Ecosystem is a self-regulatory and self-sustaining structural and functional unit of landscape (biosphere) consisting of a community of living beings and the physical environment both interacting and exchanging materials between them .A.G. Tansley (1935) coined the term ecosystem.Ecosystems are divisible into two categories, terrestrial or land ecosystem (e.g., forests, grasslands, desert, garden)and aquatic or water ecosystem (e.g., ponds, lakes,streams, estuaries, sea).
COMPONENTS OF ECOSYSTEM
Every ecosystem has both structure and function. The Structural component includes all the living organisms and their inter-relationships. The functional components include exchange of materials, energy flow, nutrient cycling, etc.Ecosystem has two major components namely, abiotic(non-living) components and biotic (living) components.
Abiotic components
Abiotic components of an ecosystem consist of non-living substances and factors. The important ones include temperature, wind, light, water, soil, minerals, etc.(Discussed in detail in previouS chapter Organisms and Population).
Biotic components
All the living organisms in an ecosystem constitute the biotic component. This biotic component is further classified as follows depending on their nutritional
trophic) relationships.
STRUCTURE OF ECOSYSTEM
Interaction of biotic and abiotic components results in a physical structure that is characteristic for each type of ecosystem.
Species composition: identification and enumeration of plant and animal species of an ecosystem gives its species composition.
Stratification : It is formation of vertical layers where vegetation is dense, e.g., 5-7 strata in tropical rain forests.Stratification is absent or rare in deserts.
Trophic structure: Trophic structure of ecosystem is a type of producer - consumer arrangement, in which each food level is called trophic level. The producers occupy the first trophic level, herbivores the second, and the carnivores constitute the third trophic level. Each ecosystem has specific food chains and food webs, e.g. grazing food chain in grassland.
Standing crop: It is the amount of living biomass present in an ecosystem. It indicates the productivity and luxuriance of growth. Dry weight is preferred over fresh weight because the latter is liable to be influenced by seasonal moist differences.
Standing state : It is the amount of inorganic nutrients present any time in the soil/water of an ecosystem. It tends to vary from season to season and ecosystem to ecosystem.
FUNCTIONS OF ECOSYSTEM
Four important functional aspects of the ecosystem are productivity, decomposition, energy fow and nutrient cycling.
Productivity-Productivity refers to the rate of biomass production ie., the rate at which sunlight is
captured by producers for the synthesis of energy rich organic compounds.It is of following types:Primary productivity,Secondary productivity,Net productivity.
Energy accumulated by plants (producers) in an ecosystem is called primary production.Primary productivity is the amount of biomass or organic matter produced per unit area over a time period by plants during photosynthesis.It can be divided into gross primary productivity and net primary productivity The rate of energy production by green plants Using organic matter during photosynthesis is the gross primary productivity (GPP).
A considerable amount of GPP is utilised by plants in respiration. The energy left after respiration and stored as organic matter in the producers is called net primary productivity (NPP).
Secondary productivity refers to the rate of assimilation of he food energy at the level of consumers, it is the amount of energy available at the consumer level for transfer to the next trophic level.
Net productivity refers to the rate of storage of organic matter not used by the heterotrophs (consumers) i.e equivalent to net primary production minus consumption by the heterotrophs
during the unit period, as a season or year etc.
Decomposition
This physical and chemical breakdown of complex organic remains with the help of organisms called decomposers.Organic remains (dead plant parts, animal remains and
excretions) are also called detrits.
The processes involved in decomposition are:-
 |
| Decomposition cycle |
Fragmentation of detritus : Detritivores (e.g termites, carrion beetles, earthworms) feed on larger
pieces and smaller fragments are left
Catabolism:The decomposers (e.g., bacteria,fungi) excrete digestive enzymes over the detritus. It changes insoluble complex organic substances into simple and soluble organic compounds and inorganic substances.
Leaching: Soluble substances formed during decomposition are subjected to leaching or passage to deeper layers of soil/ground water by percolating water
Decomposition process gives rise to humus and inorganic nutrients by humifcation and mineralisation respectively.Humification is the process of formation of humus from
detritus or organic remains. Humus is dark coloured,amorphous and organic matter rich in lignin and cellulose.It is highly resistant to microbial action and undergoes decomposition at an extremely slow rate. It is colloidal in nature, a reservoir of nutrients and helpful in maintenance
of soil moisture as well as aeration. Nutrients are released slowly as the humus is slowly decomposed.
