CBSE Class 12 Biology Ecosystem Worksheet Set A

Very Short Answer Type Questions

Question. Under what conditions would a particular stage in the process of succession revert back to an earlier stage?
Answer : Natural disturbances like fire, flood or any other natural disaster and anthropogen activities can revert back to an earlier stage of succession.

Question. Define self sustainability
Answer : Self sustainability is the maintenance of an ecosystem itself or naturally. i.e., A system that maintain itself by its own independent efforts is a self-sustainable ecosystem.

Question. Given below is a figure of an ecosystem. Answer the following questions? (i) What type of ecosystem is shown in the figure? (ii) Name any plant that is characteristic of such ecosystem.
Answer : (i) It is a tropical deciduous forest ecosystem. (ii) In India this type of forest ecosystem is characterised by Tectona, Dipterocarpus Jamun, Amla, Palas, mahua and Semul plants.

Question. What is common to earthworm, mushroom, soil mites and dung beetle in an ecosystem.
Answer : They all are designated as primary consumer in a detritus food chain and are termed as detritivores.

Question. Arrange the following as observed in vertical stratification of a forest; Grass, Shrubby plants, Teak, Amaranths.
Answer : Vertical stratification (dispersion) of species in a forest ecosystem is arranged as grass (floor of forest)® Amaranthus® Shrubby plants® Teak (tree).

Question. Name an omnivore which occurs in both grazing food chain and the decomposer food chain.
Answer : Cockroaches and crow are two omnivore, that are present in both grazing or predator and detritus food chain.

Question. Is the common edible mushroom an autotroph or a heterotroph?
Answer : Common edible mushroom (Agaricus) is achlorophyllous (does not possess chlorophyll) and is a heterotroph.

Question. Why are oceans least productive?
Answer : Oceans are least productive because (i) There is insufficient radiation as sunlight decreases with the increasing depth of the ocean. (ii) Oceans are nitrogen deficient which is an important nutrient for plants. (iii) Conditions of high salinity which is not favourable for all plants. (iv) there is no substratum to support plants.

Question. Justify the pitcher plant as a producer.
Answer : Pitcher plant (Napenthes) is an insectivores plant, which is chlorophyllus and capable of trapping solar radiations for chemical energy for photosynthesis. It is infectivorous plant which are produced in the soil lacking nitrogen thus to make up their nitrogen deficiency they trap insects.

Question. Name any two organisms which can occupy more than one trophic level in an ecosystem.
Answer : Human being (man) and birds (sparrow) can be frequently seen in more than one trophic level, in an ecosystem.

Question. In the North East region of India, during the process of jhum cultivation, forests are cleared by burning and left for regrowth after a year of cultivation. How would you explain the regrowth of forest in ecological term?
Answer : Forests that are cleared by burning and left for regrowth will show secondary succession. Since, soil is already present, the buried seeds oen to germinate. Due to wind dispersal and some other natural forces, new seeds will be brought into the area and new species will colonise the forest again.

Question. Climax stage is achieved quickly in secondary succession as compared to primary succession. Why?
Answer : The rate of ecological succession is quicker in secondary succession as compared to primary succession as the soil (substratum) is already available for further succession. While in primary succession, the succession starts from bare rocks. Which take time for first colonisation by plants as there is no nutrient holding mechanisms.

Question. Name an organism found as secondary carnivore in an aquatic ecosystem.
Answer : In an aquatic ecosystem, food chain can be drawn as follows Producer (Phytoplankton) ® Primary consumer (Zooplankton) ® Secondary consumer (small fish and water beetle) (Ist to carnivore)® Tertiary consumer (IInd carnivore like large fish, water birds like fowl and duck). So, the large fish and water birds like ducks and water fowls which are the tertiary consumers in an aquatic ecosystem occupy a position of secondary carnivore.

Question. Why are nutrient cycles in nature called biogeochemical cycles?
Answer : Nutrient cycles are called biogeochemical cycles because ions/molecules of a nutrient are transferred from the environment (rocks, air and water) to organisms (life) and then brought back to the environment in a cyclic pathway. The literal meaning of biogeochemical is bioliving organism and georocks, air and water.

