Ecosystem Class 12 Biology Revision Notes

Class 12 Notes

Please refer to Ecosystem Class 12 Biology Revision Notes for Chapter 14 provided below. These concept notes have been prepared based on the latest Class 12 Biology NCERT, CBSE, and KVS books issued for the current academic year. These notes will help you to learn all important topics given in the chapter and are important to get good marks. We have provided Class 11 Biology Notes for all chapters on our website for download in Pdf.

Class 12 Biology Chapter 14 Ecosystem Revision Notes

  • Ecosystem is a self regulatory and self sustaining structural and functional unit of landscape (biosphere) consisting of community of living beings and the physical environment, both  interacting and exchanging materials between them. A.G. Tansley (1935) coined the term ecosystem.
  • Ecosystems are divisible into two categories, terrestrial or land ecosystems (e.g., forests, grasslands, deserts, gardens) and aquatic or water ecosystems (e.g., ponds, lakes, streams, estuaries, sea). Man-made ecosystem is the one which is created and maintained by human beings, e.g., agriculture, garden, aquarium, spacecraft. Agriculture or agroecosystem is the largest man-made ecosystem. 
Ecosystem Revision Notes


Ecosystem Revision Notes

Interaction of biotic and abiotic components results in the physical structures that are characteristic for each type of ecosystem. These are :
(i) Species composition : Identification and enumeration of plant and animal species of an ecosystem gives its species composition. Maximum species composition occurs in tropical rainforest and coral reefs.
(ii) Stratification : It is formation of vertical layers where vegetation is dense, e.g., 5 – 7 strata in tropical rain forests. Stratification is absent or rare in deserts.
(iii) Trophic structure : Trophic structure of an ecosystem is a type of producer – consumer arrangement, in which each food level is called trophic level. Each ecosystem has specific food chains and food webs, e.g., grazing food chain in grassland.
(iv) Standing crop : It is the amount of living biomass present in a unit area of an  ecosystem.
(v) Standing state : It is the amount of inorganic nutrients present any time in the soil/water of ecosystem. It tends to vary from season to season and ecosystem to ecosystem.

Four important functional aspects of the ecosystem are productivity, decomposition, energy flow and nutrient cycling.

(i) Productivity : It refers to the rate of biomass production by any trophic level per unit area in unit time. It is measured as weight (e.g., g/m2/yr) or energy (e.g., kcal/m2/yr). It is of following types:
1. Primary productivity: It is the amount of biomass or organic matter produced per unit area over a time period by plants during photosynthesis. It is further divided into gross primary productivity and net primary productivity.
2. The rate of production of total organic matter by green plants by photosynthesis per unit area per unit time is known as gross primary productivity (GPP). A considerable amount of GPP is utilised by plants in respiration (R). 
3.The energy left after respiration and stored as organic matter in the producers per unit time and area is called net primary productivity (NPP). 
4. NPP = GPP – R
5. Secondary productivity refers to the rate of assimilation of the organic matter at the level of consumers. It is the amount of energy available at the consumer level for transfer to the  next trophic level. 

(ii) Decomposition : It is physical and chemical breakdown of complex organic remains (detritus) with the help of organisms called decomposers. The processes involved are fragmentation, catabolism and leaching.
1. Detrivores (e.g., termites, earthworms) feed on large pieces, and smaller fragments are left. Pulverisation occur in the digestive tract of animals as a part of it comes out undigested. The process is called fragmentation. The decomposers (e.g., bacteria, fungi) excrete digestive enzymes over the detritus. It changes insoluble complex organic substances into simple and soluble organic compounds and inorganic substances (catabolism). Soluble substances formed during decomposition are subjected to leaching or passage to deeper layers of soil /ground water by percolating water. 
2. Humification leads to formation of dark coloured amorphous substance called humus, rich in cellulose and lignin. It is highly resistant to microbial action and undergoes decomposition at an extremely slow rate. Humus is further degraded by some microbes and release of inorganic nutrients occurs by the process of mineralisation.
3. Decomposition is dependent on oxygen availability. It also depends on the chemical composition of detritus and climatic conditions. Decomposition is faster when detritus is rich in nitrogen and sugars than when detritus is rich in lignin and chitin. Warm and moist environment favours decomposition.