Mineralisation is the release of inorganic substances, both non-mineral (e.g.,CO2, H2O) and minerals (e.g., Ca2*,Mg", K", NH4) from organic matter.
Decomposition is dependent on oxygen availability. It also depends on the chemical composition of detritus and climatic conditions. Decomposition is faster when detritus is rich in nitrogen and sugars than when detritus is nch in lignin and chitin. Warm and moist environment favours decomposition.
Energy flow
The Sun is the ultimate source of energy in all ecosystems.Of the sun's energy which reaches the Earth, about 400 is reflected immediately from the clouds, dust in the atmosphere and the Earth's surface without having any heating effect. A further 15% is absorbed and converted to
heat energy in the atmosphere.The remaining 35% of incoming energy penetrates to the
Earth's surface. Just under half the radiation striking the Earth's surface is in the photosynthetically active range (PAR), the visible wavelengths. However, even under
optimum conditions only a very small proportion, about 5% of incoming radiation (10% PAR) is converted in photosynthesis into gross primary productivity (GPP).
Roughly 20% of it is consumed in respiration so that netcapture of energy (net primary productivity) is 0.8-4% of incident radiation or I.6-8% of PAR. Flow of energy in an ecosystem is unidirectional.
Trophic levels
Trophic level is a step or division of the food chain which is characterised by the method of obtaining its food. The number of trophic levels is equal to the number of steps in the food chain. The two fundamental trophic levels are producers and consumers.
Producers belong to the first trophic level or T,. They are autotrophic or photosynthetic organisms found in an ecosystem which synthesize organic nutrients from
inorganic raw materials with the help of solar radiations not only for themselves but also for heterotrophic organisms or consumers.Consumers are heterotrophic organisms which cannot
manufacture their own food. They obtain ready-made organic food from outside sources. Depending upon the mode of obtaining nourishment, heterotrophic organisms are of three main types - herbivores, carnivores and decomposers.
Herbivores or consumers of first order constitute the second trophic level or T,. Consumers of the second order or primary carnivores form third trophic level or T3.There may be 2 -3 levels of carnivores. The ultimate or top carnivores belong to T4 or Ts trophic level.Decomposers from the last or detritus trophic level (eg. To).Parasites do not have any fixed trophic level since they feed on producers, herbivores as well as carnivores of various levels, e.g., aphids, ticks, mites, leeches, mosquitoes.
Food chain
A food chain is defined as a sequence of organisms in which there is transfer of food energy through a series of repeated processes of eating and being caten. A food chain is always straight and proceeds in a progressive straight line.Usually there are 4 or 5 trophic levels in the food chain.Food chans are of three type- parasitic, detritus and
grazing.Parasitic food chain, also called auxiliary food chain begins with host and usually ends in parasites. Detritus food chain (DFC) begins with detritus or dead organic matter. Detritivores and decomposers feed over it.Therefore, food energy present in detritus passes into them.
Detritivores and decomposers are consumed by smaller carnivores which in turn become food for larger carnivores and so on. A common detritus food chain with earthworm as detritivore is
Grazing food chain (GFC) is the most common food chain. It is also called predator food chain as predation Occurs at every step. This food chain consists of producers,
consumers and decomposers.The sequence of food chain in an aquatic ecosystem is as follows
Grazing food chain (GFC) 1s the most common food chain. It is also called predator food chain as predation occurs at every step. This food chain consists of producers,consumers and decomposers.The sequence of food chain in an aquatic ecosystems as follow
Food web
Food web is a network of food chains which become interconnected at various trophic levels so as to form a number of feeding connections amongst different organisms of a biotic community.
The energy stored by plants is passed through the community of an ecosystem in a series of steps of eating and being eaten. There is a transfer of 10% of energy from one trophic level to another. This is also called the 10% law which was formulated by Raymond Lindemann (1942).
ECOLOGICAL PYRAMIDS
An ecological pyramid is a graphic representation of the relationship between the individuals present in various trophic levels of a food chain with producers forming the base and top carnivores the tip. The relationship may be in terms ot number, Biomass and energy content
ot producers and consumers. Ecological pyramids were developed by Charles Elton (1927) and are, therefore, also known as Eltonian pyramids.