Question. Give any two examples of xerarch succession.
Answer : Xerarch succession of ecological communities originates in extremely dry conditions such as sand deserts and rock deserts (as there is no water and the substratum does not absorb rain water).

Question. What does the base tier of the ecological pyramid represent?
Answer : The base tier of the ecological pyramids represent the producers or first trophic level in case of three ecological pyramids
(i) Pyramid of number (ii) Pyramid of biomass
(iii) Pyramid of energy

Question. Among bryophytes, lichens and fern which one is a pioneer species in a xeric succession?
Answer : The species that invade a bare area are called pioneer species. In a xeric succession, the pioneer species are usually lichens then bryophytes which are succeeded by ferns and some other bigger plants. Lichen produces lichen acid and carbonic acid which corrodes rock surface and release minerals required for growth. The corroded rock accumulate soil particle by wind and provides substrate for Bryophytes and ferms.

Question. What is the ultimate source of energy for the ecosystems?
Answer : Solar radiation is the ultimate source of energy for the ecosystem except deep hydrothermal ecosystem.

Question. Why is the rate of assimilation of energy at the herbivore level called secondary productivity?
Answer : The rate of assimilation of energy at herbivore level is called secondary productivity because the biomass available to the organisms of next trophic level (consumer) for further consumption is a resultant of the primary productivity which is formed by autotrophs (plants).

Question. Under what conditions would a particular stage in the process of succession revert back to an earlier stage? 
Answer. Natural or human induced disturbances like fire, deforestation, etc.

Question.Arrange the following as you observe in vertical stratification of a forest—Grass, Shrubby plants, Teak, Amaranthus. 
Answer : Grass, Amaranthus, Shrubby plants, Teak.

Question. Name the pioneer species:
(i) on a bare rock
(ii) in a water body
Answer. (i) Lichens
(ii) Phytoplanktons 

Question. Define primary production.
Answer. It is defined as the amount of biomass or organic matter produced per unit area over a certain time period by plants during photosynthesis.

Question. What is meant by humification?
Answer. The process of formation of humus from detritus or dead organic matter is called humification.

Question. Why green plants are not found beyond a certain depth in the ocean?
Answer. Beyond a certain depth in the ocean, sunlight is not able to penetrate. Due to which green plants cannot photosynthesise and thus, do not survive.

Question. Why is an earthworm called a detritivore? 
Answer. This is because earthworm breaks down detritus into smaller particles.

Question. “Man can be a primary as well as a secondary consumer.” Justify this statement.
Answer. Man has a varied diet. When on vegetarian diet, they are primary consumers and when on nonvegetarian diet, they are secondary consumers.

Question. Write the equation that helps in deriving the net primary productivity of an ecosystem.
Answer. GPP – R = NPP

Question. What is secondary productivity?
Answer. Secondary productivity is defined as the rate of formation of new organic matter by consumers.

Important Questions for NCERT Class 12 Biology Ecosystem

Question. The rate of biomass production is called productivity.
It may be primary or secondary productivity. Primary productivity does not depends on
(a) Plants species inhabitating a particular area
(b) Predation
(c) Environmental factors
(d) Photosynthetic capacity

Answer : B

Question. Which among the following is not considered as an ecological service?
(a) Generation of fertile soil
(b) Wild life habitat formation
(c) Products of antibiotics
(d) Crop pollination

Answer : C

Question. Which among the following factors are most important climatic factors that regulate decomposition through their effects on the activities of soil microbes
(a) Temp & soil moisture
(b) Temp and pH of soil
(c) Temp and oxygen
(d) pH of soil and oxygen

Answer : A

Question. Decomposition is one of the important functional aspect of ecosystem. Which of the following statement is not correct for decomposition ?
(a) Warm and moist environment favours decomposition
(b) Nitrogen and sugar component favours decomposition
(c) Low temperature and anaerobiosis favours decomposition
(d) Decomposition is largely an oxygen requiring process

Answer : C

Question. Robert Constanza and his colleagues have put price tags on natures life support services with refrence to this which of the following is not true?
(a) Fundamental ecosystem services = $ 33 trillions
(b) Soil formation = $ 16.5 trillions
(c) Nutrient cycling = less than $ 3.3 trillions
(d) Climate regulation = $ 5 trillions

Answer : D

Question. The annual net primary productivity of the whole biosphere is approximately
(a) 170 billion tons
(b) 50 billion tons
(c) 55 billion tons
(d) 710 billion tons

Answer : A

Question. By digestion and pulverisation detritus get fragmented. This fragmentation step of decomposition helps in
(a) Increasing porocity of detritus
(b) Increasing surface area of detritus
(c) Increasing rate of sedimentation
(d) All of these

Answer : B

Ques. Pyramid of numbers deals with number of
(a) species in an area
(b) individuals in a community
(c) individuals in a trophic level
(d) subspecies in a community. 

