Ecosystem

• The term ecosystem was coined by Sir A.G. Tansley in 1935
• The ecosystem consists of biotic and abiotic components.
• 1. abiotic - non living things: i) Air (ii) Water (iii) Solar radiation (iv) Temperature (v) Soil
  2. biotic - living things: i) producers ii)consumers iii)decomposers
• Vertical distribution of different species occupying different levels is called stratification
• The major functions of an ecosystem include (i) Productivity (ii) Decomposition (iii) Energy flow (iv) Nutrient cycling
• Pond Ecosystem: It is a self-sustainable unit.

  biotic:

  i. Producers some algae, floating and submerged plants) & marginal plants at edges.

  ii. Consumers (zooplanktons, free swimming, bottom dwelling forms).

  iii. Decomposers fungi, bacteria and flagellates

  abiotic:

  (i) Soil particle (ii) Water (with dissolved organic & inorganic substances) (iii) CO2, O2 dissolved (iv) Solar radiation (v) Soil (bottom of pond)
• the energy flow is unidirectional, from the sun to the earth, and from the earth to the atmosphere and goes from one animal to another.
• 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.
• Gross Primary Productivity (GPP) :It is the rate of production of organic matter during photosynthesis in an ecosystem. Some of GPP is utilised by plants for respiration and some is passed to the next trophic level.
• Net Primary Productivity: It is the available biomass for the consumption of heterotrophs. It is the weight of the organic matter stored by the producers in a unit area/volume per unit time. It is given by NPP = GPP – R where R = Respiration losses NPP is utilised by heterotrophs (herbivores & decomposers).
• Factors Affecting Primary Productivity:

  • Plant species inhabiting a particular area.
  • Environmental factors: (i) Sunlight: tropical region has maximum sunlight so it has higher productivity, (ii) Temperature: optimum temperature is required for proper functioning of enzyme, (iii) Moisture: Rain (humidity) is required for higher primary productivity.
  • Availability of nutrients: more nutrients have higher PP.
  • Photosynthetic efficiency: more efficiency to trap sunlight, so they accumulate more PP.
• Deserts have the lowest primary productivity as the soil is deficient in moisture.
• Annual Net Primary Productivity of whole biosphere is approx. 170 billion tons (dry weight) of organic matter.
• the process of breaking down complex organic matter into inorganic substances like CO2, water and nutrients is called decomposition.
• The raw materials for decomposition including dead plant remains like leaves, bark, flowers, and animal remains and their faecal matter are called detritus.
• Steps in Decomposition:

  (i) Fragmentation: The process of breaking down of detritus into smaller particles is called fragmentation, e.g., as done by earthworm (= farmer’s friend). (ii) Leaching: The process by which water-soluble inorganic nutrients go down into the soil horizon and get precipitated as unavailable salts is called leaching. (iii) Catabolism: The enzymatic process by which bacterial and fungal enzymes degrade detritus to simpler inorganic substances is called catabolism.
• Steps in Decomposition continuation:

  (iv) Humification: The process of accumulation of a dark coloured amorphous substance, called humus, that is highly resistant to microbial action and undergoes decomposition at an extremely slow rate is called humification. Humus being colloidal is reservoir of nutrients. (v) Mineralisation: The process by which humus is further degraded by some microbes to release inorganic nutrients is called mineralisation.
• Decomposition is an oxygen requiring process. Rate of decomposition is controlled by chemical composition of detritus and climatic condition.
• (i) Chemical composition of detritus:

  • The decomposition rate is slow if detritus is rich in lignin and chitin.
  • The decomposition rate is higher when detritus is rich in nitrogen and water-soluble substances like sugars. (ii) Climatic factors:
  • Warm and moist environment favors decomposition.
  • Low temperature and anaerobiosis (life in the absence of air) inhibit decomposition.
• Food chain:

  (i) The transfer of energy from producers to top consumers through a series of organisms is called food chain.

  (ii) One organism holds only one position.

  (iii) The flow of energy can be easily calculated.

  (iv) It is always straight and proceed in a progressive straight line.

  (v) Competition is limited to members of same trophic level.
• Food Web:

  (i) A number of food chains are inter-connected with each other, forming a web-like pattern is called food web.

  (ii) One organism can hold more than one position.

  (iii) The flow of energy is very difficult to calculate

  (iv) Instead of straight line it is a series of branching lines.

