. The pyramid that represents the number of organisms at each trophic level is:

   (A) Pyramid of energy   (B) Pyramid of number   (C) Pyramid of biomass   (D) Pyramid of nutrients

   Answer: (B) Pyramid of number

2. Which ecological pyramid is always upright?

   (A) Pyramid of number   (B) Pyramid of biomass   (C) Pyramid of energy   (D) All of these

   Answer: (C) Pyramid of energy

3. An inverted pyramid of biomass is commonly found in:

   (A) Forest ecosystem   (B) Grassland ecosystem   (C) Aquatic ecosystem   (D) Desert ecosystem

   Answer: (C) Aquatic ecosystem

4. The first organisms to colonise a new area are called:

   (A) Climax species   (B) Pioneer species   (C) Secondary consumers   (D) Dominant consumers

   Answer: (B) Pioneer species

5. The complete sequence of communities during succession is called:

   (A) Seral stage   (B) Sere   (C) Climax   (D) Ecad

   Answer: (B) Sere

6. The final relatively stable community in succession is called:

   (A) Pioneer community   (B) Seral community   (C) Climax community   (D) Primary community

   Answer: (C) Climax community

7. Primary succession starts in:

   (A) An established forest   (B) An area with developed soil and vegetation   (C) A previously unoccupied area   (D) A cultivated field only

   Answer: (C) A previously unoccupied area

8. Secondary succession is generally faster because:

   (A) There is no competition   (B) Soil is already present   (C) No organisms are present   (D) Energy flow stops

   Answer: (B) Soil is already present

9. Succession beginning on a dry habitat is called:

   (A) Hydrarch succession   (B) Xerarch succession   (C) Mesarch succession   (D) Aquatic succession

   Answer: (B) Xerarch succession

10. Hydrarch succession begins in:

    (A) Desert   (B) Bare rock   (C) Water body   (D) Grassland

    Answer: (C) Water body

11. A common pioneer organism on bare rock is:

    (A) Lichen   (B) Tiger   (C) Fish   (D) Large tree

    Answer: (A) Lichen

12. A community that appears during an intermediate stage of succession is called:

    (A) Pioneer community   (B) Seral community   (C) Climax community   (D) Abiotic community

    Answer: (B) Seral community

13. Which is a limitation of ecological pyramids?

    (A) They can show every food web in complete detail   (B) They may not account for organisms occupying multiple trophic levels   (C) They measure only temperature   (D) They show only abiotic factors

    Answer: (B) They may not account for organisms occupying multiple trophic levels

14. The succession of communities in an aquatic habitat is called:

    (A) Xerarch   (B) Hydrarch   (C) Lithosere   (D) Xerosere

    Answer: (B) Hydrarch

15. Which factor makes secondary succession faster than primary succession?

    (A) Complete absence of soil   (B) Presence of pre-existing soil and biological remnants   (C) Absence of nutrients   (D) Absence of organisms

    Answer: (B) Presence of pre-existing soil and biological remnants

29. Fill in the Blanks

1. The graphical representation of trophic relationships is called an ecological ________.
2. The pyramid of ________ is always upright.
3. The first organisms to colonise a new area are called ________ species.
4. The complete sequence of communities during succession is called a ________.
5. An intermediate stage of succession is called a ________ stage.
6. The final relatively stable community is called the ________ community.
7. Primary succession is generally ________ than secondary succession.
8. Succession beginning in a water body is called ________ succession.
9. Succession beginning in a dry habitat is called ________ succession.
10. ________ can act as pioneer organisms on bare rock.

Answers:

1. Pyramid
2. Energy
3. Pioneer
4. Sere
5. Seral
6. Climax
7. Slower
8. Hydrarch
9. Xerarch
10. Lichens

30. Assertion and Reason

1. Assertion: Pyramid of energy is always upright.

   Reason: Energy is lost at each successive trophic level.

   Answer: Both Assertion and Reason are true, and Reason correctly explains Assertion.

2. Assertion: Secondary succession is generally faster than primary succession.

   Reason: Soil is already present in an area undergoing secondary succession.

   Answer: Both Assertion and Reason are true, and Reason correctly explains Assertion.

3. Assertion: Pioneer species play an important role in ecological succession.

   Reason: They can modify environmental conditions and make the habitat suitable for later species.

   Answer: Both Assertion and Reason are true, and Reason correctly explains Assertion.

4. Assertion: Pyramid of biomass is always upright.

   Reason: Aquatic ecosystems can have an inverted biomass pyramid.

   Answer: Assertion is false, but Reason is true.

31. Match the Following

Column A	Column B
1. Pyramid of energy	a. First colonisers
2. Pioneer species	b. Always upright
3. Sere	c. Water body succession
4. Hydrarch succession	d. Complete succession sequence
5. Xerarch succession	e. Dry habitat succession

Answers: 1-b, 2-a, 3-d, 4-c, 5-e

32. Quick Revision

33. Must Remember for Board Exam