Answer: C

Ques. Pyramid of numbers in a pond ecosystem is
(a) irregular
(b) inverted
(c) upright
(d) spindle shaped.

Answer: C

Ques. Which of the following would appear as the pioneer organisms on bare rocks?
(a) Mosses
(b) Green algae
(c) Lichens
(d) Liverworts

Answer: C

Ques. During ecological succession
(a) the numbers and types of animals remain constant
(b) the changes lead to a community that is in near equilibrium with the environment and is called pioneer community
(c) the gradual and predictable change in species composition occurs in a given area
(d) the establishment of a new biotic community is very fast in its primary phase. 

Answer: C

Ques. Secondary succession takes place on/in
(a) newly created pond
(b) newly cooled lava
(c) bare rock
(d) degraded forest. 

Answer: D

Ques. The second stage of hydrosere is occupied by plants like
(a) Azolla
(b) Typha
(c) Salix
(d) Vallisneria.

Answer: D

Ques. Which one of the following statements is correct for secondary succession?
(a) It begins on a bare rock.
(b) It occurs on a deforested site.
(c) It follows primary succession.
(d) It is similar to primary succession except that it has a relatively fast pace. 

Answer: B

Ques. Both hydrarch and xerarch successions lead to
(a) medium water conditions
(b) xeric conditions
(c) highly dry conditions
(d) excessive wet conditions. 

Answer: A

Ques. The correct sequence of plants in a hydrosere is
(a) Volvox → Hydrilla → Pistia → Scirpus → Lantana → Oak
(b) Pistia → Volvox → Scirpus → Hydrilla → Oak → Lantana
(c) Oak → Lantana → Volvox → Hydrilla → Pistia → Scirpus
(d) Oak → Lantana → Scirpus → Pistia → Hydrilla → Volvox. 

Answer: A

Ques. An ecosystem which can be easily damaged but can recover after some time if damaging effect stops will be having
(a) low stability and high resilience
(b) high stability and low resilience
(c) low stability and low resilience
(d) high stability and high resilience. 

Answer: A

Ques. The primary succession refers to the development of communities on a
(a) forest clearing after devastating fire
(b) newly-exposed habitat with no record of earlier vegetation
(c) freshly cleared crop field
(d) pond, freshly filled with water after a dry phase. 

Answer: B

Ques. Natural reservoir of phosphorus is
(a) rock
(b) fossils
(c) sea water
(d) animal bones.

Answer: A

Ques. Which one of the following is not a gaseous biogeochemical cycle in ecosystem?
(a) Sulphur cycle
(b) Phosphorus cycle
(c) Nitrogen cycle
(d) Carbon cycle

Answer: B

Ques. About 70% of total global carbon is found in
(a) oceans
(b) forests
(c) grasslands
(d) agroecosystems.

Answer: A

Ques. Which of the following pairs is a sedimentary type of biogeochemical cycle?
(a) Phosphorus and nitrogen
(b) Phosphorus and sulphur
(c) Oxygen and nitrogen
(d) Phosphorus and carbon dioxide 

Answer: B

Ques. Match the following and select the correct option.
A. Earthworm (i) Pioneer species
B. Succession (ii) Detritivore
C. Ecosystem service (iii) Natality
D. Population growth (iv) Pollination
      A   B    C    D
(a) (i) (ii)  (iii) (iv)
(b) (iv) (i) (iii) (ii)
(c) (iii) (ii) (iv) (i)
(d) (ii) (i) (iv) (iii

Answer: D

Short Answer Questions 

Question. “Decomposition is an oxygen requiring process” Comment.
Answer. Detritus is rich in nitrogen and sugars. For oxidation of nitrogen and sugars oxygen is required by a class of aerobic microbes.