In a pyramid, the various steps of a food chain are represented sequence-wise with producers at the base,herbivores above them, followed by primary carnivores and then top carnivores constituting the top of the pyramid.
Ecological pyramids are of three general types :
Pyramid of number: shoWing the number of individual organisms at each level.
Pyramid of biomass : showing the total dry weight and other suitable measures of total amount or the living matter
Pyramid of energy: Showing the rate of energy flow and/or productivity at successive trophic Level. Pyramids of number and biomass may be upright or inverted depending upon the nature of the food chain in the particular ecosystem, whereas pyramids of energy are always upright.
In a parasitic food chain, the pyramid of numbers is always inverted. This is due to the fact that a single plant may support the growth of many herbivores and each herbivore in turn may provide nutrition to several parasites, which support many hyperparasites. Thus, from the producer towards consumers, there is a reverse position, i.e., the number of organisms gradually shows an increase, making the pyramid inverted in shape.However, In pond ecosystem, the producers are small organisms, their biomass is the least, and this value gradually shows an increase towards the apex of the pyramid, thus making the pyramid of biomass inverted in shape.
ECOLOGICAL SUCCESSION
Biotic or ecological succession is the natural development of a series of biotic communities
at the same site, one after the another till a climax community develops which does not evolve
further because it is in perfect harmony with the environment of the area.
A biotic community is influenced by biotic factors,physico-chemical factors and geographical factors.During succession some species colonise an area and their populations become more numerous, whereas populations of other species decline and even disappear.The present day communities in the world have came into existence because of the succession that has occurred over the millions of years since life had started on earth.
Actually Succession and evolution would have been
parallel processes at that time.Ecological succession is of two types: primary and secondary succession.
Primary succession is a biotic succession which occurs on a previously bare or unoccupied area, e.g., new exposed rock area, sand dunes, 1gneous rocks, deltas, newly Created pond or reservoir.
The first biotic community which develops in a bare area is called pioneer community. Climax community is the stable, self -perpetuating and final biotic community that develops at the end of biotic succession and 1S in perfect harmony with the physical environment. Climax the community is also termed as climatic climax community.
Climax community has maximum diversity and niche specialization.The various biotic communities that develop during biotic succession or the intermediate communities between the pioneer and climax communities are termed as seral stages or transitional communities.
Another general type of succession is secondary succession which starts from previously built up substrata with already existing living matter. The action of any external force, such as, a sudden change in climatic factors,biotic intervention, fire, etc., causes the exIsting community
to disappear.
Some examples of primary succession
Hydrosere
Hydrosere is an ecological succession in the newly formed poid or lake. It starts with the colonization of some phytoplanktons which form the pioneer plant community, and finally terminates into a forest, which is the climax community.
The various stages together with their chief components of plant species during primary succession in water are: Phytoplankton stage/Pioneer stage: Constitutes the
pioneer community. Some blue-green algae, green algae, diatoms and bacteria etc. were the first organisms to colonize the primitive medium of the pond. Rooted submerged stage: A new habitat suitable for the growth of rooted submerged hydrophytes like Elodea,
Utricularia, Potamogeton, Hydrilla etc.rooted floating stage : These plants colonize the
habitat with their rhizomes. They all are rooted floating hydrophytes which include Nelumbo, Limnanthemum,Trapa, Nymphaea etc. Some free floating plants such as Azolla, Lemna, Wolffia, Pistia, Salvinia etc. also become associated with the rooted plants, due to availability of salts and other minerals in abundance.Reed swamp stage: The plants of this community are
rooted but most parts of their shoots (assimilatory organs) remain exposed to air. Species of Sagittaria, Phragmites, Typha etc. are the chief plants of this stage.
Marsh or Sedge meadow stage : Because of the successive decrease in water level and further changes in the substratum, species of Carex, Juncus, Cyperus,
Eleocharis, etc. colonize this area. Woodland stage: Due to disappearance of marshy sedge-
In the meadow stage, soil becomes drier for most time of the year. This area is now invaded by terrestrial plants, which are some shrubs (Salix and Cornus) and trees (Alnus and
Populus).Climax forest stage: Forest stage is the climax community.The woodland community is gradually invaded by several trees e.g., Acer, Quercus.
