Ecosystem

Class 12 | Pure Science — Botany | Chapter 7

Class 12 Unit 7 Pure_Science

Learning Objectives

The learner will be able to:

• Describe the Structure, functions and types of ecosystems
• Draw ecological pyramids by means of number, biomass and energy
• Interpret carbon and phosphorus cycle
• Recognise pond ecosystem as a self-sufficient and self-regulating system
• Analyse ecosystem services and its management
• Discuss about the importance and conservation of ecosystem
• Explain causes, process and types of plant succession
• Classify vegetation types of India and Tamilnadu

Chapter Outline

• 7.1 Ecosystem
• 7.2 Functions of ecosystem
• 7.3 Plant succession
• 7.4 Vegetation

Have you seen lakes, ponds and pools in your surroundings? They are all called water bodies with many components in them. Can you list out the things which are found in water bodies? Mud, nutrients, clay, dissolved gases, planktons, microorganisms, plants like algae, Hydrilla, Nelumbo, Nymphaea and animals like snake, small fish, large fish, frog, tortoise and crane are the components of the water bodies which constitutes ecosystem. Further, we all know that plants and animals are prominent living components in the environment. They interact with space components such as air, water, soil, sunlight, etc. For example, you have studied in class XI, one of the life processes, photosynthesis which utilizes sunlight, water, carbondioxide, nutrients from the soil and release oxygen to the atmosphere. From this, we understand that the exchange of materials takes place between living and space components. Likewise, you can study the structure, function and types of ecosystems in this chapter.

The term 'ecosystem' was proposed by A.G. Tansley (1935), who defined it as 'the system resulting from the integration of all the living and nonliving factors of the environment'. Whereas, Odum (1962) defined ecosystem 'as the structural and functional unit of ecology'.

Parallel terms for ecosystem coined by various ecologists

• Biocoenosis – Karl Mobius
• Microcosm – S.A. Forbes
• Holocoen – Friederichs
• Geobiocoenosis – V. V. Dokuchaev, G.F. Morozov
• Biosystem – Thienemann
• Bioenert body – Vernadsky

7.1 Ecosystem

Ecosystem comprises of two major components. They are:

1) Abiotic (non-living) components

It includes climatic factors (air, water, sunlight, rainfall, temperature and humidity), edaphic factors (soil air, soil water and pH of soil), topography (latitude, altitude), organic components (carbohydrates, proteins, lipids and humic substances) and inorganic substances (Carbon, Hydrogen, Oxygen, Nitrogen and Phosphorus). Abiotic components play vital role in any ecosystem and hence the total inorganic substances present in any ecosystem at a given time is called standing quality (or) standing state.

2) Biotic (living) components

It includes all living organisms like plants, animals, fungi and bacteria. They form the trophic structures of any ecosystem. On the basis of nutritional relationships, trophic levels of an ecosystem have two components: (1) autotrophic components and (2) heterotrophic components.

(1) Autotrophic components: Autotrophs are organisms which can manufacture the organic compounds from simple inorganic components through a process called photosynthesis. In most of the ecosystems, green plants are the autotrophs and are also called producers.

(2) Heterotrophic components: Those organisms which consume the producers are called consumers and can be recognized into macro and micro consumers. Macroconsumers refer to herbivores, carnivores and omnivores (primary, secondary and tertiary consumers). Microconsumers are called decomposers. Decomposers are organisms that decompose the dead plants and animals to release organic and inorganic nutrients into the environment which are again reused by plants. Example: Bacteria, Actinomycetes and Fungi.

The amount of living materials present in a population at any given time is known as standing crop, which may be expressed in terms of number or biomass per unit area. Biomass can be measured as fresh weight or dry weight or carbon weight of organisms. Biotic components are essential to construct the food chain, food web and ecological pyramids.

7.2 Functions of Ecosystem

The function of ecosystem includes creation of energy, sharing of energy and cycling of materials between the living and nonliving components of an ecosystem. Before studying the productivity in any ecosystem, we should understand the essential role of sunlight used by producers of the first trophic level. The quantity of sunlight is directly proportional to the production of energy by plants.

7.2.1 Photosynthetically Active Radiation (PAR)

The amount of light available for photosynthesis of plants is called Photosynthetically Active Radiation (PAR) which is from 400-700 nm in wavelength. It is essential for photosynthesis and plant growth. PAR is not always constant because of clouds, tree shades, air, dust particles, seasons, latitudes and length of the daylight availability. Generally, plants absorb more blue and red light for efficient photosynthesis.

Of the total sunlight, 34 percent that reaches the atmosphere is reflected back into the atmosphere, moreover 10% is held by ozone, water vapours and atmospheric gases and the remaining 56% reaches the earth's surface. Out of this 56%, only 2 – 10% of the solar energy is used by green plants for photosynthesis while the remaining portion is dissipated as heat.

PAR is generally expressed in millimoles / square meter / second by using silicon photo voltic detectors which detect only 400 to 700 nanometer wavelength of light. PAR values range from 0 to 3000 millimoles /square meter / second. At night PAR is zero and during midday in the summer, PAR often reaches 2000 to 3000 millimoles /square meter/second.

Do You Know?

Types of Carbon

• Green carbon – carbon stored in the biosphere (by the process of photosynthesis).
• Grey carbon – carbon stored in fossil fuel (coal, oil and biogas deposits in the lithosphere).
• Blue carbon – carbon stored in the atmosphere and oceans.
• Brown carbon – carbon stored in industrialized forests (wood used in making commercial articles).
• Black carbon – carbon emitted from gas, diesel engine and coal fired power plants.

7.2.2 Productivity of an Ecosystem

The rate of biomass production per unit area in a unit time is called productivity. It can be expressed in terms of gm/m²/year or Kcal/m²/year. It is classified as given below:

1. Primary productivity
2. Secondary productivity
3. Community productivity

1. Primary Productivity

The chemical energy or organic matter generated by autotrophs during the process of photosynthesis and chemosynthesis is called primary productivity. It is the source of energy for all organisms, from bacteria to human.

a. Gross Primary Productivity (GPP): The total amount of food energy or organic matter or biomass produced in an ecosystem by autotrophs through the process of photosynthesis is called gross primary productivity.

b. Net Primary Productivity (NPP): The proportion of energy which remains after respiration loss in the plant is called net primary productivity. It is also called as apparent photosynthesis. Thus, the difference between GPP and respiration is known as NPP.

NPP = GPP − Respiration

NPP of whole biosphere is estimated to be about 170 billion tons (dry weight) per year. Out of which NPP of oceanic producers is only 55 billion tons per year.

2. Secondary Productivity

The amount of energy stored in the tissues of heterotrophs or consumers is called secondary productivity.

a. Gross secondary productivity: It is equivalent to the total amount of plant material ingested by the herbivores minus the materials lost as faeces.

b. Net secondary productivity: Storage of energy or biomass by consumers per unit area per unit time, after respiratory loss is called net secondary productivity.

3. Community Productivity

The rate of net synthesis of organic matter (biomass) by a group of plants per unit area per unit time is known as community productivity.

Factors affecting Primary Productivity

Primary productivity depends upon the plant species of an area, their photosynthetic capacity, availability of nutrients, solar radiation, precipitation, soil type, topographic factors (altitude, latitude, direction), and other environmental factors. It varies in different types of ecosystems.

7.2.3 Concept of Trophic Level in an Ecosystem

(Greek word 'trophic' = to food or feeding)

A trophic level refers to the position of an organism in the food chain. The number of trophic levels is equal to the number of steps in the food chain.

• The green plants (producers) occupying the first trophic level (T1) are called producers.
• The energy produced by the producers is utilized by the plant eaters (herbivores); they are called primary consumers and occupy the second trophic level (T2).
• Herbivores are eaten by carnivores, which occupy the third trophic level (T3). They are also called secondary consumers or primary carnivores.
• Carnivores are eaten by other carnivores, which occupy the fourth trophic level (T4). They are called the tertiary consumers or secondary carnivores.
• Some organisms which eat both plants and animals are called omnivores (Crow). Such organisms may occupy more than one trophic level in the food chain.

7.2.4 Energy Flow

The transfer of energy in an ecosystem between trophic levels can be termed as energy flow. It is the key function in an ecosystem. Part of the energy obtained from the sun by producers is transferred to consumers and decomposers through each trophic level, while some amount of energy is dissipated in the form of heat. Energy flow is always unidirectional in an ecosystem.

Laws of Thermodynamics

The storage and loss of energy in an ecosystem is based on two basic laws of thermodynamics.

i. First law of thermodynamics: It states that energy can be transmitted from one system to another in various forms. Energy cannot be destroyed or created. But it can be transformed from one form to another. As a result, the quantity of energy present in the universe is constant.