Question. What is an incomplete ecosystem? Explain with the help of a suitable example.
Answer. An ecosystem is a functional unit with biotic and abiotic factors interacting with one another resulting in a physical structure. Absence of any component will make an ecosystem incomplete as it will hinder the functioning of the ecosystem. Examples of such an ecosystem can be a fish tank or deep aphotic zone of the oceans where producers are absent.

Question. State the difference between the first trophic levels of detritus food chain and grazing food chain.
Answer. Differences between first trophic level of detritus food chain and grazing food chain

Question. Sometimes due to biotic/abiotic factor the climax remain in a particular seral stage (preclimax) without reaching the climax. Do you agree with this statement. If yes, give a suitable example. 
Answer. It is true that any change in the abiotic/biotic factor will arrest a particular seral stage leading to a pre-climax condition before the climax stage is achieved. This can happen in cases of forest fires,landslide, change in soil characteristics, increase in herbivore population leading to over-grazing.

Question. Primary productivity varies from ecosystem to ecosystem. Explain. 
Answer. Primary productivity varies from ecosystem to ecosystem because it depends on the plant species inhabiting the area and their photosynthetic activity. It also depends on various environmental factors, which vary in different ecosystems.

Question. How does primary succession start in water and lead to the climax community? Explain.
Answer. Primary succession in water
- The pioneer species are phytoplanktons.
- The phytoplanktons are replaced by free-floating angiosperms
- Then, rooted angiosperms invade sedges, grasses and finally the trees.
- At last, a stable climax forest is formed.
 -An aquatic habitat is converted into mesic habitat. 

Question. What are the shortcomings of ecological pyramids in the study of ecosystem?
Answer. The ecological pyramid assumes a simple food chain and does not accommodate food webs.
Thereby, it does not take into account the fact that species may belong to two or more trophic levels at a time. Also saprophytes despite their vital role in ecosystem are given no place in the ecological pyramids.

 Long Answer Questions

Question. Citing lake as an example of a simple aquatic ecosystem, interpret how various functions of this ecosystem are carried out. Make a food chain that is functional in this ecosystem.
Answer. (i) Productivity: Conversion of inorganic into organic material with the help of solar energy by the autotrophs.
(ii) Energy flow: Unidirectional movement of energy towards higher trophic level (and its dissipation and loss as heat to the environment).
(iii) Decomposition: Fragmentation, leaching, catabolism, humification, mineralization by bacteria, fungi and flagellates (abundant at the bottom of lake).
(iv) Nutrient cycling: Decomposition of dead matter to release the nutrients back to be re-used by the autotrophs.
Food chain in aquatic ecosystem (lake):
Phytoplanktons → Zooplanktons → Small fish → Big fish (Any other appropriate example).

Question. (a) Draw a simplified model of phosphorus cycling in a terrestrial ecosystem.
(b) Write the importance of such cycles in ecosystems.
Answer. (a) 

(b) Such cycles recycle nutrients again and again and maintain the balance in ecosystem.

Question. Draw a pyramid of biomass and pyramid of energy in sea. Give your comments on the type of pyramids drawn.
Answer. 

The pyramid of biomass in sea is inverted.
The pyramid of energy in sea is upright.

Question. Define ecological succession. Give three differences between seral stages and climax community during succession.
Answer. The sequential, gradual and predictable changes in the species composition in an area are called succession or ecological succession.
Differences between seral stages and climax community

Question. What are the limitations of ecological pyramids?
Answer. Limitations of ecological pyramids:
(ii) It assumes a simple food chain, which never exists in nature. It does not accommodate a food web.
(iii) In spite of the vital role played by saprophytes/decomposers, they are not given any position in ecological pyramids.

