CHAPTER 12 - ECOSYSTEM

 

Ecosystem

• An ecosystem is a functional unit of nature, where living organisms interact among themselves and also with the surrounding physical environment.
• Ecosystem varies greatly in size from a small pond to a large forest or a sea.
• It is divided into two basic systems -
  • Natural ecosystems -
    1. the terrestrial - e.g. forest, grassland and desert
    2. the aquatic - e.g. pond, lake, wetland, river and estuary
  • Man-made ecosystems - e.g. crop fields and an aquarium

Ecosystem - Structure and Function

• Components of the environment- abiotic and biotic
• Interaction of biotic and abiotic components result in a physical structure.
• Identification and enumeration of plant and animal species of an ecosystem gives its species composition.
• 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.
• The components of the ecosystem are seen to function as a unit in the following aspects:
  (i) Productivity;
  (ii) Decomposition;
  (iii) Energy flow; and
  (iv) Nutrient cycling
• There is unidirectional movement of energy towards the higher trophic levels and its dissipation and loss as heat to the environment.

Productivity

• Energy Source -
  • A constant input of energy is the basic requirement for any ecosystem to function and sustain. It is received from the sun.
• Primary production -
  • It is 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 m^–2).
  • Primary productivity depends on the plant species inhabiting a particular area.
  • It also depends on a variety of environmental factors, availability of nutrients and photosynthetic capacity of plants. Therefore, it varies in different types of ecosystems.
• Productivity -
  • The rate of biomass production is called productivity. It is expressed in terms of gm^–2 yr ^–1 or kcal/m²/yr to compare the productivity of different ecosystems.
  • It can be divided into two parts -
    • gross primary productivity (GPP) and
    • net primary productivity (NPP)
  • Gross primary productivity -
    • Gross primary productivity of an ecosystem is the rate of production of organic matter during photosynthesis.
  • Net primary productivity -
    • A considerable amount of GPP is utilised by plants in respiration.
    • Gross primary productivity minus respiration losses (R), is the net primary productivity (NPP).
      GPP – R = NPP
    • Net primary productivity is the available biomass for the consumption to heterotrophs (herbiviores and decomposers).
• Secondary productivity -
  • Secondary productivity is defined as the rate of formation of new organic matter by consumers.
• The annual net primary productivity of the whole biosphere is approximately 170 billion tons (dry weight) of organic matter. Of this, despite occupying about 70 per cent of the surface, the productivity of the oceans are only 55 billion tons. Rest of course, is on land.

Decomposition

• Decomposers break down complex organic matter into inorganic substances like carbon dioxide, water and nutrients and the process is called decomposition.
• Dead plant remains such as leaves, bark, flowers and dead remains of animals, including fecal matter, constitute detritus, which is the raw material for decomposition.
• The important steps in the process of decomposition are -
  • fragmentation
  • leaching
  • catabolism
  • humification and
  • mineralisation.
• Fragmentation -
  • Detritivores (e.g., earthworm) break down detritus into smaller particles. This process is called fragmentation.
• Leaching -
  • By the process of leaching, water-soluble inorganic nutrients go down into the soil horizon and get precipitated as unavailable salts.
• Catabolism -
  • Bacterial and fungal enzymes degrade detritus into simpler inorganic substances. This process is called as catabolism.
• It is important to note that all the above steps in decomposition operate simultaneously on the detritus.
• Humification and mineralisation occur during decomposition in the soil.
• Humification -
  • It leads to accumulation of a dark coloured amorphous substance called humus that is highly resistant to microbial action and undergoes decomposition at an extremely slow rate.
  • Being colloidal in nature it serves as a reservoir of nutrients.
• Mineralisation -
  • The humus is further degraded by some microbes and release of inorganic nutrients occur by the process known as mineralisation.

Factors Affecting Decomposition

• Decomposition is largely an oxygen-requiring process.
• The rate of decomposition is controlled by chemical composition of detritus and climatic factors.
• In a particular climatic condition, decomposition rate is slower if detritus is rich in lignin and chitin, and quicker, if detritus is rich in nitrogen and water-soluble substances like sugars.
• Temperature and soil moisture are the most important climatic factors that regulate decomposition through their effects on the activities of soil microbes.
• Warm and moist environment favour decomposition whereas low temperature and anaerobiosis inhibit decomposition resulting in build up of organic materials.