Example: In photosynthesis, the product of starch (chemical energy) is formed by the combination of reactants (chlorophyll, Water, Carbon dioxide). The energy stored in starch is acquired from the external sources (light energy) and so there is no gain or loss in total energy. Here light energy is converted into chemical energy.

ii. Second law of thermodynamics: It states that energy transformation results in the reduction of the free energy of the system. Usually, energy transformation cannot be 100% efficient. As energy is transferred from one organism to another in the form of food, a portion of it is stored as energy in living tissue, whereas a large part of energy is dissipated as heat through respiration. The transfer of energy is irreversible natural process.

Ten Percent Law

This law was proposed by Lindeman (1942). It states that during transfer of food energy from one trophic level to other, only about 10% is stored at every level and rest of them (90%) is lost in respiration, decomposition and in the form of heat. Hence, the law is called ten percent law.

Example: Of the 1000 Joules of Solar energy trapped by producers, 100 Joules of energy is stored as chemical energy through photosynthesis. The remaining 900 Joules would be lost in the environment. In the next trophic level herbivores, which feed on producers get only 10 Joules of energy and the remaining 90 Joules is lost in the environment. Likewise, in the next trophic level, carnivores, which eat herbivores store only 1 Joule of energy and the remaining 9 Joules is dissipated. Finally, the carnivores are eaten by tertiary consumers which store only 0.1 Joule of energy and the remaining 0.9 Joule is lost in the environment. Thus, at the successive trophic level, only ten percent energy is stored.

7.2.5 Food Chain

The movement of energy from producers up to top carnivores is known as food chain. In any food chain, energy flows from producers to primary consumers, then from primary consumers to secondary consumers, and finally secondary consumers to tertiary consumers. Hence, it shows linear network links. Generally, there are two types of food chain:

1. Grazing Food Chain

Main source of energy for the grazing food chain is the Sun. It begins with the first link, producers (plants). The second link in the food chain is primary consumers (mouse) which get their food from producers. The third link in the food chain is secondary consumers (snake) which get their food from primary consumers. Fourth link in the food chain is tertiary consumers (eagle) which get their food from secondary consumers.

2. Detritus Food Chain

This type of food chain begins with dead organic matter which is an important source of energy. A large amount of organic matter is derived from the dead plants, animals and their excreta. This type of food chain is present in all ecosystems. The transfer of energy from the dead organic matter, is transferred through a series of organisms called detritus consumers (detritivores) - small carnivores - large (top) carnivores with repeated eating and being eaten respectively. This is called the detritus food chain.

7.2.6 Food Web

The inter-locking pattern of a number of food chains form a web like arrangement called food web. It is the basic unit of an ecosystem, to maintain its stability in nature, which is also called homeostasis.

Example: In a grazing food chain of a grass land, in the absence of a rabbit, a mouse may also eat food grains. The mouse in turn may be eaten directly by a hawk or by a snake and the snake may be directly eaten by hawks. Hence, this interlocking pattern of food chains is the food web and the species of an ecosystem may remain balanced to each other by some sort of natural check.

Significance of Food Web

• Food web is constructed to describe species interaction called direct interaction.
• It can be used to illustrate indirect interactions among different species.
• It can be used to study bottom-up or top-down control of community structure.
• It can be used to reveal different patterns of energy transfer in terrestrial and aquatic ecosystems.

7.2.7 Ecological Pyramids

Graphic representation of the trophic structure and function at successive trophic levels of an ecosystem is called ecological pyramids. The concept of ecological pyramids was introduced by Charles Elton (1927). Thus, they are also called as Eltonian pyramids. There are three types:

1. Pyramid of number
2. Pyramid of biomass
3. Pyramid of energy

1. Pyramid of Number

A graphical representation of the number of organisms present at each successive trophic level in an ecosystem is called pyramids of number. There are three different shapes of pyramids: upright, spindle and inverted.

There is a gradual decrease in the number of organisms in each trophic level from producers to primary consumers and then to secondary consumers, and finally to tertiary consumers. Therefore, pyramids of number in grassland and pond ecosystem are always upright.

In a forest ecosystem the pyramid of number is somewhat different in shape, it is because the base (T1) of the pyramid occupies large sized trees (Producer) which are lesser in number. Herbivores (T2) (Fruit eating birds, elephant, deer) occupying second trophic level, are more in number than the producers. In final trophic level (T4), tertiary consumers (lion) are lesser in number than the secondary consumer (T3) (fox and snake). Therefore, the pyramid of number in forest ecosystem looks spindle shaped.

The pyramid of number in a parasite ecosystem is always inverted, because it starts with a single tree. Therefore, there is gradual increase in the number of organisms in successive trophic levels from producer to tertiary consumers.

2. Pyramid of Biomass

A graphical representation of the amount of organic material (biomass) present at each successive trophic level in an ecosystem is called pyramid of biomass.

In grassland and forest ecosystems, there is a gradual decrease in biomass of organisms at successive trophic levels from producers to top carnivores (Tertiary consumer). Therefore, these two ecosystems show upright pyramids of biomass.

However, in pond ecosystem, the bottom of the pyramid is occupied by the producers, which comprise very small organisms possessing the least biomass and so, the value gradually increases towards the tip of the pyramid. Therefore, the pyramid of biomass is always inverted in shape.

3. Pyramid of Energy

A graphical representation of energy flow at each successive trophic level in an ecosystem is called pyramid of energy. The bottom of the pyramid of energy is occupied by the producers. There is a gradual decrease in energy transfer at successive trophic levels from producers to the upper levels. Therefore, the pyramid of energy is always upright.

7.2.8 Decomposition

Decomposition is a process in which the detritus (dead plants, animals and their excreta) are broken down into simple organic matter by the decomposers. It is an essential process for recycling and balancing the nutrient pool in an ecosystem.

Nature of Decomposition

The process of decomposition varies based on the nature of the organic compounds, that is some of the compounds like carbohydrate, fat and protein are decomposed rapidly than the cellulose, lignin, chitin, hair and bone.

Mechanism of Decomposition

Decomposition is a step wise process of degradation mediated by enzymatic reactions. Detritus acts as a raw material for decomposition. It occurs in the following steps:

a. Fragmentation – The breaking down of detritus into smaller particles by detritivores like bacteria, fungi and earth worm is known as fragmentation. These detritivores secrete certain substances to enhance the fragmentation process and increase the surface area of detritus particles.

b. Catabolism – The decomposers produce some extracellular enzymes in their surroundings to break down complex organic and inorganic compounds into simpler ones. This is called catabolism.

c. Leaching or Eluviation – The movement of decomposed, water soluble organic and inorganic compounds from the surface to the lower layer of soil or the carrying away of the same by water is called leaching or eluviation.

d. Humification – It is a process by which simplified detritus is changed into dark coloured amorphous substance called humus. It is highly resistant to microbial action; therefore, decomposition is very slow. It is the reservoir of nutrients.

e. Mineralisation – Some microbes are involved in the release of inorganic nutrients from the humus of the soil, such process is called mineralisation.

Factors Affecting Decomposition

Decomposition is affected by climatic factors like temperature, soil moisture, soil pH, oxygen and also the chemical quality of detritus.

7.2.9 Biogeochemical Cycles (Nutrient Cycles)

Exchange of nutrients between organisms and their environment is one of the essential aspects of an ecosystem. All organisms require nutrients for their growth, development, maintenance and reproduction. Circulation of nutrients within the ecosystem or biosphere is known as biogeochemical cycles and also called as 'cycling of materials.' There are two basic types:

1. Gaseous cycle – It includes atmospheric Oxygen, Carbon and Nitrogen cycles.
2. Sedimentary cycle – It includes the cycles of Phosphorus, Sulphur and Calcium - which are present as sediments of earth.

Carbon Cycle

The circulation of carbon between organisms and environment is known as the carbon cycle. Carbon is an inevitable part of all biomolecules and is substantially impacted by the change in global climate. Cycling of carbon between organisms and atmosphere is a consequence of two reciprocal processes of photosynthesis and respiration. The releasing of carbon in the atmosphere increases due to burning of fossil fuels, deforestation, forest fire, volcanic eruption and decomposition of dead organic matters.

Phosphorus Cycle

It is a type of sedimentary cycle. Phosphorus is found in the biomolecules like DNA, RNA, ATP, NADP and phospholipid molecules of living organisms. Phosphorus is not abundant in the biosphere, whereas a bulk quantity of phosphorus is present in rock deposits, marine sediments and guano. It is released from these deposits by weathering.

7.2.10 Types of Ecosystems

Biosphere consists of different types of ecosystems. Though there are many types of ecosystems, only the pond ecosystem is detailed below.

