Ecosystem

 1. Ecosystem

n The term ecosystem was coined by Sir A.G. Tansley (1935).

n An ecosystem is the basic functional ecological unit in which living organisms interact among themselves and with their surrounding physical environment.

n The size of the ecosystem varies from small pond to a large forest or sea.

2. Classification of Ecosystem

3. Ecosystem Structure and Function

n The ecosystem consists of biotic and abiotic components and their interaction with each other resulting in a physical structure.

n The flow of energy takes place within these components of the ecosystem.

n The identification and enumeration of plant and animal species in an ecosystem gives it species composition.

n Vertical distribution of different species occupying different levels is called stratification, e.g., in a forest ecosystem, trees occupy top vertical strata or layer, shrubs the second, and herbs and grasses occupy the bottom layers.

n The major functions of an ecosystem include

(i) Productivity

(ii) Decomposition

(iii) Energy flow

(iv) Nutrient cycling

4. Pond Ecosystem

n It is a self-sustainable unit. A pond is a shallow water body with all the biotic and abiotic components.

n The inorganic and organic materials are conversed with the help of the radiant energy of sun by the autotrophs.

n Heterotrophs consume autotrophs.

n Decomposers decompose the dead organic matter to release minerals back for reuse by the autotrophs.

n The matter and minerals are recycled between biotic and abiotic components.

n The energy flow is unidirectional.

n Solar input, cycle of temperature, day length and other climatic conditions regulate rate of function of entire pond.

5. Productivity

n The rate of biomass or organic matter production per unit area, over a time period, by plant during photosynthesis is called productivity.

n It is expressed in kcal m–2 yr–1 or g/m2/yr or g m–2 y–1.

n The amount of biomass or organic matter produced per unit area over a time period in plants during photosynthesis is called primary production. It is expressed in terms of weight (g m–2) or energy (kcal m–2).

n The rate of formation of new organic matter by consumers is called secondary productivity.

Aspects of Primary Productivity

6. Factors Affecting Primary Productivity

n Plant species inhabiting a particular area.

n Environmental factors:

(i) Sunlight: The sunlight directly regulates the primary productivity because the plants perform photosynthesis with the help of sunlight. As tropical region receives maximum sunlight so it exhibits higher productivity.

(ii) Temperature: Temperature regulates the activity of enzyme. So, optimum temperature is required for proper functioning of enzyme.

(iii) Moisture: Rain (humidity) is required for higher primary productivity. Deserts have the lowest primary productivity as the soil is deficient in moisture.

n Availability of nutrients: Greater nutrients ensure greater primary productivity.

n Photosynthetic efficiency: Some plants have more efficiency to trap sunlight (sugarcane), so they accumulate more primary productivity.

n Annual Net Primary Productivity of whole biosphere is approx. 170 billion tons (dry weight) of organic matter. Of this, oceans occupy 70% of surface, productivity of oceans are only 55 billion tons. Rest if from land.

7. Decomposition

n The process of breaking down complex organic matter into inorganic substances like CO2, water and nutrients is called decomposition.

n The raw materials for decomposition including dead plant remains like leaves, bark, flowers, and animal remains and their faecal matter are called detritus.

Steps in Decomposition

(i) Fragmentation: The process of breaking down of detritus into smaller particles is called fragmentation, e.g., as done by earthworm (= farmer’s friend).

(ii) Leaching: The process by which water-soluble inorganic nutrients go down into the soil horizon and get precipitated as unavailable salts is called leaching.

(iii) Catabolism: The enzymatic process by which bacterial and fungal enzymes degrade detritus to simpler inorganic substances is called catabolism.

(iv) Humification: The process of accumulation of a dark coloured amorphous substance, called humus, that is highly resistant to microbial action and undergoes decomposition at an extremely slow rate is called humification. Humus being colloidal is reservoir of nutrients.

(v) Mineralisation: The process by which humus is further degraded by some microbes to release inorganic nutrients is called mineralisation.

8. Factors Affecting Decomposition

(i) Chemical composition of detritus

n The decomposition rate is sow if detritus is rich in lignin and chitin.

n The decomposition rate is higher when detritus is rich in nitrogen and water-soluble substances like sugars.

(ii) Climatic factors

n Warm and moist environment favours decomposition.

n Low temperature and anaerobiosis inhibit decomposition.

9. Food Chain and Food Web

S. No.	Food chain	Food web
(i)	The transfer of energy from producers to top consumers through a series of organisms is called food chain.	A number of food chains are inter-connected with each other, forming a web-like pattern is called food web.
(ii)	One organism holds only one position.	One organism can hold more than one position.
(iii)	The flow of energy can be easily calculated.	The flow of energy is very difficult to calculate.
(iv)	It is always straight and proceed in a progressive straight line.	Instead of straight line it is a series of branching lines.
(v)	Competition is limited to members of same trophic level.	Competition is amongst members of same and different trophic levels.

