Ecosystem Important Questions for CBSE Class 12 Biology Ecosystem-Structure and Function, Productivity and Decomposition

1. An ecosystem is a functional unit of nature, where living organisms interact among themselves and also with the surrounding physical environment. The word ‘Ecosystem’ was coined by Sir AG Tansley (1935).

2. The size of an ecosystem varies greatly from a small pond to a large forest or a sea.

3. Ecosystem can be grouped into two main categories:

  • Terrestrial ecosystem Forest, grassland, desert, etc.
  • Aquatic ecosystem Pond, lake, river, wetland, estuary, etc.

4. Crop fields and an aquarium are considered as man-made ecosystems.

5. Structure and Function of Ecosystem

(i) Each ecosystem consists of biotic (autotrophs, herbivores and carnivores) and abiotic components and their interactions with each other results in a physical structure, that is characteristic for each type of ecosystem.

(ii) Identification and enumeration of plant and animal species of an ecosystem gives its specific composition.

(iiii) Vertical distribution of different species occupying different levels is called stratification. For example, trees occupy top vertical strata or layer of a forest, shrubs the second and herbs and grasses occupy the bottom layers.

(iv) Major functional components of an ecosystem are:

(a) Productivity (b) Decomposition

(c) Energy flow (d) Nutrient cycling

(v) Let us understand these components in a pond ecosystem:

(a) A pond is a shallow water body in which all the above mentioned basic structural and functional components are present.

(b) Abiotic components are water with all the dissolved inorganic and organic materials and soil deposited at the bottom.

(c) Autotrophic components are phytoplanktons, some algae and the floating, submerged and marginal plants found at the edges.

(d) Consumers are zooplanktons, which are free swimming and bottom dwellers.

(e) Decomposers are the fungi, bacteria and flagellates found abundantly in the bottom.

(f) Functioning of pond ecosystem occurs in following steps:

  • Autotrophs convert inorganic material into organic material with the help of solar energy.
  • Heterotrophs consume autotrophs.
  • Decomposers decompose dead organic materials and mineralise it to release them back for reuse by the autotrophs.
  • The above events are repeated again and again.
  • Unidirectional movement of energy occur towards the higher trophic levels and lost in the form of heat to the environment.

6. Productivity is the rate of biomass production. It is expressed in g~2yr-1 or (kcal m~2) yr_1.

(i) The amount of biomass or organic matter produced per unit area over a time period by plants during photosynthesis is called primary production.

(ii) The primary productivity can be divided into:

  • Gross Primary Productivity (GPP) of an ecosystem is the rate of production of organic matter during photosynthesis. A considerable amount of GPP is utilised by plants in respiration.
  • Net Primary Productivity (NPP) is gross primary productivity minus respiratory losses (R).

NPP = GPP – R

NPP is the available biomass for the consumption to heterotrophs, i.e. herbivores and decomposers.
important-questions-for-class-12-biology-cbse-ecosystem-structure-and-function-productivity-and-decomposition-1

(iii) Secondary productivity is defined as the rate of formation of new organic matter by consumers.

(iv) Primary productivity of an ecosystem depends on:

  • Plant species inhabiting a particular area.
  • Availability of nutrients.
  • Photosynthetic capacity of plants.
  • Variety of environmental factors.

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

7. Decomposition is the process in which decomposers breakdown complex organic matter into inorganic substances like carbon dioxide, water and nutrients.

(i) The raw materials called detritus are dead plant remains such as leaves, barks, flowers and dead remains of animals, including faecal matter.

(ii) Process of decomposition occurs in the following steps:

  • Fragmentation is the breakdown of detritus into smaller particles by detritivores, e.g. earthworm.
  • Leaching is the process by which water soluble inorganic nutrients go down into the soil horizon and get precipitated as unavailable salts.
  • Catabolism is the process of degradation of detritus into simple inorganic substances by bacterial and fungal enzymes.
  • Humification is the process of accumulation of a dark coloured amorphous substance called humus. It is highly resistant to microbial action, undergoes decomposition at an extremely slow rate and serves as a reservoir of nutrients.
  • Mineralisation is the process by which humus is further degraded by some microbes to release inorganic substances

(iii) Decomposition rate depends on

  • Oxygen availability
  • Chemical composition of detritus
  • Climatic factors
  • Temperature and soil moisture

(iv) Decomposition rate is slower, if detritus is rich in lignin and chitin. It is quicker, if detritus is rich in nitrogen and water soluble substances like sugars.

(v) Warm and moist environment speeds up decomposition, whereas low temperature and anaerobiosis inhibits decomposition and causes the formation of organic materials.

Previous Years Examination Questions

1 Mark Questions

1. How is ‘stratification’ represented in a forest ecosystem? [Delhi 2014]

Ans. The stratification, i.e. the vertical distribution of species at different levels, in a forest ecosystem can be represented as:
important-questions-for-class-12-biology-cbse-ecosystem-structure-and-function-productivity-and-decomposition-a1

2. What does ‘R’ represent in the given equation for productivity in an ecosystem?  GPP – R = NPP    [All India 2014C]

Ans.  In the given equation for productivity in an  ecosystem GPP-R= NPP. ‘R’ represents the energy utilised by plants or producers in respiration. It is also referred to as respiration losses.