Lithosere (A xerosere on rock)
The sequence of successional stages that occur on bare rocks is called lithosere. The first inhabitants or pioneers of such a habitat are usually lichens in the temperate regions. Crustose lichen stage : The lichens of this stage are represented by the species of Rinodina, Lecanora, Rhizocarpon etc. They produce some acids which cause weathering of rocks. The dead organic matter of lichens becomes mixed with small (weathered) particles of rocks. As a result, these lichens are replaced by foliose types of lichens.
Foliose lichen stage : They can absorb and retain more water and are able to accumulate dust particles which build up the suitable substratum for the moss stage. Moss stage: A thin soil layer develops on rock surfaces,especially in crevices, and favours the growth of such xerophytic mosses as species of Tortula, Grimmia and Polytrichum. Herbs stage: This stage is constituted by shallow rootedgrasses as Festuca, Solidago, Aristida, etc.Shrub stage: Due to much accumulation of soil, the habitat becomes suitable for shrub vegetation which start migrating in the area. This habitat includes the species of Physocarpus, Rhus etc. Forest stage: From shrub stage, there develops finally a forest community.
Importance of biotic succession
It tells us how a biotic seral stage like grasses and herbs of a pasture can be maintained by not allowing the biotic succession to proceed further through interference like grazing and ire. Information gained through biotic successIon is used in having controlled growth of one or more species by preventing their superiors to invade the area, e.g.,maintenance of teak forest.
Dams are protected by preventing siltation and biotic succession to occur.it gives information about the techniques to be used during reforestation and afforestation.
Points to be noted
Autogenic succession :When the succession has begun, the vegetation itself is responsible for replacing itself by changing exISting environmental conditions.Allogenic succession: When in succession other conditions (not vegetation itself) are responsible for replacing communities, then it is called allogenic Succession.
Autotrophic succession is characterised by early and continued dominance of autotrophic organisms like green plants. It starts in a predominantly inorganic environment and the energy flow is maintained indefinitely.
Heterotrophic succession is characterised by early dominance of heterotrophs such as bacteria, fungi and actinomycetes. It begins a predominantly organic environment and there i1s a progressive decline in the energy content.
Induced succession occurs due to extensive external interference. Here the initial community has high productivity which gradually decreases. Agriculture can be deemed as an example of induced succession. flere a steady stage is maintained 1or an ultimate good harvest Due to environmental thrust and human interference the climax vegetation may retrograde into shrub land or savannah. This is referred to as retrogressive Succession.When the succession doesn 't proceed through its normal course and side tracks, the advancement line is called deflated succession.
NUTRIENT CYCLING
Biogenetic nutrients/biogeochemicals are essential elements required by organisms for their body building and metabolism which are provided by earth and return to earth after their death and decay. The amount of nutrients varies in different kinds of ecosystems and also on a seasonal basis.
There are two types of nutrient cycles: gaseous and sedimentary. The gaseous type of nutrient cycle is generally located in the atmosphere or the hydrosphere e.g., carbon, oxygen, water, nitrogen, hydrogen. In the sedimentary type, the reservoir exists in the earth 's crust,e.g., phosphorus, calcium, magnesium.
Carbon cycle
 |
Carbon is the main component of all the organic compounds of protoplasm like carbohydrates, lipids, nucleic acids,enzymes, hormones. The biospheric carbon cycle is primarily concerned with the atmospheric CO2 gas, its incorporation into organic matter by photosynthesis andits subsequent release by the respiration by all biota.
Carbon also occurs in the earth's rock predominantly as calcium and magnesium carbonates. The reservoir pool is lithosphere. The carbon fixed by producers enters the food chain and is hence passed to herbivores, carnivores and decomposers. During photosynthesis the carbon the component of the atmosphere and hydrosphere decreases.It can be replenished by various methods like:
-Respiration
-DecompoSition of organic wastes
-Burning of wood and fossil fuel
-Weathering of carbonate containing rocks
-Volcanic eruptions and hot springs.
Some carbon is being removed from circulation and added to lithosphere by hard carbonaceous shells, animal skeletons, fossiliızation, seepage of carbon rich water into interior earth and caving in of Forests during earthquakes.Major exchange in carbon cycle is between organisms and the atmosphere or hydrosphere.
Phosphorus cycle
Phosphorus is an inorganic component of nucleic acids,phospholipids, ATP, bones and teeth. It takes part in metabolic reactions involved in release of energy from food and utilization of this energy in various functions of the body. It is found in nature, in soil, as rock phosphate,in combination with calcium, iron and aluminium etc.
