Ecosystem - Stracture and Function
Category : 12th Class
The word ecosystem was coined by A.G. Tansley in 1935. According to Odum an ecosystem is the basic fundamental unit of ecology which includes both the organisms and the non-living environment each influencing the properties of the other and each is necessary for the maintenance of life.
Structure : The structure of any ecosystem is formed of two components, namely :
(1) Abiotic factors : The abiotic factors of an ecosystem include the non–living substances of the environment. e.g., Water, soil, air, light, temperature, minerals, climate, pressure etc. The biotic factors of the ecosystem depend on the abiotic factors for their survival.
(2) Biotic factors : The biotic factor include the living organisms of the environment. e.g., Plants, animals, bacteria, viruses etc. The biotic factors of an ecosystem are classified into three main groups, namely :
(i) Producers : The organisms which carry out photosynthesis constitute the producers of an ecosystem. e.g., Plants algae and bacteria.
(ii) Consumers : Consumers are organisms which eat or devour other organisms. The consumers are further divided into three or more types. They are primary consumers, secondary consumers and tertiary consumers.
(a) Primary consumers : They eat the products like plants, algae and bacteria. The primary consumers are also called herbivores.
(b) Secondary consumers : They kill and eat the herbivores. They are also called carnivores. As these carnivores directly depend on herbivores, they are specifically called primary carnivores. Fox, wolf, etc. are the secondary consumers in a terrestrial ecosystem.
(c) Tertiary consumers : They kill and eat the secondary consumers. They are also called secondary carnivores. e.g., Lion, tiger, etc.
(iii) Reducers or Decomposers : The decomposers are heterotrophs organisms that break up the dead bodies of plants and their waste products. They include fungi and certain bacteria. They secrete enzymes. The enzymes digest the dead organisms and the debris into smaller bits or molecules. These molecules are absorbed by the reducers. After taking energy, the reducers release molecules to the environment as chemical to be used again by the producers.
(iv) Other heterotrophs
(a) Scavengers or Detrivores : They feed on corpses, e.g., Vulture, Carrion Beetle. They help in quick disposal of dead bodies. In the process they also leave small fragments for decomposers.
(b) Parasites : They obtain nourishment from a living host without capturing or killing the same. Parasites obtain food from all categories of organisms. Common parasites are bacteria, fungi some worm and some insects.
Types of ecosystem
The ecosystem may be large, as large as the world or small, as small as a cow dung ecosystem. The biosphere (The total life content of the world) is the major ecosystem. It comprises all other ecosystems.
(1) Mega ecosystem : The biosphere is formed of four mega ecosystems. They are as follows :
(i) Marine ecosystem : It is the largest ecosystem of earth. Fresh water ecosystem are two types :
(a) Lotic : Running water ecosystem as river.
(b) Lentic : Still water ecosystem such as pond or lake. It includes saline-water ecosystems like oceans, seas, estuaries, brackish waters, etc.
(ii) Limnic ecosystem : It includes all fresh water ecosystems like ponds, pools, lakes, rivers, streams, etc.
(iii) Terrestrial ecosystem : It includes the ecosystems of air, forests, grasslands, deserts, etc.
(iv) Industrial or Artificial ecosystems : These are man made ecosystems. e.g., Crop land, city, town, etc.
(2) Macro ecosystems : The four mega ecosystem is further divided into sub units called macro ecosystems. e.g., Forests. The terrestrial macro ecosystem is formed of many forest ecosystems.
(3) Meso ecosystem : The macro ecosystem is further divided into meso ecosystem. For example, the forest ecosystem is formed of many meso ecosystems like deciduous forest, coniferous forests, etc.
(4) Micro ecosystem : The meso ecosystem is further divided into micro ecosystems, e.g., A low land in a forest, a mountain in a forest, etc. All ecosystems in the world are further divided into natural and artificial ecosystems.
(5) Natural ecosystems : These are self–regulating systems without much direct human interference and manipulations. e.g., Ponds, lakes, rivers, seas, oceans, grasslands, deserts, etc.
Forest ecosystem : Forests are natural plant communities with dominance of phanerogams. In India forests occupy approx. 1/10 of the land area. Indian forests can be divided into the 4 broad categories.
(1) Abiotic components : This includes inorganic and organic substances present in the atmosphere and soil. The climate (temperature, light, rainfall etc.) and soil (minerals) vary from forest to forest. In addition to minerals the occurrence of litter is the characteristic feature of majority of forests. Through litter decomposition approx. 90% energy trapped in the ecosystem by autotrophs dissipates into space as heat energy. The litter fall increases with decreasing latitudes.