Structure of Pond Ecosystem

It is a classic example for natural, aquatic, freshwater, lentic type of ecosystem. It helps us to understand the structure and function of an ecosystem. When rain water gathers in a shallow area, gradually over a period of time, different kinds of organisms (microbes, plants, animals) become part of this ecosystem. This pond ecosystem is a self-sustaining and self-regulatory fresh water ecosystem, which shows a complex interaction between the abiotic and biotic components in it.

Activity

Collect few living and nonliving components from any water body found nearby.

Abiotic Components of Pond Ecosystem

A pond ecosystem consists of dissolved inorganic (CO₂, O₂, Ca, N, Phosphate) and organic substances (amino acids and humic acid) formed from the dead organic matter. The function of pond ecosystem is regulated by few factors like the amount of light, temperature, pH value of water and other climatic conditions.

Biotic Components of Pond Ecosystem

a. Producers: A variety of phytoplanktons like Oscillatoria, Anabaena, Chlamydomonas, Pandorina, Eudorina, Volvox and Diatoms. Filamentous algae such as Ulothrix, Spirogyra, Cladophora and Oedogonium; floating plants Azolla, Salvia, Pistia, Wolffia and Eichhornia; submerged plants Potamogeton and Phragmitis; rooted floating plants Nymphaea and Nelumbo; macrophytes like Typha and Ipomoea, constitute the major producers of a pond ecosystem.

b. Consumers: The animals represent the consumers of a pond ecosystem which include zooplanktons like Paramoecium and Daphnia (primary consumers); benthos (bottom living animals) like mollusces and annelids; secondary consumers like water beetles and frogs; and tertiary consumers (carnivores) like duck, crane and some top carnivores which include large fish, hawk, man, etc.

c. Decomposers: They are also called as microconsumers. They help to recycle the nutrients in the ecosystem. These are present in mud water and bottom of the ponds. Example: Bacteria and Fungi. Decomposers perform the process of decomposition in order to enrich the nutrients in the pond ecosystem. The cycling of nutrients between abiotic and biotic components is evident in the pond ecosystem, making itself self-sufficient and self-regulating.

Do You Know?

Sea grasses and mangroves of Estuarine and coastal ecosystems are the most efficient in carbon sequestration. Hence, these ecosystems are called as "Blue carbon ecosystems". They are not properly utilized and maintained all over the world although they have rich bioresources potential.

Do You Know?

Limnology – It is the study of biological, chemical, physical and geological components of inland fresh water aquatic ecosystems (ponds, lakes, etc.).

Oceanography – It is the study of biological, chemical, physical and geological components of ocean.

Stratification of Pond Ecosystem

Based on the factors like distance from the shore, penetration of light, depth of water, types of plants and animals, there may be three zones: littoral, limnetic and profundal.

• The littoral zone, which is closest to the shore with shallow water region, allows easy penetration of light. It is warm and occupied by rooted plant species.
• The limnetic zone refers to the open water of the pond with an effective penetration of light and domination of planktons.
• The profundal zone is the deeper region of a pond below the limnetic zone with no effective light penetration and predominance of heterotrophs.
• The benthic zone is the bottom zone of a pond and is occupied by a community of organisms called benthos (usually decomposers).

The primary productivity through photosynthesis of littoral and limnetic zone is more due to greater penetration of light than the profundal zone.

7.2.11 Ecosystem Services (Benefits)

Ecosystem services are defined as the benefits that people derive from nature. Robert Constanza et al (1927) stated "Ecosystem services are the benefits provided to human, through the transformation of resources (or Environmental assets including land, water, vegetation and atmosphere) into a flow of essential goods and services". Study on ecosystem services acts as an effective tool for gaining knowledge on ecosystem benefits and their sustained use.

Do You Know?

Robert Constanza and his colleagues estimated the value of global ecosystem services based on various parameters. According to them in 1997, the average global value of ecosystems services estimated was US $ 33 trillion a year. The updated estimate for the total global ecosystem services in 2011 is US $ 125 trillion / year, indicating a four-fold increase in ecosystem services from 1997 to 2011.

Mangrove Ecosystem Services

• Offers habitat and act as nursery for aquatic plants and animals
• Provides medicine, fuel wood and timber
• Act as bridge between sea and rivers by balancing sedimentation and soil erosion
• Help to reduce water force during cyclones, tsunamis and high tide periods
• Help in wind break, O₂ production, carbon sequestration and prevents salt spray from waves

Types of Ecosystem Services

Provisioning Services	Cultural Services	Supporting Services	Regulating Services
Food, fiber and fuel	Spiritual and religious values	Primary production	Invasion resistance
Genetic resources	Knowledge system	Provision of habitat	Herbivory pollination
Bio-chemicals	Education and inspiration	Nutrient cycling	Seed dispersal
Fresh water	Recreation and aesthetic values	Soil formation and retention	Climate regulation
Medicines	Ecotourism	Production of atmospheric oxygen	Pest regulation
		Water cycling	Disease regulation
			Erosion regulation
			Water purification
			Natural hazard protection

Human Activities that Disturb or Re-engineer an Ecosystem

• Habitat destruction
• Deforestation and over grazing
• Erosion of soils
• Introduction of non-native species
• Over harvesting of plant material
• Pollution of land, water and air
• Run off pesticides, fertilizers and animal wastes

Do You Know?

Ecosystem resilience: Ecosystem is damaged by disturbances from fire, flood, predation, infection, drought, etc., removing a great amount of biomass. However, ecosystem is endowed with the ability to resist the damage and recover quickly. This ability of ecosystem is called ecosystem resilience or ecosystem robustness.

How to Protect the Ecosystem?

It is a practice of protecting ecosystem at individual, organisational and governmental levels for the benefits of both nature and humans. Threats to ecosystems are many, like adverse human activities, global warming, pollution, etc. Hence, if we change our everyday life style, we can help to protect the planet and its ecosystem. "If we fail to protect environment, we will fail to save posterity".

• Buy and use only ecofriendly products and recycle them.
• Grow more trees.
• Choose sustained farm products (vegetables, fruits, greens, etc.).
• Reduce the use of natural resources.
• Recycle the waste and reduce the amount of waste you produce.
• Reduce consumption of water and electricity.
• Reduce or eliminate the use of house-hold chemicals and pesticides.
• Maintain your cars and vehicles properly (In order to reduce carbon emission).
• Create awareness and educate about ecosystem protection among your friends and family members and ask them to find out solution to minimise this problem.

Do You Know?

Go green – It refers to the changing of one's lifestyle for the safety and benefits of the environments (Reduce, Reuse, Recycle).

Way to go green and save green:

• Close the tap when not in use.
• Switch off the electrical gadgets when not in use.
• Never use plastics and replace them with biodegradable products.
• Always use ecofriendly technology and products.

"USE ECOSYSTEM BUT DON'T LOSE ECOSYSTEM; MAKE IT SUSTAINABLE"

7.2.12 Ecosystem Management

It is a process that integrates ecological, socio economic and institutional factors into a comprehensive strategy in order to sustain and enhance the quality of the ecosystem to meet current and future needs. Ecosystem management emphasis on human role in judicious use of ecosystem and for sustained benefits through minimal human impacts on ecosystems. Environmental degradation and biodiversity loss will result in depletion of natural resources, ultimately affecting the existence of human.

Do You Know?

"By 2025, at least 3.5 billion people, nearly 50% of the world's population are projected to face water scarcity." – IUCN.

"Forests house approximately 50% of global bio-diversity and at least 300 million people are dependent on forest's goods and services to sustain their livelihood." – IUCN

Importance of Ecosystem Management

• It is used to maintain biodiversity of ecosystems.
• It helps in indicating the damaged ecosystem (Some species indicate the health of the ecosystem: such species are called a flagship species).
• It is used to recognize the inevitability of ecosystem change and plan accordingly.
• It is one of the tools used for achieving sustainability of ecosystem through sustainable development programme (or projects).
• It is also helpful in identifying ecosystems which are in need of rehabilitation.
• It involves collaborative management with government agencies, local population, communities and NGO's.
• It is used to build the capacity of local institutions and community groups to assume responsibility for long term implementation of ecosystem management activities even after the completion of the project.

Do You Know? — Urban Ecosystem Restoration Model

Adayar Poonga is located in Chennai and covers an area around a total of 358 acres of Adayar creek and estuary, of which 58 acres were taken up for eco restoration under the auspices of Government of Tamil Nadu. It is maintained by Chennai Rivers Restoration Trust (CRRT). This was a dumping site previously. Presently it has 6 species of mangroves, about 170 species of littoral and tropical dry evergreen forests (TDF) which have successfully established as a sustainable ecosystem. Restoration of plants species has brought other associated fauna such as butterflies, birds, reptiles, amphibians and other mammals of the ecosystem. Currently Adayar Poonga functions as an environmental education Centre for school and college students and the public. The entire area stands as one of the best examples for urban eco restoration in the state of Tamil Nadu.