Energy flow

• Except for the deep sea hydro-thermal ecosystem, sun is the only source of energy for all ecosystems on Earth.
• Of the incident solar radiation less than 50 per cent of it is photosynthetically active radiation (PAR).
• Plants and photosynthetic 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.
• The solar energy captured by plants flows through different organisms of an ecosystem.
• All organisms are dependent for their food on producers, either directly or indirectly.
• There is unidirectional flow of energy from the sun to producers and then to consumers.
• They need a constant supply of energy to synthesise the molecules they require, to counteract the universal tendency toward increasing disorderliness.
• The green plant in the ecosystem are called producers.
• In a terrestrial ecosystem, major producers are herbaceous and woody plants.
• Producers in an aquatic ecosystem are various species like phytoplankton, algae and higher plants.
• Starting from the plants (or producers) food chains or rather webs are formed such that an animal feeds on a plant or on another animal and in turn is food for another. The chain or web is formed because of this interdependency.
• No energy that is trapped into an organism remains in it for ever.
• The energy trapped by the producers is either passed on to a consumer or the organism.
• Death of organism is the beginning of the detritus food chain/web.
• All animals depend on plants (directly or indirectly) for their food needs. They are hence called consumers and also heterotrophs.
• If they feed on the producers, the plants, they are called primary consumers. The primary consumers will be herbivores.
  • Some common herbivores are insects, birds and mammals in terrestrial ecosystem and molluscs in aquatic ecosystem.
• If the animals eat other animals which in turn eat the plants (or their produce) they are called secondary consumers.
  • The consumers that feed on these herbivores are carnivores, or more correctly primary carnivores (though secondary consumers).
• Those animals that depend on the primary carnivores for food are labelled secondary carnivores.
• A simple grazing food chain (GFC) is depicted below:
  Grass ----------------------------------->Goat ------------------------------->Man
  (Producer) (Primary Consumer) (Secondary consumer)
• Saprotrophs -
  • The detritus food chain (DFC) begins with dead organic matter.
  • It is made up of decomposers which are heterotrophic organisms, mainly fungi and bacteria.
  • They meet their energy and nutrient requirements by degrading dead organic matter or detritus. These are also known as saprotrophs (sapro: to decompose).
  • Decomposers secrete digestive enzymes that breakdown dead and waste materials into simple, inorganic materials, which are subsequently absorbed by them.
• In an aquatic ecosystem, GFC is the major conduit for energy flow.
• In a terrestrial ecosystem, a much larger fraction of energy flows through the detritus food chain than through the GFC.
• The natural interconnection of food chains make it a food web.
• Based on the source of their nutrition or food, organisms occupy a specific place in the food chain that is known as their trophic level.
• Producers belong to the first trophic level, herbivores (primary consumer) to the second and carnivores (secondary consumer) to the third.
• The important point to note is that the amount of energy decreases at successive trophic levels.
• Each trophic level has a certain mass of living material at a particular time called as the standing crop.
• The standing crop is measured as the mass of living organisms (biomass) or the number in a unit area. The biomass of a species is expressed in terms of fresh or dry weight.
• The number of trophic levels in the grazing food chain is restricted as the transfer of energy follows 10% law – only 10% of the energy is transferred to each trophic level from the lower trophic level.
• In nature, it is possible to have so many levels – producer, herbivore, primary carnivore, secondary carnivore in the grazing food chain.

Ecological pyramids

• Shape of a pyramid - The base of a pyramid is broad and it narrows towards the apex.
• The base of each pyramid represents the producers or the first trophic level while the apex represents tertiary or top level consumer.
• The three types of ecological pyramids that are usually studied are
  • (a) pyramid of number
  • (b) pyramid of biomass and
  • (c) pyramid of energy.
• A given organism may occupy more than one trophic level simultaneously. Hence the trophic level represents a functional level, not a species as such.
• In most ecosystems, all the pyramids, of number, of energy and biomass are upright, i.e., producers are more in number and biomass than the herbivores, and herbivores are more in number and biomass than the carnivores. Also energy at a lower trophic level is always more than at a higher level.
• The pyramid of biomass in sea is generally inverted because the biomass of fishes far exceeds that of phytoplankton.
• Pyramid of energy is always upright, can never be inverted, because when energy flows from a particular trophic level to the next trophic level, some energy is always lost as heat at each step.
• Each bar in the energy pyramid indicates the amount of energy present at each trophic level in a given time or annually per unit area.
• Saprophytes are not given any place in ecological pyramids even though they play a vital role in the ecosystem.

One Mark Questions

Q1. What is the ultimate source of energy in ecosystem?

Ans. The sun.

Q2. Define food chain.

Ans. The sequential feeding of one organism by other in one direction is called food chain.

Q3. Define food web.

Ans. The interconnecting food chains are termed as food web.

CHAPTER 10 – BIOTECHNOLOGY AND ITS APPLICATIONS

CHAPTER 11 – ORGANISMS AND POPULATIONS

CHAPTER 13 – BIODIVERSITY AND CONSERVATION