10. Types of Food Chains

n Two types of food chains can be observed in the ecosystem:

(i) Grazing Food Chain (GFC), e.g.,

(1)Grass→Rabbit→Lion(2)Grass(Producer)→Goat(Pr.Consumer)→Man(Sec.Consumer)

(ii) Detritus Food Chain (DFC), e.g., Dead leaves  Woodlouse  Blackbird

n In an aquatic ecosystem, GFC is the major conduit for energy flow.

n In terrestrial ecosystem, DFC may be connected with GFC at some levels. Some of the organisms of DFC are prey to GFC animals in a natural ecosystem. Some animals like cockroaches, crows, etc. are omnivores.

 

S. No.	Grazing food chain (GFC)	Detritus food chain (DFC)
(i)	It starts with green plants called producers as first trophic level.	It begins with dead organic matter and decomposers called saprophytes as first trophic level. Decomposers secrete digestive enzymes that breakdown dead and waste into simple, inorganic materials which are absorbed by them.
(ii)	A much less fraction of energy flows through this type of food chain.	A much large fraction of energy flows through this type of food chain.
(iii)	Energy for food chain comes from sun.	Energy for the food chain comes from organic remain or detritus.

 

11. Trophic Level

n In an ecosystem, an organism occupies a specific place in the food chain called trophic level.

n Each trophic level has a certain mass of living material at a particular time called the standing crop.

n The standing crop is measured as the biomass of living organisms (biomass), or the number in a unit area. Biomass is expressed interms of fresh or dry weight.

12. Energy Flow

n The sun is the only source of energy for all ecosystems on earth.

n Out of the total incident solar radiation, only 50 per cent of it is photosynthetically active radiation (PAR).

n Plants capture only 2–10 per cent of the PAR and this small amount of energy sustains the entire living world. So, there is unidirectional flow of energy from the sun to producers and then to consumers.

n The energy is transferred in an ecosystem, in the form of food which is degraded and lose major part of food energy as heat during metabolic activities and only a very small fraction becomes stored as biomass.

n This is correlated to second law of thermodynamics, i.e., ecosystems need a constant supply of energy to synthesize molecules they require, to counteract universal tendency towards increasing disorderliness.

n The green plants in the ecosystem which can trap solar energy to convert it into chemical bond energy are called producers.

n All the animals that depend for food on plants are called consumers or heterotrophs.

n Consumers are divided into the following categories:

(i) Primary consumers: Animals which feed directly on plants, i.e., herbivores.

(ii) Secondary consumers: Consumers that feed on primary consumers, i.e., carnivores.

(iii) Tertiary consumers: Consumers that feed on secondary consumers.

n Lindeman’s 10 per cent law: At each step of food chain, when food energy is transferred from one trophic level to the next higher trophic level, only about 10 per cent of energy is passed on to the next trophic level. This is known as Lindeman’s 10 per cent law given by Lindeman in 1942.

13. Ecological Pyramid

n The relationship between producers and consumers at different trophic levels in an ecosystem can be graphically represented in the form of a pyramid called ecological pyramid.

n Structure: The base always represents the producers or the first trophic level and the apex represents top level consumer or the last trophic level.

n Ecological pyramids are of three types:

(i) Pyramid of number

(ii) Pyramid of biomass

(iii) Pyramid of energy

(i) Pyramid of number:

The relationship between producers and consumers in an ecosystem can be represented in the form of a pyramid in terms of number of organisms at different trophic levels called pyramid of number.

It is inverted, when you count number of insects feeding on a big tree.

(ii) Pyramid of biomass:

The relationship between producers and consumers in an ecosystem can be represented in the form of a pyramid in terms of biomass called pyramid of biomass. It can be

(a) Upright, e.g., in case of grassland ecosystem; or

(b)Inverted, e.g., in case of pond ecosystem as biomass of fishes for exceeds that of phytoplanktons.

 

 

(iii) Pyramid of energy:

The relationship between producers and consumers in an ecosystem can be represented in the form of a pyramid, in terms of flow of energy called pyramid of energy. It is always upright because energy is always lost as heat at each step and as it follow 10% law.

 

n Limitations of ecological pyramids:

(i) It never takes into account the same species belonging to two or more trophic levels.

(ii) It assumes a simple food chain, which never exists in nature. It does not accommodate a food web.

(iii) In spite of the vital role played by saprophytes/decomposers, they are not given any position in ecological pyramids.

14. Ecological Succession

n The sequential, gradual and predictable changes in the species composition in an area are called succession or ecological succession.

n It mainly focuses on changes in vegetation that in turn affect types of animals.

n The entire sequence of communities that successively changes in a given area are called sere(s).

n The individual transitional communities are termed as seral stages or seral communities.

n The community that is in near equilibrium with the environment is called a climax community.

n The species that invade a bare area are called pioneer species.

n The changes that occur in successive seral stages to reach a climax community are:

(i) changes in the diversity of species of organisms.

(ii) increase in the total biomass.