Question. (a) Explain primary productivity and the factors that influence it.
(b) Describe how do oxygen and chemical composition of detritus control decomposition.
Answer. (a)
- The rate of biomass or organic matter production per unit area, over a time period, by plant during photosynthesis is called productivity.
- It is expressed in kcal m^–2 yr^–1 or g/m²/yr or g m^–2 y^–1.
- The amount of biomass or organic matter produced per unit area over a time period in plants during photosynthesis is called primary production. It is expressed in terms of weight (g m^–2) or energy (kcal m^–2).
- The rate of formation of new organic matter by consumers is called secondary productivity.
(b) Decomposition of detritus is slow if it contains lignin, chitin, tannins and cellulose, whereas it is quicker if detritus is made up of nitrogenous compounds and water-soluble substances like sugars. This is because the latter are easy to degrade.
Oxygen is required for the activity of decomposers and detritivores. Therefore, a reduced oxygen amount will slow down the process of decomposition.

Question. Draw and complete the following model of carbon cycle filling a, b, c, d, e and f.

Answer. (a) Respiration
(b) Photosynthesis
(c) Respiration
(d) Combustion of fossil fuels
(e) Aquatic food chain
(f) Coal, oil.

Question. (a) Colonisation of a rocky terrain is a natural process. Mention the group of organisms which invade this area first. Give an example.
(b) Over the years, it has been observed that some of the lakes are disappearing due to urbanisation. In absence of human interference, depict by making a flow chart, how do the successional series progress from hydric to mesic condition.
(c) Identify the climax community of hydrarch and xerarch succession.
Answer. (a) Pioneer species invade the area first. For example, lichen
(b) Phytoplankton – (hydric) → Submerged plant stage → Submerged free floating plant stage
→ Reed swamp stage → Marsh – meadow stage → Scrub stage → Forest stage (Mesic condition)
(c) Forest is the climax community for both successions.

1 What does the base tier of the ecological pyramid represent?

2 In the North East region of India, during the process of jhum cultivation, forests are cleared by burning and left for regrowth after a year of cultivation. How would you explain the regrowth of forest in ecological term?

3 Climax stage is achieved quickly in secondary succession as compared to primary succession. Why?

4 Why are oceans least productive?

5 Organisms at a higher trophic level have less energy available. Comment.

6 What are the shortcomings of ecological pyramids in the study of ecosystem?

7 How do you distinguish between humification and mineralisation?

8 List three parameters used for constructing ecological pyramids. Describe any one instance where the pyramid may look inverted.

9 Differentiate between a detritivore and a decomposer giving an example of each.

10 Construct an ideal pyramid of energy when 1,00,000 joules of sunlight is available. Label all its trophic levels.

11 Construct a pyramid of biomass starting with phytoplanktons. Label 3 trophic levels. Is the pyramid upright or inverted? Why?

12 Explain with the help of two examples how the pyramid of number and the pyramid of biomass can look inverted.

13 Justify the given statements:

a) Energy flow in an ecosystem is unidirectional

b) Secondary succession is faster than primary succession

14 What will happen to an ecosystem if:

a) All producers are removed

b) All organisms of herbivore level are eliminated

c) All top carnivore population is removed 

An ecosystem is a functional unit of nature, where living organisms interact each other and with the physical environment.

ECOSYSTEM – STRUCTURE & FUNCTION

Types of ecosystems
• Terrestrial ecosystem: Forest, grassland, desert etc.
• Aquatic ecosystem: Pond, lake, wetland, river & estuary.
• Man-made ecosystem: Crop fields and aquarium.
- Entire biosphere is regarded as global ecosystem.
- In an ecosystem, biotic and abiotic components interact and function as a unit.
- Vertical distribution of different species occupying different levels is called stratification. E.g. in a forest, trees occupy top strata (layer), shrubs the second and herbs & grasses the bottom layers.

Pond (Aquatic ecosystem)
A pond is a shallow, simple, self-sustainable water body that exhibits all basic components of an ecosystem.

• Abiotic components: Water and soil deposit.
• Climatic conditions: Solar input, cycle of temperature, day-length etc.
• Autotrophic components: Phytoplankton, some algae and the floating, submerged and marginal plants.
• Consumers (heterotrophs): Zooplankton, free swimming and bottom dwelling forms.
• Decomposers: Fungi, bacteria and flagellates. Pond performs all the functions of an ecosystem. E.g.
o Autotrophs convert inorganic into organic material using solar radiant energy.
o Heterotrophs consume the autotrophs.
o Decomposition and mineralization of the dead matter to release them back for reuse by the autotrophs.

