environmental science,  Video Notes

Ecosystem – Structure and Function

The term ‘ecosystem’ was proposed by a British ecologist A.G. Tansley (1953). It represents the basic fundamental, functional unit of ecology which comprises of the biotic community together with its abiotic (non-living) environment.

Ecosystem is the functional unit of nature where living organisms interact with each other and with their environment.

Ecosystems can be recognized as self-regulating and self-sustaining units of landscapes that may be terrestrial or aquatic. Forests, grasslands and deserts are examples of terrestrial ecosystems. The aquatic ecosystems can be either freshwater (ponds, lakes, streams) or saltwater (marine estuaries) type.

Ecosystem may be natural (forest, sea) if developed under natural conditions or artificial (garden, aquarium, agriculture) if created by man.

Ecosystem is normally an open system because there are continuous and variable entry and loss of energy and materials. Ecosystem is known by different terms i.e., biogeocoenosis or geobiocoenosis or microcosm or ecosom or biosystem etc., the whole earth can be called biosphere or ecosphere.

Ecosystem is composed of a variety of abiotic (non-living) and biotic (living organisms) components that function in an interrelated fashion.

Ecosystem – Formed of two components

  1. Abiotic Component
    1. Physical or climatic components Like temperature, water, soil, light, humidity, pressure etc.
    2. Inorganic compounds Like, water, minerals (sulphur, nitrogen, phosphorus etc) and atmospheric gases (O2, N2, CO2 etc).
    3. Organic Compounds Includes organic substances present in dead bodies of plants and animals
  2. Biotic Component – Classified into 3 main groups on the basis of mode of nutrition
    1. Producers – Organisms which carry out photosynthesis. E.g. plants, algae and bacteria. Also called autotrophs.
    2. Consumers – Organisms which derive their food directly or indirectly from the producers. Also called heterotrophs.
      1. Primary consumers – Also called herbivores. Directly feed on producers (green plants). E.g. Protozoans, grasshoppers, caterpillar etc.
      2. Secondary consumers – Also called carnivores or primary carnivores. They prey upon herbivores. E.g. water insects,
        deer rabbit, cattle, goat etc.
      3. Tertiary consumers – They feed on primary carnivores, like snakes eat frogs, a bird eats all types of fishes. Also called secondary carnivores.
    3. Decomposers – Also called reducers or micro consumers or saprobes. They obtain their nutrition from organic remains. E.g. Fungi and certain bacteria.

ECOSYSTEM – STRUCTURE AND FUNCTION

Ecosystem is self-sustained functional units.

The structure of an ecosystem can be expressed by the following terms –

  • Species compositor: Plant and animal species found in an ecosystem.
  • Stratifi cation: Vertical layers of plants.
  • Standing crop: Amount of biomass.
  • Standing state: Amount of inorganic substances.

Species composition

It differs from one ecosystem to another depending upon geography, topography and
climate.

Each ecosystem has a biotic community composed of a particular grouping of species.

Maximum species composition occurs in tropical rainforests and coral reefs. Minimum occurs in deserts and arctic regions.

Stratifications

Stratification is the occurrence of vertical zonation in the ecosystem & indicates the presence of favourable environmental conditions, for e.g., trees occupy top vertical strata or layer of a forest, shrubs the second & herbs & grasses occupy the bottom layers.

Stratification helps in accommodation of large number & types of plants in the same area. It also provides a number of microhabitat & niches for various types of animals.

It is absent or poor where environmental conditions are unfavourable, e.g. desert
ecosystems have very few trees & shrubs.

Standing crop

Standing crop is the amount of living biomass in an ecosystem. It indicates the productivity & luxuriance of growth.

It is expressed in the form of number or biomass of organisms per unit area.

A terrestrial ecosystem with high standing crop possesses a forest while the one with
low standing crop occurs in grassland followed by arid ecosystem.

Standing state

The amount of nutrients, e.g., nitrogen, phosphorus & calcium present in the soil at any given time is known as standing state.

The proper functioning of an ecosystem takes place through the following processes:

  • Productivity
  • Decomposition
  • Relationship of producers and consumers
  • Flow of energy through different trophic levels, and
  • Cycling of nutrients.

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