(2) Biotic components
(i) Producers : They are represented mainly by trees but shrubs and ground flora are also found. Depending upon the kinds of forest the flora varies.
(ii) Consumers
(a) Primary consumers (herbivores) are small animals feeding on tree leaves, include ants, beetles, flies, bugs, spiders, leaf hoppers etc. neelgai, deer, elephants, moles, squirrels and fruit bats are large animals which feed upon shoots and/or fruits.
(b) Secondary consumers are different kinds of birds, snakes, lizards, feeding on primary consumers.
(c) Tertiary consumers are tiger and lion are the top carnivores.
(iii) Decomposers : Streptomyces (sps. of Angiococus, Bacillus and Pseudomonas) and Fungi (Aspergillus, Polyporus, Alternaria, Fusarium, Trichonderma) are helpful in decomposing the litter. Further litter decomposition found to be slow in cooler and drier areas, therefore, sometimes its accumulation on the soil surface makes a thick layer.
Fresh water pond ecosystem : A pond is a suitable example for ecosystem. It is a lentic fresh-water ecosystem. It contains shallow standing water. The pond ecosystem is formed of abiotic factors and biotic factors.
(1) Abiotic factors : The abiotic factors of the pond ecosystem are water, \[C{{O}_{2}},\,\,{{O}_{2}}\] inorganic compounds, organic compounds, light, temperature, pressure, pH etc.
(2) Biotic factors : The biotic factors of the pond ecosystem are producers, consumers and reducers.
(i) Producers : The producers synthesize the energy from abiotic substances. The producers of a pond include phytoplankton like diatoms, blue green algae (Oscillatoria), green algae, green flagellates (Volvox, Euglena, Chlamydomonas), rooted plants, submerged plants and floating plants.
(ii) Consumers : Consumers eat other organisms. The organisms which depend on producers are called primary consumers or herbivores. e.g., Zooplankton (Cyclops, Daphnia, larvae of Chironomus etc), Dysticus (insect), Lymnaea (snail) etc. The primary consumers are eaten by the secondary consumers or carnivores. These carnivores are called primary carnivores because they are the first carnivores in the food chain. e.g., Small fishes, frogs etc. The secondary consumers are eaten by the tertiary consumers or secondary carnivores. e.g., large fish.
(iii) Reducers or Decomposers : Several bacteria, fungi and actinomycetes represent this group e.g., Aspergillus, Saprolegnia.
Dynamics of ecosystem : The various components of the ecosystem constitute an interacting system. They are connected by energy, nutrients and minerals. The continuous survival of the ecosystem depends on the flow of energy and the circulation of nutrients and minerals in the ecosystem. Thus the dynamics (functions) of the ecosystem includes the following :
(1) Energy : Energy is the ability to do work. The flow of energy is unidirectional in the ecosystem. The main source of energy for an ecosystem is the radiant energy or light energy derived from the sun. The amount of solar radiation reaching the surface of the earth is 2 cals/sq.cm/min. It is more or less constant and is called solar constant or solar flux. About 95 to 99% of the energy is lost by reflection. Plants utilize only 0.02% of the energy reaching earth. The light energy is converted into chemical energy in the form of sugar by photosynthesis.
\[6{{H}_{2}}O+6C{{O}_{2}}+Light\to 6{{C}_{6}}{{H}_{12}}{{O}_{6}}+6{{O}_{2}}\]
(2) Primary production : Plants convert light energy into chemical energy in the form of sugar by photosynthesis. The total amount of sugar and other organic materials produced in plants per unit area per unit time is called gross primary production. During photosynthesis respiration is also going on side by side. During respiration some amount of sugar is oxidised. Hence it is not easy to measure gross primary production. The total organic material actually present (biomass) in plants is called net primary production.
Net primary production = Gross primary production - Respiration. i.e.,
\[Pn=Pg-R\]
\[\therefore Pg=Pn+R\]
Where, Pg = Gross primary production
Pn = Net primary production
R = Respiration
Thus the amount of organic material produced during a given period of time per unit area is called primary production. The productivity is generally expressed in terms of grams or kilocalories per square meter per day or per year.
(3) Secondary production : The energy trapped by the producers (primary production) is utilized by the consumers. The producers are directly consumed by the herbivores that are eaten by the primary carnivores that in turn are consumed by the secondary carnivores. The consumers store some amount of energy in their tissues. This energy, stored by the consumers, is called secondary production. Only about 10 to 20% of the primary production is converted into secondary production. The remaining 80 to 90% is lost by the consumers in the form of faeces.