7.3 Plant Succession

We very often see that forests and lands in our areas are drastically affected by natural calamities (Flood, earthquake) and anthropogenic activities (Fire, over grazing, cutting of trees). Due to these reasons all plants of an area are destroyed and the areas become nude. When we observe this area, over a period of time we can see that it will be gradually covered by plant community again and become fertile.

Such successive replacement of one type of plant community by the other of the same area/place is known as plant succession. The first invaded plants in a barren area are called pioneers. On the other hand, a series of transitional developments of plant communities one after another in a given area are called seral communities. At the end a final stage and a final plant community gets established which are called as climax and climax community respectively.

7.3.1 Causes of Succession

Ecological succession is a complex process. There are three types of causes for any ecological succession:

Types of Causes

a. Initiating causes – Activity of abiotic (light, temperature, water, fire, soil erosion and wind) and biotic factors (competition among organisms) leads to formation of a barren area or destruction of the existing community of an area, initiating primary or secondary succession respectively.

b. Continuing causes – The processes of migration, aggregation, competition, reaction etc, are the continuing causes which lead to change the plant communities and nature of the soil in an area.

c. Stabilizing causes – The stabilization of the plant community in an area is primarily controlled by climatic factors rather than other factors.

7.3.2 Characteristics of Ecological Succession

• It is a systematic process which causes changes in specific structure of plant community.
• It is resultant of changes of abiotic and biotic factors.
• It transforms unstable community into a stable community.
• Gradual progression in species diversity, total biomass, niche specialisation, and humus content of soil takes place.
• It progresses from simple food chain to complex food web.
• It modifies the lower and simple life form to the higher life forms.
• It creates inter-dependence of plants and animals.

7.3.3 Types of Succession

1. Primary Succession

The development of plant community in a barren area where no community existed before is called primary succession. The plants which colonize first in a barren area is called pioneer species or primary community or primary colonies. Generally, Primary succession takes a very long time for the occurrence in any region.

Example: Microbes, Lichen, Mosses.

2. Secondary Succession

The development of a plant community in an area where an already developed community has been destroyed by some natural disturbance (Fire, flood, human activity) is known as secondary succession. Generally, this succession takes less time than the time taken for primary succession.

Example: The forest destroyed by fire and excessive lumbering may be re-occupied by herbs over a period of time.

Differences between Primary and Secondary Succession

S.No.	Primary Succession	Secondary Succession
1	Developing in a barren area	Developing in disturbed area
2	Initiated due to a biological or any other external factors	Starts due to external factors only
3	No soil, while primary succession starts	It starts where soil covers are already present
4	Pioneer species come from outside environment	Pioneer species develop from existing environment
5	It takes more time to complete	It takes comparatively less time to complete

3. Allogenic Succession

Allogeneic succession occurs as a result of abiotic factors. The replacement of existing community is caused by other external factors (soil erosion, leaching, etc.) and not by existing organisms.

Example: In a forest ecosystem soil erosion and leaching alter the nutrient value of the soil leading to the change of vegetation in that area.

4. Autotrophic Succession

If the autotrophic organisms like green plants are dominant during the early stages of succession it is called autotrophic succession. This occurs in the habitat which is rich in inorganic substances. Since green plants dominate in the beginning of this succession, there is a gradual increase in organic matter and subsequently the energy flow in the ecosystem.

7.3.4 Process of Succession

There are a number of sequential processes in primary autotrophic succession:

Sequential Processes

1. Nudation – This is the development of a barren area without any form of life. The barren area may be developed due to topographic (soil erosion, wind action), climatic (hails, storm, fire), and biotic (human activities, epidemics, etc.) factors.

2. Invasion – If species invade or reach a barren area from any other area it is called invasion. When the seeds, spores or other propagules of plant species reach the barren area, by air, water and various other agents, it is known as migration.

3. Ecesis (Establishment) – After reaching a new area (invasion), the successful establishment of the species, as a result of adjustment with the conditions prevailing in the area, is known as ecesis. If the establishment is complete, the plant will be able to reproduce sexually in that particular area.

4. Aggregation – The successful establishment of species, as a result of reproduction and increase in population of the species than the earlier stage is called aggregation.

5. Competition – It refers to the aggregation of a particular species in an area which leads to inter specific and intraspecific competition among the individuals for water, nutrient, radiant energy, CO₂, O₂ and space, etc.

6. Reaction – The species occupying a habitat gradually modify the environmental condition, where the existing species community is displaced or replaced by another. This is called reaction. The community which is replaced by another community is called seral community.

7. Stabilization (Climax stage) – The final establishment of plant community is called stabilization. This establishment of a plant community which maintains itself in equilibrium with climax of the area and not replaced by others is known as climax community and the stage is climax stage.

7.3.5 Classification of Plant Succession

Hydrosere

The succession in a freshwater ecosystem is also referred to as hydrosere. Succession in a pond, begins with colonization of the pioneers like phytoplankton and finally ends with the formation of climax community like forest stage. It includes the following stages:

Stages of Hydrosere

1. Phytoplankton stage – It is the first stage of succession consisting of the pioneer community like blue green algae, green algae, diatoms, bacteria, etc. The colonization of these organisms enrich the amount of organic matter and nutrients of pond due to their life activities and death. This favors the development of the next seral stages.

2. Submerged plant stage – As the result of death and decomposition of planktons, silt brought from land by rain water, lead to a loose mud formation at the bottom of the pond. Hence, the rooted submerged hydrophytes begin to appear on the new substratum. Example: Chara, Utricularia, Vallisneria and Hydrilla etc. The death and decay of these plants will build up the substratum of pond to become shallow. Therefore, this habitat now replaces another group of plants which are of floating type.

3. Submerged free-floating stage – During this stage, the depth of the pond will become almost 2-5 feet. Hence, the rooted hydrophytic plants and with floating large leaves start colonising the pond. Example: Rooted floating plants like Nelumbo, Nymphaea and Trapa. Some free-floating species like Azolla, Lemna, Wolffia and Pistia are also present in this stage. By death and decomposition of these plants, further the pond becomes shallower.

4. Reed-swamp stage – It is also called an amphibious stage. During this stage, rooted floating plants are replaced by plants which can live successfully in aquatic as well as aerial environment. Example: Typha, Phragmites, Sagittaria and Scirpus etc. At the end of this stage, water level is very much reduced, making it unsuitable for the continuous growth of amphibious plants.

5. Marsh meadow stage – When the pond becomes swallowed due to decreasing water level, species of Cyperaceae and Poaceae such as Carex, Juncus, Cyperus and Eleocharis colonise the area. They form a mat-like vegetation with the help of their much-branched root system. This leads to an absorption and loss of large quantity of water. At the end of this stage, the soil becomes dry and the marshy vegetation disappears gradually and leads to shrub stage.

6. Shrub stage – As the disappearance of marshy vegetation continues, soil becomes dry. Hence, these areas are now invaded by terrestrial plants like shrubs (Salix and Cornus) and trees (Populus and Alnus). These plants absorb large quantity of water and make the habitat dry. Further, the accumulation of humus with a rich flora of microorganisms produces minerals in the soil, ultimately favouring the arrival of new tree species in the area.

7. Forest stage – It is the climax community of hydrosere. A variety of trees invade the area and develop any one of the diverse types of vegetation. Example: Temperate mixed forest (Ulmus, Acer and Quercus), Tropical rain forest (Artocarpus and Cinnamomum) and Tropical deciduous forest (Bamboo and Tectona).

In the 7 stages of hydrosere succession, stage 1 is occupied by pioneer community, while the stage 7 is occupied by the climax community. The stages 2 to 6 are occupied by seral communities.

Lithosere

Lithosere is a type of xerosere initiating on a barren rock surface. The barren rock is devoid of water and organic matter. A barren rock surface gets mineral deposits due to weathering. This results in the colonization of pioneer organisms like crustose lichens. Through a series of successive seral stages, forest stage (Climax community) is achieved finally.

Stages of Lithosere

1. Crustose lichen stage – The pioneers like crustose lichens (Rhizocarpon and Lecanora) secrete some acids which enhance the weathering of rock. Due to this continuous process, small particles of rocks are formed, which together with decaying lichen make the first thin layer of soil on rock surface. However, this process is very slow. At the end, these habitats become less fit for existing plants and is gradually replaced by another type of lichens called foliose lichen.

2. Foliose lichen stage – Gradually crustose lichens are replaced by foliose lichen like Parmelia and Dermatocarpon etc. These plants have leaf like structures. They also secrete some acids which further loosen the rocks into small soil particles. This process enhances water retaining capacity of the habitat and causes further accumulation of soil particles and humus. Gradual changes make the area less favourable for existing foliose lichen.

