Structure and function of an ecosystem: EVS Notes, PDF, Books
EVS Notes Unit I Multidisciplinary nature, Natural Resources, Forest resources, Water resources, Mineral resources, Food resources, Energy resources, Land resources, Role of an individual Unit II Introduction structure-function, Forest eco-system; Grassland ecosystem; Desert ecosystem; Aquatic ecosystems Unit III Air pollution, Water pollution, Soil pollution
The term ecosystem is defined as the system resulting from the integration of all the living and non-living factors of the environment. The terms biocoenosis, microcosm, biocoenosis or geobiocoenosis, holocoen, biosystem, bioinert body, and ecocosm, respectively are used to express similar ideas.
However, the term ecosystem is most preferred, where eco refers to the environment, and system implies an interacting and interdependent complex. The organisms of any community besides interacting among themselves always have a functional relationship with the environment. This structural and functional system of communities and the environment is called an ecological system or ecosystem. It is the basic functional unit in ecology, since it includes both biotic and abiotic environments, influencing each other for the maintenance of life.
An ecosystem may, in its simplest form, be defined as a self-sustained community of plants and animals existing in its own environment. An ecosystem may be as small as a drop of pond water (micro-ecosystem) or as large as an ocean. It can be of temporary nature, e.g., a fresh pool or a field of cultivated crops, or permanent e.g., a forest or an ocean. A balanced aquarium may be thought of as an artificially established self-sustained ecosystem.
Characteristics of Ecosystem
According to Smith (1966), the ecosystem has the following general characteristics:
- It is a major structural and functional unit of ecology.
- Its structure is related to its species diversity; the more complex ecosystems have high species diversity and vice versa.
- Its function is related to energy flow and material cycling through and within the system.
- The relative amount of energy needed to maintain an ecosystem depends on its structure. The more complex the structure, the lesser the energy it needs to maintain itself.
- It matures by passing from fewer complexes to more complex states. The early stages of each succession have an excess of potential energy and a relatively high energy flow per unit of biomass. Later (mature) stages have less energy accumulation and they flow through more diverse components.
- Both the environment and the energy fixation in any given ecosystem are limited and cannot be exceeded without causing serious undesirable effects.
- Alternations in the environment represent selective pressures upon the population to which it must adjust.
- Organisms that are unable to adjust to the changing environment must necessarily vanish.
Kinds of Ecosystems
Artificially ecosystems may be classified as follows:
These operate under natural conditions without any major interference by man. On the basis of the type of habitat, these may be further divided as:
Forest, grassland, desert, etc.
Fresh water – which may be lotic (e.g., running water as spring, stream, or river) or lentic (e.g., standing water as a lake, pond, pools, puddles, ditch, swamp, etc.).
Marine – such deep bodies as the ocean or shallow ones as seas or an estuary, etc.
Artificial (Man – engineered) ecosystems
These are maintained artificially by man whereby in addition of energy and planned manipulation, the natural balance is disturbed regularly, e.g. cropland ecosystem.
Structure of the Ecosystem
All ecosystems, whether terrestrial, freshwater, marine, or man-engineered, consist of the following major components:
Trophic organization—the relationship of food between various layers
Nutrients—required for living organisms
Biotic (living) components
This comprises all the living organisms. From the nourishment (or trophic) standpoint, they may be divided into two categories:
The autotrophs (autotrophic = self-nourishing)
These are green plants and certain photosynthetic or chemosynthetic bacteria which can convert the light energy of the sun into potential chemical energy in the form of organic compounds
needed by plants for their own growth and development. Oxygen is produced as a by-product of photosynthesis, needed by all living organisms for respiration. These green plants are also
known as producers because they produce food for all the other organisms.
The heterotrophs (heterotrophic = other nourishing)
They are dependent directly or indirectly upon the autotrophs for their food. The organisms involved are also known as consumers because they consume the materials built up by
producers. These may be subdivided into two kinds:
Macroconsumers (or Phagotrophs, Phago = to eat)
These are organisms that ingest food and digest it inside their bodies.
They may be herbivores (plant-eating), carnivores (= animal eating), or omnivores (= eating all kinds of food).
The herbivores are primary consumers. For example, insects like grass hopers, chewing up stems and leaves, animals like goats, cows, deer, and rabbits eat up the entire aerial portion of green plants,
and man-eating up plant products, are all primary consumers. The frog, a carnivore, is a secondary consumer as it eats the herbivores, and the snake that eats the frog is a tertiary consumer, there is
also a class of top consumers, which are not killed and eaten by any other animals e.g. lion, tiger, leopard, vulture, etc.
Microconsumers (Saprotrophs, sapro = to decompose, or osmotrophs, osmo = to pass through a membrane)
These are the organisms that secrete digestive enzymes to break down food into simpler substances and then absorb the digested food. They are mostly parasitic and saprophytic bacteria, actinomycetes, and fungi. They are also known as decomposers because of their role in the decomposition of dead organic matter. However, the parasites are not decomposers, and also some consumers (e.g. insects and such small animals) also help in decomposition by breaking down the organisms into small bits. Keeping this in view, Wiegert and Owens (1970) suggested the classification of heterotrophs into two categories, biophages (= feeding on living organisms) and saprophages (= feeding on dead organic matter).
Decomposers break down the complex compounds of dead or living protoplasm, absorb some of the decomposition products and release inorganic nutrients which are cycled back to the soil and the atmosphere from where they are once again made available to the primary producers.
Such a division of organisms based on the type of nutrition gives rise to the trophic structure of the ecosystem and the energy source used which is one kind of producer-consumer arrangement, where each food level is known as a trophic level. The amount of living material in different trophic levels or in a component population is known as the standing crop, a term applicable to both, plants as well as animals. The standing crop may be expressed in terms of the organism’s mass, which can be measured as living weight, dry weight, ash-free dry weight or carbon weight or calories, or any other convenient unit suitable for comparative purposes.
In nature, simple food chains occur only rarely. There are several food chains linked together, and intersecting each other to form a network known as a food web.
Structurally abiotic components include –
Climate regime: Precipitation, temperature, light, and other physical factors.
Inorganic substances: Elements such as C, N, H, O, P, S, etc., are involved in material cycles.
Organic Compounds: Carbohydrates, proteins, lipids, and humic substances that link the abiotic components with the biotic components (for details see any elementary book on ecology).
The minerals and atmospheric gases keep on cycling. They enter into biotic systems and after the death and decay of organisms return to the soil and atmosphere. This is known as the biogeochemical cycle. This circulation of materials involves trapping of the solar energy by the green plants which are ultimately lost by the organisms in several ways. The amount of abiotic materials present in an ecosystem is called the standing stage.
The function of the ecosystem is to allow the flow of energy and cycling of materials which ensures the stability of the system and continuity of life. These two ecological processes include interaction between the abiotic environment and the communities. For the sake of convenience, the ecosystem dynamics may be analyzed in terms of the following:
(i) food chains,
(ii) food pyramids,
(iii) energy flow,
(iv) nutrient cycles,
(v) development and evolution of the ecosystem, and
(vi) homeostasis and stability of the ecosystem.
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