(4) Food chain : The biotic factors of the ecosystem are linked together by food. For example, the producers form the food for the herbivores. The herbivores form the food for the carnivores. The sequence of the eaters being eaten is called food chain to another trophic level.
Producers\[\to \]Herbivores\[\to \]Carnivores
The various steps in a food chain are called trophic levels. Owing to repeated eating being eaten, the energy is transferred from one trophic level.
This transfer of energy from one trophic level to another is called energy flow. A typical food chain can be seen in a pond ecosystem. The algae and phytoplakton are eaten by the zooplankton. The zooplankton are eaten by fishes which are eaten by snakes.
Types of food chains : The food chains are of two types, namely :
(i) Grazing food chain : This food chain starts from plants, goes through herbivores and ends in carnivores.
Plants\[\to \]Herbivores\[\to \]Primary Carnivores\[\to \]Sec. Carnivores
This type of food chain depends on the autotrophs which capture the energy from solar radiation. A few chains are given below :
Grass\[\to \]Grasshopper\[\to \]Lizard\[\to \]Hawk
Grass\[\to \]Mouse\[\to \]Snake\[\to \]Hawk
Phytoplankton\[\to \]Zooplankton\[\to \]Fish\[\to \]Snake.
The grazing food chain is further divided into two types, namely : (a) Predator (b) Parasitic.
(ii) Detritus food chain : It starts from dead organic matter and ends in inorganic compounds. There are certain groups of organisms which feed exclusively on the dead bodies of animals and plants. These organisms are called detritivores. The detritivores include algae, bacteria, fungi, protozoans, insects, millipeds, centipeds, crustaceans, mussels, clams, annelid worms, nematodes, ducks, etc.
(5) Food web : In an ecosystem the various food chains are interconnected with each other to form a net work called food web. The interlocking of many food chains is called food web. Simple food chains are very rare in nature. This is because each organism may obtain food from more than one tropic level. In other words, one organism forms food for more than one organisms of the higher trophic level.
Example : In a grassland ecosystem
Significance of food web : Food webs are very important in maintaining the stability of an ecosystem. For example, the deleterious growth of grasses is controlled by the herbivores. When one type of herbivores increase in number and control the vegetation.
Similarly, when one type of herbivorous animal becomes extinct, the carnivore predating on this type may eat another type of herbivore.
(6) Trophic levels : Each food chain contains many steps like producers, herbivores, primary carnivores and so on. Each step of the food chain is called trophic level. The number of trophic levels in a food chain in restricted to 5 or 6. Green plants make first trophic level.
\[{{T}_{1}}\to \]Producers - (Trees, Plants, Grass)
\[{{C}_{1}}\] or \[{{T}_{2}}-\] Herbivorous – (Cow, Grass hopper, Zooplankton)
\[{{C}_{2}}\] or \[{{T}_{3}}-\]Primary carnivorous (Dog, Frog, Lizard)
\[{{C}_{3}}\] or \[{{T}_{4}}\]Secondary carnivorous (Hawk, Fox, Snake)
\[{{C}_{4}}\]or \[{{T}_{5}}\] Tertiary carnivorous or Top carnivorous (Tiger, Lion, Man)
Phytoplankton\[\to \]Zooplankton\[\to \]Fishes\[\to \]Snakes
\[Tr.\text{ }{{L}_{1}}Tr.\text{ }{{L}_{2}}~~~~~~~~Tr.\text{ }{{L}_{3}}~~~~~~Tr.\text{ }{{L}_{4}}\]
(7) Energy flow : The transfer of energy from one trophic level to another trophic level is called energy flow. The flow of energy in an ecosystem is unidirectional. That is, it flows from the producer level to the consumer level and never in the reverse direction. Hence energy can be used only once in the ecosystem. But the minerals circulate and recirculate many times in the ecosystem. A large amount of energy is lost at each trophic level. It is estimated that 90% of the energy is lost when it is transferred from one trophic level to another.
Hence the amount of energy available decreases from step to step. Only about 10% of the biomass is transferred from one trophic level to the next one is a food chain. And only about 10% chemical energy is retained at each trophic level. This is called 10% law of Lindeman (1942). When the food chain is short, the final consumers may get a large amount of energy. But when the food chain is long, the final consumer may get a lesser amount of energy.