3. Moss stage – When the habitat is changed, the existing foliose lichen starts disappearing and favours the growth of some xerophytic moss like Polytrichum, Tortula and Grimmia. The luxurious growth of moss competes with lichens. Due to the death and decay of mosses, further addition of humus and moisture to the habitat takes place. Therefore, the next seral community tries to replace the moss community.

4. Herb stage – With the gradual disappearance of moss stage, herbaceous plant communities like Aristida, Festuca and Poa, etc., invade the habitat. The extensive growth of these herbs alters the habitat. The decaying leaves, stems, root and other parts of these plants get deposited on the soil surface in the form of humus. It further increases the water holding capacity of soil. These conditions become more suitable for shrubs.

5. Shrub stage – The habitat changes results in the invasion of shrubs like Rhus, Zizyphus, Capparis and dominated by herbaceous plants. The death and decaying of shrubs further enrich the habitat with soil and humus. Therefore, the shrubs are replaced by trees which constitute the climax community.

6. Forest stage – The trees capable of growing in xerophytic condition try to invade the area which was occupied previously by shrubs. Further increasing the humus content of the soil favours the arrival of more trees and vegetation finally become mesophytes. As the trees are deeply rooted and much branched, they absorb more quantity of water and nutrients. After a long interval, a complete harmony is established among the plant communities. The climax stage remains unchanged unless some major environmental changes disturb it.

Of the 6 stages of lithosere succession, stage 1 is occupied by pioneer community and the stage 6 is occupied by climax community. The stages 2 to 5 are occupied by seral communities.

7.3.6 Significance of Plant Succession

• Succession is a dynamic process. Hence an ecologist can access and study the seral stages of a plant community found in a particular area.
• The knowledge of ecological succession helps to understand the controlled growth of one or more species in a forest.
• Utilizing the knowledge of succession, even dams can be protected by preventing siltation.
• It gives information about the techniques to be used during reforestation and afforestation.
• It helps in the maintenance of pastures.
• Plant succession helps to maintain species diversity in an ecosystem.
• Patterns of diversity during succession are influenced by resource availability and disturbance by various factors.
• Primary succession involves the colonization of habitat of an area devoid of life.
• Secondary succession involves the reestablishment of a plant community in disturbed area or habitat.
• Forests and vegetation that we come across all over the world are the result of plant succession.

7.4 Vegetation

Vegetation refers to the plant cover of an area. Geographically, India is a tropical country and also has strong monsoon climate and differs from other tropical regions of the World. India has four major climatic regions such as wet zone, intermediate zone, dry zone and arid zone, these regions are characterized by different types of natural vegetation. Nature of vegetation is also determined by several factors like altitude, types of plants, animals, climate, soil type, etc. Vegetation in Indian sub-continent is influenced by biotic factors and the existing human culture for a long time. The influence of man on plant formation and distribution is called anthropogenic effect on vegetation.

Tamil Nadu has a rich biodiversity right from the Gulf of Mannar to Western Ghats. Tamil Nadu shares the Western Ghats with states of Kerala, Karnataka, Goa, Maharashtra, Gujarat while Eastern Ghats is shared with the State of Andhra Pradesh. Of the 10 geographic zones in India, Coramandel (or) East Coast and Western Ghats are from Tamil Nadu.

7.4.1 Vegetation Types of India and Tamil Nadu

Vegetation of India and Tamil Nadu consists of variety of plant communities and also possesses rich bio-diversity. It is classified into the following four types:

• Forest vegetation
• Grassland vegetation
• Riparian vegetation
• Aquatic and semi aquatic vegetation

Forest Vegetation

Champion and Seth (1968) recognized a total of 16 forest types in India, whereas 9 types of them in Tamil Nadu.

I. Moist Tropical Forests

It is in the warmer plains. It is characterised by very dense, multi-storeyed diverse trees, shrubs, lianas and scrub jungles. These areas experience a high rainfall and dry climate. These are further classified into the following types on the basis of wetness.

1. Tropical wet evergreen forests
This type is found at an altitude of nearly 1500 m on the slopes of hills and mountains. These are also called tropical rain forests or tropical wet evergreen forests, where annual rainfall is more than 250 cm. Vegetation consists of luxuriantly growing huge trees of more than 45 meter in height, shrubs, lianas and abundant epiphytes. The common plants are Dipterocarpus, Artocarpus, Mangifera, Emblica and Ixora. These forests occur in Andaman and Nicobar Islands, Western Coasts, Aannaaimalai hills and Assam. This type is also found in western ghats of Thirunelveli, Kanyakumari, Annaaimalai Hills of Tamil Nadu.

2. Tropical semi-evergreen forests
This type occurs on the slopes of hills and mountain usually up to 1000 meter altitude. The annual rainfall in these forests is between 200 to 250 centimeter. Vegetation consists of luxuriantly growing evergreen species of giant trees and shrubs. The common tree species are Terminalia, Bambusa, Ixora, Artocarpus, Michelia, Eugenia, and Shorea. Orchids, ferns, some grasses, and herbs are also dominant. These forests are found in Western Coasts, Eastern Orissa and Upper Assam. This type is also present in Coimbatore, Tirunelveli and Kanyakumari District of Tamil Nadu.

3. Tropical moist deciduous forests
The annual rainfall of these forests is 100 to 200 cm with short dry periods. These are spread over an extensive part of the country. Many of the plants shed their leaves in the hot summer. Some are evergreen and semi-evergreen. The common plant species are Terminalia, Grewia, Adina, Melia, Albizzia, Dalbergia and Shorea. The most dominant plants are Tectona and Sal. These are found in Kerala, Karnataka, South Madhya Pradesh, northern parts of Uttar Pradesh, Bihar, Bengal, Orissa and Assam. This type is also present in Kanyakumari, Theni, Gudalur, Dindigul, Madurai and Nilgiris of Tamil Nadu.

4. Littoral and swamp forests
These include beach forests, tidal forests, mangrove forests and fresh water swamp forests.

a. Beach forests: These are found all along the sea coasts and river deltas. These areas have sandy soil which consists of large amount of lime and salts but poor in nitrogen and other mineral nutrients. The rainfall varies from 75 cm to 500 cm with moderate temperature. The common tree species are Casuarina, Borassus, Phoenix, Pandanus, Morinda and Thespesia with many twiners and climbers.

b. Tidal or mangrove forests: Tidal forests grow near the estuaries, swampy margins of islands and along sea coasts. The plants are halophytes characterized by the presence of stilt roots, pneumatophores and viviparous germinations of seed. The common plants are Rhizophora, Avicennia, and Sonneratia. These are found near sea coast, Gujarat, Ganges, delta regions of Mahanadhi, Godavari, Krishna, Sundarbans and Pulicat, Pichavaram, Ramanathapuram of Tamil Nadu.

c. Fresh water swamp forests: These forests grow in low lying land areas where rain or river water gets collected for some time. So, the water table is closer to the earth surface. The common plants are Salix, Acer, Ficus and all varieties of grasses and sedges. These forests are found in wetlands of Kanchipuram, Kanniyakumari of Tamil Nadu.

II. Dry Tropical Forests

These are classified into three types: Tropical dry deciduous forests, tropical thorn forests and tropical dry evergreen forests.

5. Tropical dry deciduous forests
These forests are found at about 400 to 800 meters Mean Sea Level (MSL). These forests are found in the areas where annual rainfall is usually low, ranging between 70 and 100 centimetres. The largest forest area of the country is occupied by tropical dry deciduous forest. The dry season is long and most of the trees remain leafless during this season. The forest trees are not dense, and grow up to 10 to 15 meter in height. The common plant species are Dalbergia, Diospyros, Terminalia, Acacia, Chloroxylon, Bauhinia and Zizyphus. Some common Climbers are Combretum, Hiptage; herbs like Abutilon, Achyranthes and Tribulus. These are found in Andhra Pradesh, Punjab, Uttar Pradesh, Bihar, Orissa, Madhya Pradesh and also found in all districts of Tamil Nadu at lower elevations.

6. Tropical thorn forests
These forests extend from plains upto 400 Meter. Occur in the areas where annual rainfall is between 20 and 70 centimeter. The dry season is hot and very long. The vegetation is of open type consisting of small trees (8 to 10 meter length) and thorny or spiny shrubs with a stunted growth. The plants remain leafless for most of the year and many species have latex. In rainy season, there is a luxuriant growth of ephemeral herbs and grass. The most common plant species are Acacia, Cassia, Calotropis, Albizzia, Zizyphus, Dichrostachys, Euphorbia, Capparis, and including unpalatable species. They are found in Karnataka, Andhra Pradesh, Maharashtra, South Punjab, most parts of Rajasthan and part of Gujarat and Thirunelveli in Tamil Nadu.