3. Mention any two reasons why the primary productivity varies in different types  of ecosystems?  [All India 2014c].

Ans. The primary productivity varies in different types of ecosystems as:

(i) It depends upon plant species  (producers) of a given ecosystem and their photosynthetic capacity.

(ii) It is dependent on various environmental factors like availability of nutrients.

4. Why are green algae not likely to be found in the deepest strata of the ocean?  [All India 2013]

Ans. Green algae survive by utilisation of food  synthesised by themselves through photosynthesis. At deepest layer in ocean, light is absent. So, photosynthesis does not take place and hence, green algae are not found in deep strata.

5. Write a difference between net primary productivity and gross primary  productivity.     [All India 2011]

Ans
important-questions-for-class-12-biology-cbse-ecosystem-structure-and-function-productivity-and-decomposition-5

6. What is secondary productivity? [Delhi 2009]

Ans. Secondary productivity is defined as the  rate of formation of new organic matter by the consumers.

7. All the primary productivity is not available to a herbivore, give one reason.  [Delhi 2009C]

Ans. All the primary productivity is not available  to a herbivore because a considerable amount is utilised by the plants in respiration, while some is lost as heat into the environment.

8. What is net primary productivity of an ecosystem? [Delhi 2008C]

Ans. The amount of remaining energy or  biomass in a producer after meeting the cost of its respiration is called net primary productivity of an ecosystem.

2 Marks Question

9. Differentiate between a detritivore and a decomposer giving an example of each.  [Delhi 2008]

Ans

important-questions-for-class-12-biology-cbse-ecosystem-structure-and-function-productivity-and-decomposition-9

 

3 Marks Question

10.(i) What is primary productivity? Why does it vary in different types of ecosystems?

(ii) State the relation between gross and net primary productivity.    [Delhi 2014]

Ans. The amount of biomass or organic matter produced per unit area over a time period by plants during photosynthesis is called primary productivity. It is expressed in terms of g_2yr~1.

The primary productivity varies in different types of ecosystem as it depends upon different plant species present in a given ecosystem and each of their photosynthetic efficiency. Also, the environmental factors, availability of various nutrients vary in different ecosystems, leading to variations in primary productivity.

(ii) The relationship between gross and net primary productivity can be explained by equation:

GPP- R= NPP

While, GPP is total amount of organic matter produced during photosynthesis and NPP is same amount of organic matter (biomass) for the consumption of heterotrophs, except for some respiratory losses, i.e. energy utilised by plants during respiration.

 

5 Marks Questions

11. How is detritus decomposed step-by-step by different agents and made available as  nutrients to the plants? Explain.   [Delhi 2013c]

or

Describe the process of decomposition of detritiis under the following heads fragmentation, leaching, catabolism, humification and mineralisation. [All India 2008C]

Ans. Detritus is the raw material for decomposition. It includes dead remains of plants (leaves, bark and flowers) and of animals including faecal matter. It is largely an aerobic process, i.e. requires oxygen for its processing.

Different steps involved in the process of decomposition are:

(i) Fragmentation is the process of breaking down of detritus into smaller particles.

(ii) Leaching is the process by which water soluble inorganic nutrients go down into the soil horizons and gets precipitated as unavailable salts.

(iii) Catabolism is the process of degradation of detritus into simple organic material by the action of bacterial and fungal enzymes and their further conversion into inorganic compounds.

(iv) Humification is a process that leads to an accumulation of a dark coloured, amorphous and colloidal substance called humus which is highly resistant to microbial action and decompose at a very slow rate. It serves as a reservoir of nutrients.

(v) Mineralisation is the process by which humus is further degraded by microbial action and release inorganic nutrients.

12. (i) Explain primary productivity and the factors that influence it.(ii) Describe how do oxygen and chemical composition of detritus control  decomposition?   [Delhi 20U]

(i) Primary productivity can be defined as the amount of biomass or organic matter produced per unit area over a time period by plants during photosynthesis. It is expressed in terms of weight (gm-2) or energy (kcal cm2).

It can be divided as:

(a) Gross Primary Productivity (GPP) It is the rate of production of biomass/organic matter by the producers during photosynthesis.

(b) Net Primary Productivity (NPP)

It is. the biomass/organic matter available at the producer level to the primary consumers, i.e. GPP respiratory losses.

Factors affecting primary productivity:

  • Availability of nutrients.
  • Quality and duration of sunlight.
  • Water availability.
  • Temperature of given place.
  • Type of plant species.
  • Photosynthetic capacity of plants.

(ii) (a) Oxygen composition of detritus

  • Decomposition is an oxygen consuming process. Anaerobic conditions inhibit decomposition.

(b) Chemical composition of detritus

  • Decomposition is fast when detritus is rich in nitrogen and water soluble substances like sugars.
  • It is slow, when detritus is rich in lignin and chitin.

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