4 basic components of functioning of an ecosystem:
1) Productivity
2) Decomposition
3) Energy flow
4) Nutrient cycling

1. PRODUCTIVITY

Solar energy is the basic requirement for an ecosystem to function and sustain.
- Amount of biomass (organic matter) produced per unit area over a time period by plants during photosynthesis is called primary production. It is expressed in weight (g–2) or energy (kcal m–2).
- The rate of biomass production is called productivity. It is expressed in g–2 yr–1 or (kcal m–2) yr–1.
- It is divided into gross primary productivity (GPP) and net primary productivity (NPP).

- Gross primary productivity (GPP): It is the rate of production of organic matter during photosynthesis. A considerable amount of GPP is used by plants in respiration.

- Net primary productivity (NPP): It is the available biomass for the consumption to heterotrophs (herbivores & decomposers). i.e., NPP is the Gross primary productivity minus respiration losses (R).

NPP = GPP – R

- Secondary productivity: It is the rate of formation of new organic matter by consumers.
- Primary productivity varies in different ecosystems because it depends on
o The plant species inhabiting an area.
o Environmental factors.
o Availability of nutrients.
o Photosynthetic capacity of plants.

- Annual net primary productivity of whole biosphere is about 170 billion tons (dry weight) of organic matter. Of this, despite occupying about 70 % of the surface, the productivity of the oceans is only 55 billion tons.

2. DECOMPOSITION

- It is the breakdown of complex organic matter by decomposers into inorganic substances like CO₂, water and nutrients. It is largely an oxygen-requiring process.
- Raw material for decomposition is called Detritus. E.g. dead plant remains (leaves, bark, flowers etc.), dead remains of animals, fecal matter etc.

Steps of decomposition
a. Fragmentation: It is the breakdown of detritus into smaller particles by detritivores (e.g. earthworm).
b. Leaching: Water soluble inorganic nutrients go down into soil horizon and precipitate as unavailable salts.
c. Catabolism: Degradation of detritus into simpler inorganic substances by bacterial and fungal enzymes.
The above three processes occur simultaneously.
d. Humification: Accumulation of humus (dark amorphous substance) in soil. Humus is resistant to microbial action and so decomposes very slowly. Being colloidal, it serves as a reservoir of nutrients.
e. Mineralization: It is the release of inorganic nutrients due to the degradation of humus by some microbes.

Factors influencing decomposition
• Chemical composition of detritus:
o Decomposition is slow in detritus rich in lignin & chitin.
o It is quicker in detritus rich in nitrogen and water-soluble substances like sugars.
• Climatic factors (temperature & soil moisture):
o Warm and moist environment favour decomposition.
o Low temperature & anaerobiosis inhibit decomposition resulting in buildup of organic material 

3. ENERGY FLOW

- Sun is the only source of energy for all ecosystems (except deep sea hydro-thermal ecosystem).
- Of the incident solar radiation, less than 50% is photosynthetically active radiation (PAR).
- Plants and photosynthetic bacteria (autotrophs), fix solar radiant energy to make food.
- Plants capture only 2-10% of the PAR. This energy sustains the entire living world.
- Ecosystems obey 2nd Law of thermodynamics. They need a constant supply of energy to synthesize the molecules. It helps to counteract the entropy.

Producers (Autotrophs):
- These are organisms that synthesize food.
- In a terrestrial ecosystem, major producers are herbaceous and woody plants. Primary producers in an aquatic ecosystem are phytoplankton, algae and higher plants.
- The energy trapped by the producer is passed on to a consumer or the organism dies.

Consumers (heterotrophs):
- These are animals that directly or indirectly depend on plants for food. They include:
o Primary consumers (herbivores): Feed on plants. E.g. insects, birds, mammals, molluscs etc.
o Secondary consumers (primary carnivores): Feed on herbivores. E.g. frog, fox, man etc.
o Tertiary consumers (secondary carnivores): Feed on primary carnivores. E.g. tiger, lion etc.
- The chain of feeding relationship between different organisms is called a food chain. It is 2 types:
• Grazing Food Chain (GFC): Here, primary consumer feeds on living plants (producer). E.g.