7. Tropical dry evergreen forests
This type of vegetation is found in areas where annual rainfall is in plenty but the dry season is comparatively longer. The trees are dense, evergreen, short and about 10-to-15-meter height. The common plant species are Manilkara, Walsura, Diospyros and Memexylon. These types of forests are found in the eastern parts of Tamil Nadu, East coast of Andhra Pradesh. They are also found in all coastal districts in Tamil Nadu from Thiruvallur to Nagapatinum districts.

III. Montane Subtropical Forests

This type of vegetation occurs in the areas with fairly high rainfall but where the climate is cooler than the tropical and warmer than the temperate forests. They are found in the altitude between 1000 meter and 2000 meter. The common plants are Eugenia, Syzygium and Toona are mostly evergreens. Many epiphytes including orchids and ferns are present. These are found in Nilgiri, Mahabaleswar, Assam and Manipur. In Eastern Ghats, it is found in the upper slopes and plateau of Shervaroys, Kollimalai and Pachamalai of Tamil Nadu.

These are further classified into:

• 8. Sub-tropical broad leaved hill forests (Tamil Nadu, Kerala, Karnataka and Assam)
• 9. Sub-tropical pine forests (Punjab, U.P and a part of Sikkim)
• 10. Sub-tropical dry evergreen forests (Shivaliks and foot hills of western Himalayas)

IV. Montane Temperate Forests

This type of vegetation occurs where humidity and temperature are comparatively low. These forests are very dense with an extensive growth of grass and evergreen trees of 15 – 45 meters tall. The common plants are Artocarpus, Balanocarpus, Pterocarpus, Myristica and woody climbers besides ferns and epiphytes. It is also called mountain wet temperate forests. They are found in mountains of Himalayas. These are further classified into:

• 11. Montane wet temperate forests
• 12. Himalayan moist temperate forests
• 13. Himalayan dry temperate forests

In Tamil Nadu montane forest is mostly confined to moist and sheltered valleys, glens and hollows as in the Anamalis, Nilgiri and Palani hills tops at above 1000 meter. They are known in Tamil as 'Sholas'. The common vegetation of sholas are Ilex, Syzygium, Michelia, Eurya and Rhododendron.

V. Sub-Alpine Forests

14. Sub-Alpine Forests
This type of vegetation is found in the altitude ranging between 2900 meter to 3500 meter, where snow fall occurs for several weeks in a year with less than 65 centimeter annual rainfall. Hence, strong winds and below 0 degree celsius temperature prevail for greater part of the year. The common tree species are Abies, Pinus, Betula, Quercus, Salix, Rhododendron with plenty of epiphytic orchids, mosses and lichens. They occur in Himalayas from Ladakh in the West to Arunachal in the East Bengal, Uttar Pradesh, Assam, Jammu and Kashmir.

VI. Alpine Scrub

This type of vegetation is found in the Himalayas at an altitude ranging from 3600 m to 4900 m. The height of the trees decreases with increasing altitudes. The common plants are small sized plants such as Sedum, Primula, Saxifraga, Rhododendron, Juniperus and with many types of lichens. These are further classified into:

• 15. Moist alpine scrubs
• 16. Dry alpine scrubs

2. Grassland Vegetation

Grassland refers to the vegetation community predominated by graminoids (that is grass and grass like plants). These are found in the altitude ranging from 150 to 2000 meter and above mean sea level. The major plant families are Poaceae, Cyperaceae, Fabaceae, Gentianaceae and Asteraceae. The grassland not only comprises plants but also serves as habitats to a variety of micro and macro fauna. Based on the range of altitude, grasslands are categorized into: low altitude grasslands and high-altitude grasslands.

Do You Know?

Grasslands created and maintained by human are called anthropogenic grasslands.

a. Low Altitude Grasslands

This type of grasslands are found at an altitude upto 1000 meter. The common plant species are Halopyrum, Wild Saccharum, Arundinella, Heteropogon and Chrysopogon. These types of grasslands are spread over coastal areas, riverline and alluvial areas of Deccan plateau, Chota Nagpur plateau, Gangetic, Brahmaputra valley and Eastern Ghats. In Tamil Nadu, these are found in the Eastern Ghats. These are scattered and intermixed with local forests. They are exposed to considerable biotic interference. Fire is common during dry months.

b. Higher Altitude Grasslands

This type of grasslands are found in altitude above 1000 meter. The common plants species are Chrysopogon, Arundinella, Andropogon, Heteropogon, Cymbopogon, Imperata, Festuca, and Agrostis. It is spread over the southern slopes of Himalayas, sub-Himalayan ranges, Nagaland, Himachal Pradesh and Western Ghats. In Tamil Nadu, these grasslands are found in higher regions of western ghats and are found between the sholas forest patches that occur in the depressions and furrows created by water courses flowing in these rolling downs are called as rolling grassland and also called shola grassland. It shows different types of vegetation like grasses, herbs, few shrubs and stunted trees.

Do You Know?

Existence of two climax communities under the influence of same climatic conditions are found in higher mountain hill tops, above 7000 feet MSL (Mean Sea Level) of Nilgiris. Example: Sholas and grasslands.

3. Riparian Vegetation

This type of vegetation is located along streams and rivers. The most common species are, Terminalia, Diospyros, Salix, Ficus and grasses. They are found on the banks of Godavari, Krishna, Ganga, Brahmaputra, Narmadha, Yamuna and riverbeds of Cauvery and Thamirabharani in Tamil Nadu.

Activity

Visit nearby forest and water bodies, observe the species found, describe and then identify the various types of vegetation.

4. Aquatic and Semi-Aquatic Vegetation

This type of vegetation is found in lakes, ponds, puddles and marshy places. The common plant species are Nelumbo, Nymphaea, Bacopa, Typha, Pandanus, Cyperus, Aeschynomene, Hydrilla, Aponogeton and Potomogeton. It is found in various parts of Tamil Nadu.

Summary

Key Points

• The interaction between biotic and abiotic components in an environment is called ecosystem.
• Autotrophs and heterotrophs are the producers and consumers respectively.
• The function of ecosystem refers to creation of energy, flow of energy and cycling of nutrients.
• The amount of light available for photosynthesis is called Photosynthetically Active Radiation. It is essential for increase in the productivity of ecosystem.
• The rate of biomass production per unit area/time is called productivity. It is classified as primary productivity, secondary productivity and community productivity.
• The transfer of energy in an ecosystem can be termed as energy flow. It is explained through the food chain, food web, ecological pyramids (pyramid of number, biomass and energy) and biogeochemical cycle.
• Cycling of nutrients between abiotic and biotic components is evident in the pond ecosystem, making itself self-sufficient and self-regulating.
• Ecosystem protected for the welfare of posterity is called ecosystem management.
• Successive replacement of one type of plant community by the other of the same area/place is known as plant succession.
• The first invaded plants in a barren (nude) area are called pioneers (pioneers' communities). A series of transitional developments of plant communities one after another in a given area are called seral communities.
• Succession is classified as primary succession, secondary succession, allogeneic succession and autotrophic succession.
• Plant succession is classified into hydrosere (Initiating on water bodies), Mesosere and xerosere. Further xerosere is subdivided into Lithosere (Initiating on a barren rock), Halosere and Psammosere.
• Vegetation refers to the plant cover of an area. Geographically, India and Tamil Nadu show tropical climate. Hence it has rich vegetation (Forest vegetation, Grassland vegetation, Riparian vegetation, Aquatic and semi aquatic vegetation).
• According to Champion and Seth (1968), forest vegetation of India and Tamil Nadu has been classified into 16 and 9 types respectively.

Evaluation

Exercise A. Choose the Most Suitable Answer

Choose the most suitable answer from the given four alternatives and write the option code and the corresponding answer.

