Functions of Ecosystem


  • Ecosystem functions can be defined by “the ecological processes that control the fluxes of energy, nutrients and organic matter through an environment”
  • Interaction of biotic and abiotic components result in a physical structure that is characteristic for each type of ecosystem
  • The components of the ecosystem are seen to function as a unit when following aspects are considered:
      • A constant input of solar energy is the basic requirement for any ecosystem to function and sustain
      • Primary production 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 energy (kcal m^2).
        • The rate of biomass production is called productivity
      • It can be divided into
        • Gross primary productivity of an ecosystem is the rate of production of organic matter during photosynthesis. A considerable amount of GPP is utilised by plants in respiration
        • Gross primary productivity minus respiration losses (R), is the net primary productivity (NPP). Net primary productivity is the available biomass for the consumption to heterotrophs ( herbivores and decomposers)
      • 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. The annual net primary productivity of the whole biosphere is approximately 170 billion tons (dry weight) of organic matter.
      1. Decomposers break down complex organic matter into inorganic substances like carbon dioxide, water and nutrients and the process is called decomposition
      2. Dead plant remains such as leaves, bark, flowers and dead remains of animals, including faecal matter, constitute detritus, which are the raw material for decomposition.
    • The important steps in the process of decomposition are fragmentation, leaching, catabolism, humification and mineralisation
      1. Detritivores (e.g., earthworm) break down detritus into smaller particles. This process is called fragmentation.
      2. By the process of leaching, water soluble inorganic nutrients go down into the soil horizon and get precipitated as unavailable salts.
      3. Bacterial and fungal enzymes degrade detritus into simpler inorganic substances. This process is called as catabolism
      4. Humification and mineralisation occur during decomposition in the soil.
        • Humification 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
        • The humus is further degraded by some microbes and release of inorganic nutrients occur by the process known as mineralisation
        • Decomposition is largely an oxygen-requiring process
        • 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

Decomposition in Terrestrial Ecosystem

      • 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 suns’ radiant energy to make food from simple inorganic materials
      • So, all organisms are dependent for their food on producers, either directly or indirectly.
        • Hence, there exists an unidirectional flow of energy from the sun to producers and then to consumers
      • Hence, 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
      • All animals depend on plants (directly or indirectly) for their food needs. They are hence called consumers and also heterotrophs
      • Further, 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
      • In this perspective, Organisms occupy a place in the natural surroundings or in a community according to their feeding relationship with other organisms.
        • 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. When any organism dies it is converted to detritus or dead biomass that serves as an energy source for decomposers.
      • Organisms at each trophic level depend on those at the lower trophic level for their energy demands.
        • 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 number of trophic levels in the grazing food chain is restricted as the transfer of energy follows 10 per cent law – only 10 per cent of the energy is transferred to each trophic level from the lower trophic level.

Energy flow in an Ecosystem

      • One gets the shape of a pyramid, when food or energy relationship between organisms at different trophic level are expressed
        • Thus, relationship is expressed in terms of number, biomass or energy
      • The base of each pyramid represents the producers or the first trophic level while the apex represents tertiary or top level consumer
      • This concept can be better understood by assessing the following pyramids:

Pyramid of number

      • The below diagram is that of a Grassland Ecosystem. Only the top-three carnivores are supported in an ecosystem, based on production of plants


      • Pyramid of Biomass
        • This shows a sharp decrease in biomass at higher trophic levels
        • Biomass refers to the total weight of living matter per unit area. In an ecosystem the biomass decreases from the producer level to the consumer level
        • In a grassland the biomass of grasses is the maximum and it gradually decreases towards the consumer level.

      • Inverted Pyramid
        • The pyramid of biomass is inverted in a pond or lake ecosystem.
        • The biomass of phytoplankton is less as compared with that of the small herbivorous fish that feed on these producers.
        • The biomass of large carnivorous fish that depends on small fishes is still greater.

      • Pyramid of energy
        • In an ecosystem, the energy flow from the producer to the consumer level will be decreasing. In a grassland, grass plants trap the maximum sun light energy.
        • The energy gradually decreases towards the top consumer level.
        • The chemical energy is transformed into kinetic energy.

  • However, 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