1. AN ECOSYSTEM IS A FUNCTIONAL UNIT OF NATURE where living organisms interact among themselves and also with the surrounding physical environment.
2. ENTIRE BIOSPHERE A global ecosystem, as a composite of all local ecosystems on Earth.
3. GLOBAL ECOSYSTEM two categories: Aquatic and Terrestrial ecosystem.
4. Forest, grassland and desert Terrestrial Ecosystems.
5. Pond, lake, wetland, river and estuary Aquatic Ecosystems.
6. Crop fields and an aquarium may also be considered as MAN-MADE ECOSYSTEMS.
o Interaction of biotic and abiotic components result in a physical structure that is characteristic for each type of ecosystem.
8. VERTICAL DISTRIBUTION OF DIFFERENT SPECIES OCCUPYING DIFFERENT LEVELS IS CALLED STRATIFICATION.
o For example, trees occupy top vertical strata or layer of a forest, shrubs the second and herbs and grasses occupy the bottom layers.
9.The components of the ecosystem are seen to function as a unit when we consider the following aspects:
(iii) Energy flow; and
(iv) Nutrient cycling.
A constant input of solar energy is the basic requirement for any ecosystem to function and sustain.
Primary production is the amount of biomass or organic matter produced per unit area over a time period by plants during photosynthesis.
o THE RATE OF BIOMASS PRODUCTION IS CALLED PRODUCTIVITY.
It is expressed in terms of g–2 yr–1 or (kcal m–2) yr–1 to compare the productivity of different ecosystems.
It can be divided into ●
gross primary productivity (GPP) and ●
net primary productivity (NPP).
Gross primary productivity of an ecosystem is the rate of production of organic matter during photosynthesis.
A considerable amount of GPP is utilised by plants in respiration.
NPP is Gross primary productivity minus respiration losses (R), ■
GPP – R = NPP .
oNet primary productivity is the available biomass for the consumption to heterotrophs (herbivores and decomposers).
Secondary productivity is the rate of formation of new organic matter by consumers.
oPrimary productivity depends on the plant species inhabiting a particular area.
oIt also depends on a variety of environmental factors, availability of nutrients and photosynthetic capacity of plants.
Therefore, it varies in different types of ecosystems.
The annual net primary productivity of the whole biosphere is approximately 170 billion tons (dry weight) of organic matter.
The annual NPP of terrestrial ecosystem is 115 billion tons(dry weight) of organic matter.
Despite occupying about 70 per cent of the surface, the productivity of the oceans are only 55 billion tons.
complex organic compounds of detritus are converted to carbon dioxide, water and inorganic nutrients by the decomposers.
Decomposition involves three processes, namely FRAGMENTATION OF DETRITUS, LEACHING AND CATABOLISM.
Dead plant remains such as leaves, bark, flowers and dead remain of animals, including faecal matter, constitute detritus, which is the raw material for decomposition.
o DETRITIVORES (E.G., EARTHWORM) BREAK DOWN DETRITUS INTO SMALLER PARTICLES. THIS PROCESS IS CALLED FRAGMENTATION.
BY THE PROCESS OF LEACHING, water-soluble inorganic nutrients go down into the soil horizon and get precipitated as unavailable salts.
oBacterial and fungal enzymes degrade detritus into simpler inorganic substances. THIS PROCESS IS CALLED AS CATABOLISM.
oIt is important to note that all the above steps in decomposition operate simultaneously on the detritus.
Humification and mineralisation occur during decomposition in the soil.
leads to accumulation of a dark coloured amorphous substance called HUMUS which is highly resistant to microbial action and undergoes decomposition at an extremely slow rate. Being colloidal in nature it serves as a reservoir of nutrients.
Further degradation of humus by some microbes and release of inorganic nutrients from humus occur by the process known as MINERALISATION
Decomposition is largely an oxygen-requiring process.
The rate of decomposition is controlled by chemical composition of detritus and climatic factors.
In a particular climatic condition, decomposition rate is slower if detritus is rich in lignin and chitin, and quicker, if detritus is rich in nitrogen and water- soluble substances like sugars.