1. Which of the following is not an abiotic component of the ecosystem?

a) Bacteria b) Humus c) Organic compounds d) Inorganic compounds

2. Which of the following is / are not a natural ecosystem?

a) Forest ecosystem b) Rice field c) Grassland ecosystem d) Desert ecosystem

3. Pond is a type of

a) forest ecosystem b) grassland ecosystem c) marine ecosystem d) fresh water ecosystem

4. Pond ecosystem is

a) not self-sufficient and self-regulating b) partially self-sufficient and self-regulating c) self-sufficient and not self-regulating d) self-sufficient and self-regulating

5. Profundal zone is predominated by heterotrophs in a pond ecosystem, because of

a) with effective light penetration b) no effective light penetration c) complete absence of light d) a and b

6. Solar energy used by green plants for photosynthesis is only

a) two to eight percentage b) two to ten percentage c) three to ten percentage d) two to nine percentage

7. Which of the following ecosystem has the highest primary productivity?

a) Pond ecosystem b) Lake ecosystem c) Grassland ecosystem d) Forest ecosystem

8. Ecosystem consists of

a) decomposers b) producers c) consumers d) all of the above

9. Which one is in descending order of a food chain

a) Producers → Secondary consumers → Primary consumers → Tertiary consumers b) Tertiary consumers → Primary consumers → Secondary consumers → Producers c) Tertiary consumers → Secondary consumers → Primary consumers → Producers d) Tertiary consumers → Producers → Primary consumers → Secondary consumers

10. Significance of food web is / are

a) it does not maintain stability in nature b) it shows patterns of energy transfer c) it explains species interaction d) b and c

11. The following diagram represents

a) pyramid of number in a grassland ecosystem b) pyramid of number in a pond ecosystem c) pyramid of number in a forest ecosystem d) pyramid of biomass in a pond ecosystem

12. Which of the following is / are not the mechanism of decomposition

a) Eluviation b) Catabolism c) Anabolism d) Fragmentation

13. Which of the following is not a sedimentary cycle

a) Nitrogen cycle b) Phosphorous cycle c) Sulphur cycle d) Calcium cycle

14. Which of the following are not regulating services of ecosystem services
i) Genetic resources   ii) Recreation and aesthetic values   iii) Invasion resistance   iv) Climatic regulation

a) i and iii b) ii and iv c) i and ii d) i and iv Exercise B. Short Answer Questions

15. Productivity of profundal zone will be low. Why?

16. Discuss the gross primary productivity is more efficient than net primary productivity.

17. Pyramid of energy is always upright. Give reasons.

18. Write some plants are found in sub alpine forest.

19. What will happen if all producers are removed from ecosystem?

20. Construct the food chain with the following data. Hawk, plants, frog, snake, grasshopper.

21. Name of the food chain which is generally present in all type of ecosystem. Explain and write their significance.

Exercise C. Long Answer Questions

22. Shape of pyramid in a particular ecosystem is always different in shape. Explain with example.

23. Generally human activities are against to the ecosystem, whereas you a student how will you help to protect ecosystem?

24. Generally in summer the forest are affected by natural fire. Over a period of time, it recovers itself by the process of successions. Find out the types of succession and explain.

25. Draw a pyramid from following details and explain in brief. Quantities of organisms are given - Hawks-50, plants-1000, rabbit and mouse-250 + 250, pythons and lizard- 100 + 50 respectively.

26. Various stages of succession are given below. From that rearrange them accordingly. Find out the type of succession and explain in detail.
Reed-swamp stage, phytoplankton stage, shrub stage, submerged plant stage, forest stage, submerged free floating stage, marsh meadow stage.

Reed-swamp stage Phytoplankton stage Shrub stage Submerged plant stage Forest stage Submerged free floating stage Marsh meadow stage

Glossary

Ecosystem Study of interaction between living and non-living components Standing quality Total inorganic substances present in any ecosystem at a given time and given area Standing crops Amount of living material present in a population at any time Biomass Can be measured as fresh weight or dry weight of organisms Benthic Bottom zone of the pond Trophic Refers to the position of organisms in food chain Omnivores Those that eat both plants and animals Food chain Refers to movement of energy from producers up to top carnivores Food web Interlocking pattern of food chain Pyramid of number Refers to number of organisms in a successive trophic level Pyramid of biomass Refers to quantitative relationship of the standing crops Pyramid of energy Refers to transformation of energy at successive trophic levels Ten percent law Refers to only 10 percent of energy being stored in each successive trophic level Biogeochemical cycle Exchange of nutrients between organisms and environments Carbon cycle Circulation of carbon among organisms and environments Guano It is accumulated excrement of sea birds and bats Phosphorus cycle Circulation of Phosphorus among organisms and environments Succession Successive replacement of one type of plant communities by other on barren or disturbed area Pioneers Invaded plants on barren area Primary succession Plants colonising on barren area Secondary succession Plants colonising on disturbed area Climax communities Final establishment of plant communities which are not replaced by others

ICT Corner — ECO SYSTEM

Steps:

• Type the URL or scan the QR code to open the activity page then Introduction page will open.
• Click on the Learn icon in the introduction page to know in detail.
• Click on the Flashcards icon in the introduction page to know about the topics easily.
• Click on the Test icon to write a quiz test — finally it displays the marks you scored.

URL: https://play.google.com/store/apps/details?id=com.ksolveecologyfree

* Pictures are indicative only

Unit IX — Plant Ecology

Book-back Exercises — Questions & Answers

Answers generated strictly from the Samacheer Kalvi textbook content. Attempt each question first, then click "Show Answer" to check.

I. Choose the Most Suitable Answer

1. Which of the following is not an abiotic component of the ecosystem?

a) Bacteria b) Humus c) Organic compounds d) Inorganic compounds

Answer: a) Bacteria

Explanation: Biotic (living) components include all living organisms like plants, animals, fungi and bacteria. Humus, organic compounds and inorganic compounds are all part of the abiotic (non-living) component.

2. Which of the following is / are not a natural ecosystem?

a) Forest ecosystem b) Rice field c) Grassland ecosystem d) Desert ecosystem

Answer: b) Rice field

Explanation: Rice field is an artificial/man-made ecosystem, whereas forest, grassland and desert ecosystems are natural ecosystems.

3. Pond is a type of

a) forest ecosystem b) grassland ecosystem c) marine ecosystem d) fresh water ecosystem

Answer: d) fresh water ecosystem

Explanation: A pond ecosystem is a classic example for natural, aquatic, freshwater, lentic type of ecosystem.

4. Pond ecosystem is

a) not self-sufficient and self-regulating b) partially self-sufficient and self-regulating c) self-sufficient and not self-regulating d) self-sufficient and self-regulating

Answer: d) self-sufficient and self-regulating

Explanation: The cycling of nutrients between abiotic and biotic components is evident in the pond ecosystem, making itself self-sufficient and self-regulating.

5. Profundal zone is predominated by heterotrophs in a pond ecosystem, because of

a) with effective light penetration b) no effective light penetration c) complete absence of light d) a and b

Answer: b) no effective light penetration

Explanation: The deeper region of a pond below the limnetic zone is called profundal zone with no effective light penetration and predominance of heterotrophs.

6. Solar energy used by green plants for photosynthesis is only

a) two to eight percentage b) two to ten percentage c) three to ten percentage d) two to nine percentage

Answer: b) two to ten percentage

Explanation: Out of the 56% of solar energy that reaches the earth's surface, only 2–10% of the solar energy is used by green plants for photosynthesis while the remaining portion is dissipated as heat.

7. Which of the following ecosystem has the highest primary productivity?

a) Pond ecosystem b) Lake ecosystem c) Grassland ecosystem d) Forest ecosystem

Answer: d) Forest ecosystem

Explanation: Primary productivity depends upon the plant species of an area, their photosynthetic capacity, availability of nutrients, solar radiation and other factors. Forest ecosystems, with dense multi-storeyed diverse vegetation, have the highest primary productivity among the listed options.

8. Ecosystem consists of

a) decomposers b) producers c) consumers d) all of the above

Answer: d) all of the above

Explanation: Biotic components include all living organisms forming the trophic structures — autotrophic components (producers) and heterotrophic components (consumers and decomposers).

9. Which one is in descending order of a food chain

a) Producers → Secondary consumers → Primary consumers → Tertiary consumers b) Tertiary consumers → Primary consumers → Secondary consumers → Producers c) Tertiary consumers → Secondary consumers → Primary consumers → Producers d) Tertiary consumers → Producers → Primary consumers → Secondary consumers

Answer: c) Tertiary consumers → Secondary consumers → Primary consumers → Producers

Explanation: In a food chain, energy flows from producers to primary consumers, then to secondary consumers, and finally to tertiary consumers. The descending order reverses this sequence.

10. Significance of food web is / are

a) it does not maintain stability in nature b) it shows patterns of energy transfer c) it explains species interaction d) b and c

Answer: d) b and c

Explanation: Food web is constructed to describe species interaction (direct interaction) and can be used to reveal different patterns of energy transfer in terrestrial and aquatic ecosystems. It also maintains stability (homeostasis) in nature.

11. The following diagram represents

a) pyramid of number in a grassland ecosystem b) pyramid of number in a pond ecosystem c) pyramid of number in a forest ecosystem d) pyramid of biomass in a pond ecosystem

Answer: c) pyramid of number in a forest ecosystem

Explanation: In a forest ecosystem, the pyramid of number is spindle shaped because the base (T1) is occupied by large sized trees (fewer in number), herbivores (T2) are more in number than producers, and tertiary consumers (T4) are fewer than secondary consumers (T3).

12. Which of the following is / are not the mechanism of decomposition

a) Eluviation b) Catabolism c) Anabolism d) Fragmentation

Answer: c) Anabolism

Explanation: The mechanisms of decomposition described in the chapter are Fragmentation, Catabolism, Leaching (Eluviation), Humification and Mineralisation. Anabolism is not listed as a mechanism of decomposition.