Grass -----→             Goat ------→                Man -------→
Producer      (Primary Consumer)     (Secondary Consumer)

• Detritus Food Chain (DFC): Here, primary consumer feeds on dead organic matter (detritus). Death of organism is the beginning of the DFC.
- Detritus is made up of decomposers (saprotrophs) such as fungi & bacteria. They secrete digestive enzymes that breakdown detritus into simple, inorganic materials, which are absorbed by them. Thus, they get energy & nutrients.
- In an aquatic ecosystem, GFC is the major conduit for energy flow.
- In a terrestrial ecosystem, a much amount of energy flows through the DFC than through the GFC.
- DFC may be connected with GFC at some levels. Some organisms of DFC are prey to the GFC animals. Some animals (cockroaches, crows, human etc.) are omnivores. Such interconnections of food chains are called food web.
- A specific place of organisms in the food chain is known as their trophic level.

- The amount of energy decreases at successive trophic levels. When an organism dies it becomes dead biomass (detritus). It is an energy source for decomposers.
- Organisms at each trophic level depend on those at the lower trophic level for their energy.
- The amount of living material in a trophic level at a given time is called standing crop. It is measured as the biomass (mass of living organisms) or the number in a unit area.
- Biomass of a species is measured in terms of fresh or dry weight. Dry weight is more accurate because it is the exact mass of body which remains constant.
- Number of trophic levels in GFC is restricted as it follows 10% law (only 10% of energy is transferred to each trophic level from the lower trophic level).

ECOLOGICAL PYRAMIDS

- The representation of a food chain in the form of a pyramid is called ecological pyramid.
- The base of a pyramid represents producers (first trophic level). The apex represents tertiary or top-level consumer.
- Ecological pyramids are 3 types: Pyramid of number, Pyramid of biomass and Pyramid of energy.

a) Pyramid of number: E.g. grassland ecosystem.

b) Pyramid of biomass: It shows a sharp decrease in biomass at higher trophic levels.

c) Pyramid of energy: Primary producers convert only 1% of the energy in the sunlight available to them into NPP.

- Any calculations of energy content, biomass, or numbers has to include all organisms at that trophic level.
- A trophic level represents a functional level, not a species as such. A species may occupy more than one trophic level in the same ecosystem at the same time. E.g. A sparrow is a primary consumer when it eats seeds, fruits, peas. It is a secondary consumer when it eats insects & worms.
- In most ecosystems, all the pyramids are upright, i.e., producers are higher in number, biomass and energy than the herbivores, and herbivores are higher in number, biomass and energy than the carnivores.
- But in some cases, inverted pyramids for number and biomass are present.
- Inverted pyramid of number: E.g. Insects feeding on a tree.
- Inverted pyramid of biomass: E.g.
o Small standing crop of phytoplankton supports large standing crop of zooplankton.
o Pyramid of biomass in sea is inverted because the biomass of fishes far exceeds that of phytoplankton

- Pyramid of energy is always upright because some energy is always lost as heat at each trophic level. So energy at a lower trophic level is always more than at a higher level.

Limitations of ecological pyramids
o It does not consider the same species belonging to two or more trophic levels.
o It assumes a simple food chain that never exists in nature. It does not accommodate a food web.
o Saprophytes are not included

ECOLOGICAL SUCCESSION

- It is a gradual, slow and predictable change in the species composition of an area leading to a climax community (community that is in equilibrium with the environment).
- In this, some species colonize an area and increase in number, whereas other species decline and disappear.
- The entire sequences of communities that successively change in an area are called sere. Individual transitional communities are termed seral stages (seral communities).
- The species invading a bare area are called pioneer species.
- During succession, there is a change in species diversity, increase in number of species and organisms and an increase in total biomass.
- Present-day communities are due to succession of millions of years. Succession and evolution were parallel processes.

- Succession is 2 types:

o Primary: The succession taking place in areas where no living organisms ever existed. E.g. newly cooled lava, bare rock, newly created pond or reservoir. To establish a biotic community, fertile soil must be formed. So primary succession is a very slow process.

o Secondary: The succession taking place in an area after the existed organisms are lost. E.g. abandoned farm lands, burned or cut forests, lands that are flooded.
Since some soil or sediment is present, succession is faster than primary succession. The species that invade depend on the nature of the soil,
availability of water etc.
- In succession, changes in vegetation affect food & shelter of animals. Thus, succession leads to change in number and types of animals & decomposers.
- Natural or human induced disturbances (deforestation, fire etc.) convert a particular seral stage to an earlier stage.
They create new conditions that encourage some species and discourage or eliminate other species.