(iii) Energy flow : Sun is the ultimate source of energy in all ecosystems. Of the incident solar radiation, less than 50 per cent of it is photosynthetically active radiation (PAR). Plants and photosynthetic and chemosynthetic bacteria (autotrophs), fix sun’s radiant energy to make food from simple inorganic materials. Plants capture only 2-10 per  cent of the PAR and  this small amount of energy sustains the entire living world. Flow of energy takes place from producers to different consumers in an ecosystem which constitute different trophic levels.

  • A food chain involves a nutritive interaction between the living organisms (biotic components) of an ecosystem. Here occurs repeated eating i.e., each group eats the other and is subsequently eaten by some other group of organisms. The number of steps in a food chain is limited to four or five and at each step a large portion of the energy is lost as heat. In a food chain, there is unidirectional flow of energy. There is transfer of 10% energy from one trophic level to another. This is called 10% law formulated by Lindemann in 1942.
  • Food web is a network of food chains which become interconnected at various trophic levels so as to form a number of feeding connections amongst the different organisms of a biotic community.
  • An ecological pyramid is a graphical representation of ecological parameters like biomass, energy and number of individuals present in the various trophic levels of a food chain with producers forming the base and top carnivores the tip. Ecological pyramids were developed by Charles Elton (1927) and are therefore also known as Eltonian pyramids.
  • Calculations of energy content, biomass, or numbers has to include all organisms at that trophic level. No generalisations will be true if few individuals are considered at any trophic level into account.
  • A given species may occupy more than one trophic level in the same ecosystem at the same time; for example, sparrow is a primary consumer when it eats seeds, fruits peas, and a secondary consumer when it eats insects and worms.

Types of food chain  

Ecosystem Revision Notes

Types of ecological pyramids 

Ecosystem Revision Notes
  • There are certain limitations of ecological pyramids such as it does not take into account the same species belonging to two or more trophic levels. It assumes a simple food chain, something that almost never exists in nature; it does not accommodate a food web. Moreover, saprophytes are not given any place in ecological pyramids even though they play a vital  role in the ecosystem.

(iv) Nutrient cycling : Biogeochemical cycles or nutrient cycles are cyclic exchanges, transfer and storage of biogenetic nutrients through various components of ecosystem (biotic and abiotic), so that the nutrients can be used repeatedly again and again. Reservoir pool and cycling pool are  two stores of nutrients. Reservoir pool is the reservoir of biogenetic nutrients from which the nutrients are transferred to cycling pool.

Types of Biogeochemical Cycles 

Ecosystem Revision Notes
Ecosystem Revision Notes


  • Ecological succession is the observed process of changes in the species structure of an ecological community over time. The first group of organisms establishing during an ecological succession are known as the pioneers, primary community or primary colonisers. Climax community is the stable, self perpetuating and final biotic community that develops at the end of biotic succession and is in perfect harmony with the physical environment. Climax community has maximum diversity and niche specialisation. The various biotic communities that develop during biotic succession are termed as seral or transitional communities.
  • Primary succession starts from the primitive substratum, on a previously sterile or primarily bare area. Secondary succession starts from previously built up substrata with already existing living matter. In secondary succession since soil is already there, the rate of succession is much faster and hence, climax is also reached more quickly.
  • Hydrosere (hydrarch) refers to the biotic succession in a newly formed pond or lake. The stages that occur in hydrosere are: Plankton stage, rooted submerged stage, rooted floating stage, free floating stage, reed swamp stage, sedge or marsh meadow stage, woodland stage, climax forest stage.
  • Xerosere (xerarch) refers to the sucession that occurs at places where moisture is present in minimal amounts such as dry deserts, rocks, etc. The stages that occur in xerosere are : crustose lichen stage, foliose lichen stage, moss stage, herb stage, shrub stage, forest stage. 

Healthy ecosystems are the base for a wide range of economic, environmental and aesthetic goods and services. The products of ecosystem processes are thus named as ecosystem  services, for example, healthy forest ecosystems purify air and water, mitigate droughts and floods, recycle nutrients, generate fertile soils, provide habitat to wildlife, maintain biodiversity, pollinate crops, provide storage site for carbon and also provide aesthetic, cultural and spiritual values.

Ecosystem Class 12 Biology Revision Notes