TEMPERATURE AND SOIL MOISTURE are the most important climatic factors that regulate decomposition through their effects on the activities of soil microbes.
Warm and moist environment favour decomposition whereas low temperature and anaerobiosis inhibit decomposition resulting in build up of organic materials.
Except for the deep-sea hydro-thermal ecosystem (chemosynthesis: hydrogen sulphide is the source of energy), sun is the only source of energy for all ecosystems on Earth.
Incident solar radiation less than 50 per cent of it is PHOTOSYNTHETICALLY ACTIVE RADIATION (PAR).
oPlants and photosynthetic bacteria (autotrophs), fix sun’s radiant energy to make food from simple inorganic materials.
oPlants capture only 2-10 per cent of the PAR and this small amount of energy sustains the entire living world. So, it is very important to know how the solar energy captured by plants flows through different organisms of an ecosystem.
oAll organisms are dependent for their food on producers, either directly or indirectly. So, THERE IS UNIDIRECTIONAL FLOW OF ENERGY FROM THE SUN TO PRODUCERS AND THEN TO CONSUMERS.
oECOSYSTEM NEED A CONSTANT SUPPLY OF ENERGY to synthesise the molecules they require, to counteract the universal tendency toward increasing disorderliness.
oFirst, plants capture solar energy and then, food is transferred from the producers to decomposers.
oOrganisms of different trophic levels in nature are connected to each other for food or energy relationship forming a food chain/food web.
oThe energy trapped by the producer, hence, is either passed on to a consumer or the organism dies.
oDeath of organism is the beginning of the detritus food chain/web.
oAll animals depend on plants (directly or indirectly) for their food needs.
They are called consumers and also heterotrophs.
oIf they feed on the producers, the plants, they are called primary consumers (herbivore), and if the animals eat other animals they are called secondary consumers (carnivore/primary carnivore).
oSecondary carnivore depends on primary carnivore for food.
oTHE DETRITUS FOOD CHAIN (DFC) BEGINS WITH DEAD ORGANIC MATTER.
It is made up of decomposers which are heterotrophic organisms, mainly fungi and bacteria.
They meet their energy and nutrient requirements by degrading dead organic matter or detritus.
These are also known as saprotrophs (sapro: to decompose).
13.A simple grazing food chain (GFC) is depicted below:
(Producer) >(Primary Consumer) > (Secondary consumer)
14.Decomposers secrete digestive enzymes that breakdown dead and waste materials into simple, inorganic materials, which are subsequently absorbed by them.
15. IN AN AQUATIC ECOSYSTEM, GFC IS THE MAJOR CHANNEL FOR ENERGY FLOW WHEREAS IN A TERRESTRIAL ECOSYSTEM, A MUCH LARGER FRACTION OF ENERGY FLOWS THROUGH THE DETRITUS FOOD CHAIN THAN THROUGH THE GFC.
oSome of the organisms of DFC are prey to the GFC animals so the GFC and DFC are connected at some level.
16.Some animals like cockroaches, crows, etc., are omnivores.
17.These natural interconnection of food chains make it a food web.
18. FOOD WEB IS MORE STABLE THAN FOOD CHAIN.
19. BASED ON THE SOURCE OF THEIR NUTRITION OR FOOD, ORGANISMS OCCUPY A SPECIFIC PLACE IN THE FOOD CHAIN THAT IS KNOWN AS THEIR TROPHIC LEVEL.
20. Producers belong to the first trophic level, herbivores (primary consumer) to the second and carnivores (secondary consumer) to the third trophic level and tertiary consumer (top carnivore) occupies the fourth trophic level.
21.THE AMOUNT OF ENERGY DECREASES AT SUCCESSIVE TROPHIC LEVELS.BIOLOGICAL CLASSIFICATION
22.When any organism dies it is converted to detritus or dead biomass that serves as an energy source for decomposers.
23.Organisms at each trophic level depend on those at the lower trophic level for their energy demands.
24.Each trophic level has a certain mass of living material at a particular time called as the STANDING CROP.
oThe standing crop is measured as the mass of living organisms (biomass) or the number in a unit area.