Succession of Plants

- Based on the nature of the habitat, succession of plants is 2 types: hydrarch and xerarch.

o Hydrarch succession: It takes place in wetter areas. It progresses from hydric to mesic conditions.

o Xerarch succession: It takes place in dry areas. It progresses from xeric to mesic conditions.
- Hence, both hydrarch & xerarch successions lead to medium water conditions (mesic, the climax community).
- Primary succession on rocks (xerophytic habitat):
Lichens (pioneer species. They secrete acids to dissolve rock, helping in weathering & soil formation) → small plants like bryophytes (they need only small amount of soil) → bigger plants → forest (mesophytic).
The climax community (forest) remains stable if the environment remains unchanged.

- Primary succession in water:
Phytoplankton (pioneers) → rooted-submerged plants → rooted-floating angiosperms → free-floating plants →
reed-swamp → marsh-meadow → scrub → trees (climax community is a forest). With time, the water body is converted into land.

4. NUTRIENT CYCLING

- Amount of nutrients (C, N, P, Ca etc.) present in the soil in a given time is called the standing state. It varies in different kinds of ecosystems and also on a seasonal basis.
- Nutrients are never lost from the ecosystems. They are recycled again and again.
- The movement of nutrient elements through various components of an ecosystem is called nutrient cycling (biogeochemical cycles).

- Nutrient cycles are 2 types:

a. Gaseous cycle: For this, the reservoir exists in the atmosphere. E.g. Nitrogen & Carbon cycles.

b. Sedimentary cycle: For this, the reservoir is located in Earth’s crust. E.g. Sulphur & Phosphorus cycles.
- Environmental factors (soil, moisture, pH, temperature, etc.) regulate the rate of release of nutrients into the atmosphere. The reservoir meets with the deficit of nutrients due to imbalance in the rate of influx and efflux.

Carbon Cycle

- Reservoir of carbon: Atmosphere (about 1%), organisms (49% of dry weight), oceans (71% dissolved carbon. It regulates the amount of atmospheric CO₂), fossil fuel etc.
- Carbon cycling occurs through atmosphere, ocean and through living and dead organisms.
- 4×10¹³ kg of carbon is fixed in the biosphere through photosynthesis annually.
- A major amount of carbon returns to the atmosphere as CO₂ through respiration.
- Processing of wastes & dead organic matter by decomposers also release CO₂.
- Some amount of the fixed carbon is lost to sediments and removed from circulation.
- Burning of wood, forest fire and combustion of organic matter, fossil fuel and volcanic activity are other sources for releasing CO₂ in the atmosphere.
- Role of human activities in carbon cycle: Deforestation, burning of fossil fuel etc. has increased the rate of release of CO₂ into the atmosphere.

Phosphorus Cycle
- Phosphorus is a constituent of biological membranes, nucleic acids & cellular energy transfer systems. Many animals use phosphorus to make shells, bones and teeth.
- The natural reservoir of phosphorus is rock (in the form of phosphates).
- When rocks are weathered, minute amounts of phosphates dissolve in soil solution and are absorbed by the plants.
Herbivores and other animals obtain this from plants. The waste products and the dead organisms are decomposed by phosphate-solubilising bacteria releasing phosphorus.

ECOSYSTEM SERVICES

- The products of ecosystem processes are called ecosystem services.
- E.g. forest ecosystems purify air and water, mitigate droughts and floods, cycle nutrients, generate fertile soils, provide wildlife habitat, maintain biodiversity, pollinate crops, provide storage site for carbon and provide aesthetic, cultural & spiritual values.
- Robert Costanza and his colleagues have tried to put price tags on nature’s life-support services.
- Researchers have put an average price tag of US $ 33 trillion a year on fundamental ecosystems services. This is nearly twice the value of the global gross national product GNP (US $ 18 trillion).
- Out of this total cost, soil formation accounts for about 50%.
- Contributions of other services like recreation & nutrient cycling are less than 10% each.
- The cost of climate regulation and habitat for wildlife are about 6 % each.