13. Which of the following is not a sedimentary cycle

a) Nitrogen cycle b) Phosphorous cycle c) Sulphur cycle d) Calcium cycle

Answer: a) Nitrogen cycle

Explanation: Gaseous cycle includes atmospheric Oxygen, Carbon and Nitrogen cycles. Sedimentary cycle includes the cycles of Phosphorus, Sulphur and Calcium which are present as sediments of earth.

14. Which of the following are not regulating services of ecosystem services: i) Genetic resources ii) Recreation and aesthetic values iii) Invasion resistance iv) Climatic regulation

a) i and iii b) ii and iv c) i and ii d) i and iv

Answer: c) i and ii

Explanation: Genetic resources belong to Provisioning services and Recreation and aesthetic values belong to Cultural services. Invasion resistance and Climate regulation are Regulating services of ecosystem.

II. Short Answer Questions (2-mark type)

15. Productivity of profundal zone will be low. Why?

Answer: The profundal zone is the deeper region of a pond below the limnetic zone. It has no effective light penetration and hence photosynthesis cannot take place. Since primary productivity through photosynthesis is absent, the overall productivity of the profundal zone is very low compared to the littoral and limnetic zones.

16. Discuss the gross primary productivity is more efficient than net primary productivity.

Answer: Gross Primary Productivity (GPP) is the total amount of food energy or organic matter or biomass produced in an ecosystem by autotrophs through photosynthesis. Net Primary Productivity (NPP) is the proportion of energy which remains after respiration loss in the plant (NPP = GPP − Respiration). Since NPP is always less than GPP (as respiratory losses are subtracted), GPP is always greater (more efficient) than NPP.

17. Pyramid of energy is always upright. Give reasons.

Answer: The pyramid of energy is always upright because the bottom of the pyramid is occupied by producers which have the maximum energy. There is a gradual decrease in energy transfer at successive trophic levels from producers to the upper levels. According to the ten percent law, only 10% of energy is stored at each successive trophic level while 90% is lost as heat, respiration and decomposition. Hence, energy always decreases upward, making the pyramid always upright.

18. Write some plants are found in sub alpine forest.

Answer: Sub-Alpine forests are found at an altitude ranging between 2900 m to 3500 m. The common tree species found in sub-alpine forests are Abies, Pinus, Betula, Quercus, Salix and Rhododendron, with plenty of epiphytic orchids, mosses and lichens.

19. What will happen if all producers are removed from ecosystem?

Answer: Producers (green plants) are the autotrophs that manufacture organic compounds through photosynthesis and occupy the first trophic level. They are the source of energy for all organisms from bacteria to human. If all producers are removed, there will be no food energy available for primary consumers (herbivores), secondary consumers, tertiary consumers and decomposers. The entire food chain and food web will collapse, leading to the extinction of all living organisms in the ecosystem.

20. Construct the food chain with the following data. Hawk, plants, frog, snake, grasshopper.

Answer: Plants → Grasshopper → Frog → Snake → Hawk

This is a grazing food chain where plants are the producers (T1), grasshopper is the primary consumer (T2), frog is the secondary consumer (T3), snake is the tertiary consumer (T4) and hawk is the top carnivore (T5). Energy flows from producers up to top carnivores in this linear network.

21. Name of the food chain which is generally present in all type of ecosystem. Explain and write their significance.

Answer: The Detritus food chain is generally present in all types of ecosystems. It begins with dead organic matter (detritus) which is an important source of energy. A large amount of organic matter is derived from dead plants, animals and their excreta. Energy is transferred through a series of organisms called detritivores – small carnivores – large (top) carnivores with repeated eating and being eaten. Its significance is that it plays a major role in recycling nutrients and decomposing dead organic matter, thereby maintaining the nutrient pool of any ecosystem.

22. Shape of pyramid in a particular ecosystem is always different in shape. Explain with example.

Answer: The shape of ecological pyramids varies with the type of ecosystem. The pyramid of number in grassland and pond ecosystems is always upright (gradual decrease in number from producers to consumers). In forest ecosystem, it is spindle shaped because trees (producers) are fewer, herbivores (T2) are more, and tertiary consumers are fewer again. In a parasite ecosystem, it is always inverted because it starts with a single tree and organism number increases at each successive level. The pyramid of biomass in a pond ecosystem is inverted (producers have least biomass), while in grassland and forest ecosystems it is upright.

23. Generally human activities are against to the ecosystem, where as you a student how will you help to protect ecosystem?

Answer: As a student, I can help protect the ecosystem by following these practices mentioned in the chapter: Buy and use only ecofriendly products and recycle them; grow more trees; reduce the use of natural resources; recycle waste and reduce the amount of waste produced; reduce consumption of water and electricity; reduce or eliminate the use of household chemicals and pesticides; never use plastics and replace them with biodegradable products; create awareness and educate friends and family members about ecosystem protection and ask them to find solutions to minimise environmental problems.

III. Detail Answer Questions (5-mark type)

24. Generally in summer the forest are affected by natural fire. Over a period of time, it recovers itself by the process of successions. Find out the types of succession and explain.

Answer: When a forest is destroyed by natural fire, the type of succession that occurs is Secondary Succession. Secondary succession is defined as the development of a plant community in an area where an already developed community has been destroyed by some natural disturbance such as fire, flood or human activity. Generally, this succession takes less time than primary succession because the soil cover is already present and pioneer species develop from the existing environment. Example: The forest destroyed by fire and excessive lumbering may be re-occupied by herbs over a period of time. The process involves nudation (barren area created by fire), invasion of new species, ecesis (establishment), aggregation, competition, reaction and finally stabilization into a climax community. The succession proceeds through seral communities until the original or a new climax community is re-established.

25. Draw a pyramid from following details and explain in brief. Quantities of organisms are given — Hawks-50, plants-1000, rabbit and mouse-250+250, pythons and lizard-100+50 respectively.

Answer: Based on the given data, this is a Pyramid of Number for a grassland ecosystem (upright pyramid).

Trophic levels and number of organisms:

• T1 – Producers (Plants): 1000
• T2 – Primary Consumers (Rabbit + Mouse): 250 + 250 = 500
• T3 – Secondary Consumers (Pythons + Lizard): 100 + 50 = 150
• T4 – Tertiary Consumers (Hawks): 50

The pyramid is upright in shape as there is a gradual decrease in the number of organisms from producers (1000) at the base to tertiary consumers (Hawks: 50) at the top. This demonstrates that in grassland ecosystems, the pyramid of number is always upright, as defined by Charles Elton (1927) who introduced the concept of ecological pyramids (Eltonian pyramids).

26. Various stages of succession are given below. From that rearrange them accordingly. Find out the type of succession and explain in detail. Reed-swamp stage, phytoplankton stage, shrub stage, submerged plant stage, forest stage, submerged free floating stage, marsh meadow stage.

Answer: The correct rearranged order is:

1. Phytoplankton stage → 2. Submerged plant stage → 3. Submerged free-floating stage → 4. Reed-swamp stage → 5. Marsh meadow stage → 6. Shrub stage → 7. Forest stage

This is Hydrosere — succession in a freshwater ecosystem (pond). It begins with pioneer communities like phytoplankton (blue green algae, green algae, diatoms) and ends with a climax community of forest stage. The phytoplankton stage enriches organic matter; submerged plants like Chara and Hydrilla follow; then rooted floating plants like Nelumbo and Nymphaea appear in the submerged free-floating stage. Reed-swamp stage brings amphibious plants like Typha; marsh meadow stage brings Cyperaceae and Poaceae species like Carex and Juncus. Shrub stage follows with plants like Salix and Alnus, and finally forest stage with trees like Ulmus, Acer and Quercus forms the climax community. Stages 2 to 6 represent seral communities, while stage 1 is the pioneer community and stage 7 is the climax community.

📝 Summary

1. An ecosystem is defined as "the structural and functional unit of ecology" (Odum, 1962), a term first proposed by A.G. Tansley (1935), and consists of the integration of all living and non-living factors of the environment.
2. Ecosystems have two major components: Abiotic (non-living) components such as climatic factors, edaphic factors, topography, and inorganic substances, and Biotic (living) components including plants, animals, fungi, and bacteria.
3. Biotic components are classified into autotrophic components (producers like green plants that manufacture organic compounds through photosynthesis) and heterotrophic components (consumers and decomposers that depend on producers for nutrition).
4. Decomposers such as Bacteria, Actinomycetes, and Fungi break down dead plants and animals to release organic and inorganic nutrients back into the environment, which are reused by plants, completing the nutrient cycle.
5. Key ecological concepts covered in this chapter include ecological pyramids (number, biomass, and energy), carbon and phosphorus cycles, pond ecosystem as a self-sufficient system, plant succession, ecosystem services, and vegetation types of India and Tamil Nadu.