(iii) increase in the number of species and organisms.

n Ecological succession is of two types:

(i) Primary succession: It begins in areas where no living organisms ever existed. Therefore, the establishment of a biotic community is very slow, e.g., newly cooled lava, bare rock, newly created pond or reservoir. It takes natural processes several hundred to thousand years to produce fertile soil on bare rock.

(ii) Secondary succession: It begins in areas where natural biotic communities have been destroyed, e.g., abandoned farm lands, buried or cut forests. Since soil is available, it is a faster process. The species that invade depend on the condition of soil, available of water, environment and seeds or other propagules present.

15. Succession of Plants

n The plant succession is of two types:

(i) Hydrarch succession: The plant succession which takes place in wet area or water, leading to successional series, progress from hydric to the mesic conditions.

(ii) Xerarch succession: The plant succession which takes place in a dry area, leading to successional series from xeric to mesic conditions.

(i) Primary succession in water

l The pioneer species are phytoplanktons.

l The phytoplanktons are replaced by free-floating angiosperms.

l Then, rooted angiosperms invade sedges, grasses and finally the trees.

l At last, a stable climax forest is formed.

l An aquatic habitat is converted into mesic habitat.

(ii) Primary succession on rocks

l Lichens are the pioneer species on a bare area.

l The lichen secretes some acids to dissolve rock and help in weathering and soil formation.

l Later, some small bryophytes invade and hold the small amount of soil.

l The bryophytes are succeeded by herbs, shrubs and ultimately big trees.

l At last, a stable climax forest is formed.

l The xerophytic habitat gets converted into a mesophytic one.

16. Nutrient Cycle

n The movement of nutrient elements through various components (abiotic + biotic) of an ecosystem is called nutrient cycling or biogeochemical cycle.

n The total amount of nutrients like carbon, phosphorus, calcium, etc., present in soil at any time is called standing state.

n Standing state varies with the kind of ecosystem and season.

n The nutrient reservoir meets the deficit arising due to imbalance in the rate of influx and efflux.

n Soil, moisture, PH, temperature regulate the rate of release of nutrients into the atmosphere.

n The nutrient cycles are of two types:

(i) Gaseous cycle

(ii) Sedimentary cycle.

 

S. No.	Gaseous cycle	Sedimentary cycle
(i)	The reservoir is the atmosphere.	The reservoir is the earth crust.
(ii)	The nutrient occurs as gas or vapour.	The nutrient is non-gaseous.
(iii)	It is comparatively quick or fast.	It is quite slow.
(iv)	For example, carbon cycle, nitrogen cycle.	For example, phosphorus cycle, sulphur cycle.

 

17. Carbon Cycle

n Carbon constitutes 49 per cent of dry weight of an organism.

n 71 per cent of the carbon is found dissolved in oceans which is responsible for its regulation in atmosphere.

n The carbon cycle occurs through atmosphere, oceans and through living and dead organisms.

n It is estimated that 4 × 1013 kg of carbon is fixed in the biosphere through photosynthesis annually.

n Carbon is returned to atmosphere as CO2 by animals and plants through respiration and the activities of decomposers.

n Some amount of fixed carbon is lost as sediments and removed from circulation.

n Burning of wood, forest fire, volcanic activity and combustion of organic matter and fossil fuels are some additional sources for releasing CO2 in the atmosphere.

n Human activities like deforestation and vehicular burning of fossil fuels have caused an increase in the amount of CO2 in atmosphere.

18. Phosphorus Cycle

n Importance of phosphorus:

(i) It is a major constituent of biological membranes, nucleic acids and cellular energy transfer systems.

(ii) It is required for making shells, bones and teeth.

n Rocks are the natural reservoirs of phosphorus.

n During weathering of rocks, minute amounts of these phosphates dissolve in soil solution and are absorbed by plants through roots.

n Herbivores and carnivores obtain this element from plants directly or indirectly.

n The waste products and the dead organisms are decomposed by phosphate-solubilising bacteria and in turn release phosphorus.

n Gaseous exchange of phosphorus between organisms and environment are negligible.

19. Ecosystem Services

n The products of ecosystem processes (economic, environmental and aesthetic) are called ecosystem services.

n Forests are the major source of ecosystem services. They

(i) purify air and water,

(ii) mitigate droughts and floods,

(iii) cycle nutrients,

(iv) generate fertile soils,

(v) provide wildlife habitat,

(vi) maintain biodiversity,

(vii) pollinate crops,

(viii) provide storage site for carbon,

(ix) provide aesthetic, cultural and spiritual values.

n Robert Constanza and his colleagues tried to put price tags on nature’s life-support services which came up to US$ 33 trillion a year which are taken for granted. This is nearly twice the value of the global gross national product, (GNP) which is US $18 trillion.

n Out of the total cost of various ecosystem services, the soil formation accounts for 50%; recreation and nutrient cycling contribute less than 10% each. Cost of climate regulation and wildlife habitat are about 6% each.