  (v) Competition is amongst members of same and different trophic levels.
• • Two types of food chains can be observed in the ecosystem:

  (i) Grazing Food Chain (GFC), e.g., (1) Grass ⮕ Rabbit ⮕ Lion

  (2) Grass (producer) ⮕ Goat (Pr. Consumer) ⮕ Man (Sec. Consumer)

  (ii) Detritus Food Chain (DFC), e.g., Dead leaves ⮕ Woodlouse ⮕ Blackbird.

  • aquatic ecosystem, GFC is the major conduit for energy flow.
  • terrestrial ecosystem, DFC may be connected with GFC at some levels. Some of the organisms of DFC are prey to GFC animals in a natural ecosystem.
• Grazing food chain (GFC):

  (i) It starts with green plants called producers as first trophic level.

  (ii) A much less fraction of energy flows through this type of food chain

  (iii) Energy for food chain comes from sun.
• Detritus food chain (DFC):

  • Death of an organism is the beginning of DFC.
  • (i) It begins with dead organic matter and decomposers called saprophytes as first trophic level. Decomposers secrete digestive enzymes that breakdown dead and waste into simple, inorganic materials which are absorbed by them.

  (ii) A much large fraction of energy flows through this type of food chain.

  (iii) Energy for the food chain comes from organic remain or detritus.

  • Saprophytes like fungi and bacteria meet their energy and nutrients by degrading dead organic matter or detritus.
• • In an ecosystem, an organism occupies a specific place in the food chain called trophic level.
  • Each trophic level has a certain mass of living material at a particular time called the standing crop.
  • The standing crop is measured as the biomass of living organisms (biomass), or the number in a unit area. Biomass is expressed in terms of fresh or dry weight.
• Energy Flow:

  The sun is the only source of energy.

  Out of the total incident solar radiation, only 50 per cent of it is photosynthetically active radiation (PAR).

  Plants capture only 2–10 per cent of the PAR and this small amount of energy sustains the entire living world. So, there is unidirectional flow of energy from the sun to producers and then to consumers.

  The energy is transferred in an ecosystem, in the form of food which is degraded and lose major part of food energy as heat during metabolic activities and only a very small fraction becomes stored as biomass.
• Energy flow is correlated to second law of thermodynamics, i.e., ecosystems need a constant supply of energy to synthesize molecules they require, to counteract universal tendency towards increasing disorderliness.
• • The green plants in the ecosystem which can trap solar energy to convert it into chemical bond energy are called producers.
  • All the animals that depend for food on plants are called consumers or heterotrophs.
  • Consumers are divided into the following categories: (i) Primary consumers: Animals which feed directly on plants, i.e., herbivores.

  (ii) Secondary consumers: Consumers that feed on primary consumers, i.e., carnivores.

  (iii) Tertiary consumers: Consumers that feed on secondary consumers.
• Lindeman’s 10 per cent law:

  At each step of food chain, when food energy is transferred from one trophic level to the next higher trophic level, only about 10 per cent of energy is passed on to the next trophic level. This is known as Lindeman’s 10 per cent law given by Lindeman in 1942.
• Each trophic level has a certain mass pf living material at a particular time called as standing crop. it is measured as the mass of living organisms(biomass) or number in a unit area. Biomass of a species is expressed in terms of fresh or dry weight.
• Ecological Pyramid:

  • The relationship between producers and consumers at different trophic levels in an ecosystem can be graphically represented in the form of a pyramid called ecological pyramid.
  • The base always represents the producers or the first trophic level and the apex represents top level consumer or the last trophic level.
  • Ecological pyramids are of three types:

  (i) Pyramid of number (ii) Pyramid of biomass (iii) Pyramid of energy
• (i) Pyramid of number: The relationship between producers and consumers in an ecosystem can be represented in the form of a pyramid in terms of number of organisms at different trophic levels called pyramid of number.

  It is inverted, when you count number of insects feeding on a big tree.

  (ii)Pyramid of biomass: The relationship between producers and consumers in an ecosystem can be represented in the form of a pyramid in terms of biomass called pyramid of biomass.
• Continuation:

  It can be (pyramid of biomass)

  (a) Upright, e.g., in case of grassland ecosystem; or

  (b) Inverted, e.g., in case of pond ecosystem as biomass of fishes for exceeds that of phytoplanktons.

  (iii) Pyramid of energy: The relationship between producers and consumers in an ecosystem can be represented in the form of a pyramid, in terms of flow of energy called pyramid of energy. It is always upright because energy is always lost as heat at each step and as it follow 10% law.
• Limitations of ecological pyramids:

  (i) It never takes into account the same species belonging to two or more trophic levels.

  (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