25.The biomass of a species is expressed in terms of fresh or dry weight.
oMeasurement of biomass in terms of dry weight is more accurate.
26.The number of trophic levels in the grazing food chain is restricted as the transfer of energy follows 10 per cent law – only 10 per cent of the energy is transferred to each trophic level from the lower trophic level.
27.A given species may occupy more than one trophic level in the same ecosystem at the same time; for example, a sparrow is a primary consumer when it eats seeds, fruits, peas, and a secondary consumer when it eats insects and worms.
In most ecosystems, all the pyramids, of number, of energy and biomass are upright, i.e., producers are more in number and biomass than the herbivores, and herbivores are more in number and biomass than the carnivores. Also, energy at a lower trophic level is always more than at a higher level.
The pyramid of number for a tree ecosystem is initially inverted at producer level then become upright. On one tree there are many insects (primary consumer) then fewer birds (secondary consumer) then very few large birds (tertiary consumer).
The pyramid of biomass in sea is also generally inverted because the biomass of fishes far exceeds that of phytoplankton.
PYRAMID OF ENERGY IS ALWAYS UPRIGHT, can never be inverted, because when energy flows from a particular trophic level to the next trophic level, some energy is always lost as heat at each step.
Decomposer are not given any place in ecological pyramids even though they play a vital role in the ecosystem.
oPyramid shows a simple food chain, something that almost never exists in nature; it does not accommodate a food web where a species presents in more than one trophic level.
29. ECOLOGICAL SUCCESSION:
o Composition and structure of community constantly change in response to the changing environmental conditions. This change is orderly and sequential, parallel with the changes in the physical environment.
o These changes continue till the establishment of CLIMAX COMMUNITY.
o Climax community is in near equilibrium with the environment.
o The climax community remains stable as long as the environment remains unchanged.
o The gradual and fairly predictable change in the species composition of a given area is called ecological succession.
o During succession some species colonise an area and their populations become more numerous, whereas populations of other species decline and even disappear.
o The entire sequence of communities that successively change in a given area are called sere(s).
o The individual transitional communities are termed seral stages or seral communities.
o In the successive seral stages there is a change in the diversity of species of organisms, increase in the number of species and organisms as well as an increase in the total biomass.
30. Succession and evolution would have been parallel processes at that time.
31. Succession is a process that starts where no living organisms are there.
32. PRIMARY SUCCESSION:
o These could be areas where no living organisms ever existed.
o Example- newly cooled lava, bare rock, newly created pond or reservoir.
o The establishment of a new biotic community is generally slow.
o Before a biotic community of diverse organisms can become established, there must be soil.
o Depending mostly on the climate, it takes natural processes several hundred to several thousand years to produce fertile soil on bare rock.
33. SECONDARY SUCCESSION
o takes place in areas that somehow, lost all the living organisms that existed there.
o Example – abandoned farm lands, burned or cut forests, lands that have been flooded.
o Since some soil or sediment is present, succession is faster than primary succession.
o During ecological succession, change in vegetation also affect the food and shelter of that area so change in animal community also accompanied with it.
o AT ANY TIME DURING PRIMARY OR SECONDARY SUCCESSION, NATURAL OR HUMAN INDUCED DISTURBANCES (FIRE, DEFORESTATION, ETC.), CAN CONVERT A PARTICULAR SERAL STAGE OF SUCCESSION TO AN EARLIER STAGE.
o Also, such disturbances create new conditions that encourage some species and discourage or eliminate other species.
34. SUCCESSION OF PLANTS:
o HYDRARCH SUCCESSION takes place in wetter areas and the successional series progress from hydric to the mesic conditions (medium water condition).
o XERARCH SUCCESSION takes place in dry areas and the series progress from xeric to mesic conditions.
o THE SPECIES THAT INVADE A BARE AREA ARE PIONEER SPECIES.
o In primary succession on rocks these are usually lichens which secretes acids to dissolve rock, helping in weathering and soil formation.
o After some soil formation, some very small plants like bryophytes grow. Then they are succeeded by bigger plants, and after several more stages, ultimately a stable climax forest community is formed.
o THE CLIMAX COMMUNITY remains stable as long as the environment remains unchanged.