Phosphate circulates in the abiotic environment in the lithosphere as well as in the hydrosphere. Phosphate present in the soil may occur in insoluble form. It is dissolved by chemicals secreted by microorganisms and plant roots.The dissolved phosphate is absorbed by the plants and changed to organic form. Phosphate fertilizers are added to the soil to increase its availability.From plants, phosphorus travels to animals along with the food chain. Animal excretions and dead bodies of organisms are acted upon by decomposers.
 |
| Phosphorus cycle |
Phosphorus is released in the process. The same becomes available for re-utilization by plants. Inside the soil, some phosphorus is lost through leaching.Similarly, a sufficient amount of phosphorus combines with calcium, iron or aluminium and becomes insoluble. It settles down at the bottom of a lake or ocean as sediment. Bone and teeth may also remain undegraded. Such phosphorus becomes part of the lithosphere. It is released after a very long interval when these rocks containing them are exposed to weathering agencies or are mined.
Nitrogen cycle
 |
| Nitrogen cycle |
The conversion of organic nitrogenous substances in soil into NH3 (ammonia) is called ammonification, which is due to ammonifying bacteria, e.g., Bacillus mycoides, B. vulgaris and B. ramosus, etc.The conversion of NH3 in soil into nitrates and nitrites is called nitrification, which is done by nitrifying bacteria,e.g., Nitrosomonas, Nitrosococcus (convert NH3 into nitrites) and Nitrobacter (convert nitrites into nitrates).The conversion of nitrates and nitrites in soil into atmospheric N2 is called denitrification, which is done by denitrifying bacteria, e.g., Micrococcus denitrificans and Bacillus denitrificans
Sulphur cycle
In nature, sulphur mainly occurs in elemental form as metal sulphides and sulphates. SO2 comes back to earth as H;SO4 after getting dissolved in rainwater.Starting with photos, athesis, plants pick up sulphur in the form of sulphate which is utilized by them in the synthesis of amino acids like cysteine, cystine and methionine.From plants, it is transferred to the consumers. Excretion and dead bodies of organisms carry it back to the earth. The same is broken by the decomposers to release HaS under anaerobic conditions; sulphates under aerobic conditions or occasionally as elemental sulphur.photosynthetic green bacteria (e.g., Chlorobium limicola) can carry the reduction of H;S to elemental sulphur. The purple bacteria (eg. Thiopedea rosea, Rhodopseudomonas palustris) oxidises H2S to sulphates which may be recirculated and taken up by the producers.
 |
| Sulphur cycle |
ECOSYSTEM SERVICES
Healthy ecosystems are the base for a wide range of economic, environmental and aesthetic goods and services. The products of ecosystem processes are named as ecosystem services, for example, healthy forest ecosystems purify air and water, mitigate droughts and
floods, cycle nutrients.For optimum utility of ecosystems they must be healthy.
For example:
Soil: Soil formation and soil protection are the major ecosystem services accounting for nearly 50% of their total worth.Plant cover protects the soil from drastic changes in temperature.
Perennial water : Plant litter and humus prevent run off water, hold water like sponge and allow
percolation of water. A lot of water is held in the soil which slowly passes towards the perched water table. It comes out as springs. 1 hey are a source of perennial fresh water which is quite pure.
Air : Plant cover of natural ecosystems absorb pouring gases, cause settling of suspended particulate matter, removes CO, and releases O,Wetlands : Ecosystems protect the land from floods,remove sediments and other pollutants and recharge ground water.
Chimate: lIncrease in atmospheric humidity and good rainfall have moderate effects on climate.
Biodiversity: Ecosystem maintains the biodiversity which is of enormous benefits for mankind.
Nutrient cycling : Due to the ecosystem, there is no depletion of nutrients but the same are repeatedly circulated and recirculated. It keeps the fertility of soil intact.
Wildlife habitat : Ecosystems are habitat of wildlife.
Pollination : Bees and other insects of natural ecosystems visit nearby farmlands and pollinate the crop plants.
Tribals : A large number of tribals live in forests.Grazing grounds: Ecosystems are grazing areas for numerous cattle.Other values : Natural ecosystems are a source of spiritual, cultural and aesthetic values.