(8) Ecological pyramids : The number, biomass and energy of organisms gradually decrease from the producer level to the consumer level. This can be represented in the form of a pyramid called ecological pyramid. Ecological pyramid is the graphic representation of the number, biomass, and energy of the successive trophic levels of an ecosystem. The use of ecological pyramid was first described by Charles Elton in 1927. In the ecological pyramid, the producer forms the base and the final consumer occupies the apex. There are three types of ecological pyramids, namely :
(i) The pyramid of number : The number of individuals at the trophic level decreases from the producer level to the consumer level. That is, in an ecosystem the number of producers is far high. The number of herbivores is lesser than the producers. Similarly, the number of carnivores is lesser than the herbivores.
In a cropland ecosystem : In croplands the crops are more in numbers. The grasshoppers feeding on crop plants are lesser in number. The frogs feeding on grasshopper are still lesser in number. The snakes feeding on frogs are fewer in number.
Crop\[\to \]Grasshopper\[\to \]Frogs\[\to \]Snakes\[\to \]Hawks
In a grassland ecosystem : In a grassland the grasses are there in large numbers. The consumers decrease in the following order.
Grass\[\to \]Grasshopper\[\to \]Lizard\[\to \]Hawk
Grass\[\to \]Rabbit\[\to \]Fox\[\to \]Lion
In a pond ecosystem : The number in a pond ecosystem decreases in the following order.
Phytoplankton\[\to \]Zooplankton\[\to \]Fishes\[\to \]Snakes
(ii) The pyramid of biomass : 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 : In a grassland the biomass of grasses is the maximum, and it gradually decreases towards the consumer level in the following order.
Grass\[\to \]Mouse\[\to \]Snake\[\to \]Hawk
Grass\[\to \]Grasshopper\[\to \]Lizard\[\to \]Hawk
In a forest : In a forest the biomass or trees is the maximum and the biomass of the top consumer is the minimum. The decrease in weight occurs in the following order :
Plants\[\to \]Deer\[\to \]Fox\[\to \]Tiger
Plants\[\to \]Rabbit\[\to \]Fox\[\to \]Lion
(iii) Pyramid of energy : The energy flow in an ecosystem from the producer level to the consumer level. At each trophic level 80 to 90% of energy is lost. Hence the amount of energy decreases from the producer level to the consumer level. This can be represented in a pyramid of energy level to the consumer level. This can be represented in a pyramid of energy.
In a grassland : In a grassland green plants trap the maximum light energy. The energy gradually decreases towards the top consumer level.
Grass\[\to \]Grasshopper\[\to \]Lizard\[\to \]Hawk
Grass\[\to \]Rabbit\[\to \]Fox\[\to \]Lion
Grass\[\to \]Mouse\[\to \]Snake\[\to \]Hawk
In a pond : In a pond maximum energy is trapped by the phytoplankton. Then the amount of energy decreases towards the top-consumer level.
Phytoplankton\[\to \]Zooplankton\[\to \]Fish\[\to \]Snake
Phytoplankton\[\to \]Zooplankton\[\to \]Small fish\[\to \]Large fish
Inverted pyramid of numbers : When the ecosystem contains lesser number of producers and more number of consumers, the pyramid will be inverted in shape. Inverted pyramid occurs in a tree ecosystem. A single tree (producer) contains many fruit eating birds (primary consumers). The birds contain numerous parasites (secondary consumers).
Inverted pyramid of biomass : When the biomass of producers is less and that of consumers is more the pyramid will have inverted shape. It occurs in a pond or lake ecosystem. Here the biomass of diatoms and phytoplankton are negligible as compared to that of crustaceans and small fishes.
Importance of ecosystem
(1) Energy : Study of ecosystems provides information about amount of energy flowing into them, its harvesting and availability at various levels.
(2) Biogeochemical cycling : Density of ecosystem is governed by degree of biogeochemical cycling and the amount of inorganic nutrients entering the ecosystem from outside.
(3) Food webs : Each ecosystem has a number of food webs. The knowledge of food webs is helpful to restore a degraded ecosystem and prevent unscientific exploitation of different ecosystems.
(4) Protection : Each ecosystem whether natural or man-made requires protection from pollutants and pests.
(5) Inter-relationships : Study of ecosystems gives information about inter-relationships amongst various types of organisms as well as between organisms and their abiotic environment.
(6) Carrying Capacity : By knowing the carrying capacity of an ecosystem, it can be known as to the number of producers and consumers which can be supported by that ecosystem.
(7) Inputs : The shortage of inputs can be known and corrected.
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