o With time the xerophytic habitat gets converted into a mesophytic one.
o IN HYDROSERE, the pioneers are the small phytoplankton, they are replaced with time by free-floating angiosperms, then by rooted hydrophytes, sedges, grasses and finally the trees. The climax again would be a forest.
o With time the water body is converted into land.
o IN SECONDARY SUCCESSION the species that invade depend on the condition of the soil, availability of water, the environment as also the seeds or other propagules present.
o Since soil is already there, the rate of succession is much faster and hence, climax is also reached more quickly.
o In primary succession, establishment of climax community takes thousands of years whereas secondary succession is much faster and hence, climax is also reached more quickly.
o All succession whether taking place in water or on land, either primary or secondary proceeds to a similar climax community – the mesic.
The amount of nutrients, such as carbon, nitrogen, phosphorus, calcium, etc., present in the soil at any given time.
It varies in different kinds of ecosystems and also on a seasonal basis.
36. The movement of nutrient elements through the various components of an ecosystem is called NUTRIENT CYCLING/BIOGEOCHEMICAL CYCLES.
37. AMOUNT OF NUTRIENTS NEVER LOST IN ANY ECOSYSTEM.
38. NUTRIENT CYCLES ARE OF TWO TYPES: (A) GASEOUS AND (B) SEDIMENTARY.
39.Nitrogen and carbon cycles are example of gaseous cycle where atmosphere serves as reservoir.
40.Sulphur and phosphorus cycles are the example of sedimentary cycle where earth crust serves as reservoir.
41.Environmental factors, e.g. soil, moisture, pH, temperature etc., regulate the rate of release of nutrients into the atmosphere.
42.The function of the reservoir is to meet with the deficit which occurs due to imbalance in the rate of influx and efflux.
carbon constitutes 49 per cent of dry weight of organisms and is next only to water.
71 per cent of total global carbon is found dissolved in oceans. This oceanic reservoir regulates the amount of carbon dioxide in the atmosphere.
The atmosphere contains only about 1per cent of total global carbon.
FOSSIL FUEL ALSO REPRESENT A RESERVOIR OF CARBON.
Carbon cycling occurs through atmosphere, ocean and through living and dead organisms.
According to one estimate 4 × 1013 kg of carbon is fixed in the biosphere through photosynthesis annually.
Out of which considerable amount of carbon returns to the atmosphere as CO2 through respiratory activities of the producers and consumers.
Decomposers also contribute substantially to CO2 pool by their processing of waste materials and dead organic matter of land or oceans.
Some amount of the fixed carbon is lost to sediments and removed from circulation.
Burning of wood, forest fire and combustion of organic matter, fossil fuel, volcanic activity are additional sources for releasing CO2 in the atmosphere.
Human activities have significantly influenced the carbon cycle.
Rapid deforestation and massive burning of fossil fuel for energy and transport have significantly increased the rate of release of carbon dioxide into the atmosphere leading to greenhouse effect in.
o Phosphorus is a major constituent of biological membranes, nucleic acids and cellular energy transfer systems.
o Many animals also need large quantities of this element to make shells, bones and teeth.
o THE NATURAL RESERVOIR OF PHOSPHORUS IS ROCK.
o Phosphorus is present in the form of phosphate in rock.
o When rocks are weathered, minute amounts of these phosphates dissolve in soil solution and are absorbed by the roots of the plants then available to consumers via plant.
o The waste products and the dead organisms are decomposed by phosphate- solubilising bacteria releasing phosphorus.
o The major and important differences between carbon and phosphorus cycle are firstly, atmospheric inputs of phosphorus through rainfall are much smaller than carbon inputs, and, secondly, gaseous exchanges of phosphorus between organism and environment are negligible.
o 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, generate fertile soils, provide wildlife habitat, maintain biodiversity, pollinate crops, provide storage site for carbon and also provide aesthetic, cultural and spiritual values.
o ROBERT CONSTANZA AND HIS COLLEAGUES HAVE VERY RECENTLY TRIED TO PUT PRICE TAGS ON NATURE’S LIFE-SUPPORT SERVICES.
Biodiversity class 12 NCERT PDF