Ecosystem | CBSE Biology Class XII Notes

➢  INTRODUCTION

✔     An ecosystem can be visualised as a functional unit of nature, where living organisms interact among themselves and also with the surrounding physical environment.

✔     Ecosystem is the interaction of living things among themselves and with their surrounding environment.

✔     There are two basic ecosystems

o   Terrestrial

▪    Forest, grassland and desert ecosystem

o   Aquatic

▪    Pond, lake, wetland, river and estuary ecosystem

➢  ECOSYSTEM- STRUCTURE AND FUNCTION

✔     The interactions between the various biotic and abiotic factors of an ecosystem lead to the maintenance of the ecosystem.

✔     Stratification :  Vertical distribution of different species occupying different levels.

o   trees occupy top vertical strata or layer of a forest,

o   shrubs the second and

o   herbs and grasses occupy the bottom layers.

✔     The components of the ecosystem

o   Productivity;

o   Decomposition;

o   Energy flow; and

o   Nutrient cycling

✔     Example of pond ecosystem:

o   Abiotic component: the water with all the dissolved inorganic and organic substances and the rich soil deposit at the bottom of the pond.

o   Producers : autotrophic components that include the phytoplankton, some algae and the floating, submerged and marginal plants found at the edges.

o   Decomposers: the fungi, bacteria and flagellates especially abundant in the bottom of the pond.

o   The pond performs all the functions of any ecosystem and of the biosphere as a whole,i.e.,

▪    Conversion of inorganic into organic material with the help of the radiant energy of the sun by the autotrophs;

▪    Consumption of the autotrophs by heterotrophs;

▪    decomposition and mineralisation of the dead matter to release them back for reuse by the autotrophs.

o   There is unidirectional movement of energy towards the higher trophic levels and its dissipation and loss as heat to the environment.

➔  PRODUCTIVITY

✔     A constant input of solar energy is the basic requirement for any ecosystem to function and sustain.

✔     Primary production: The amount of biomass or organic matter produced per unit area over a time period by plants during photosynthesis. [Unit: weight (g – 2 ) or energy (kcal m – 2 )].

✔     Productivity: The rate of biomass production. [Unit: g –2 yr –1 or (kcal m – 2 ) yr –1]

✔     Gross primary productivity (GPP): The rate of production of organic matter during photosynthesis.

✔     Net primary productivity (NPP): Gross primary productivity minus respiration losses (R). GPP – R = NPP

✔     Secondary productivity: The rate of formation of new organic matter by consumers.

✔     Primary productivity depends upon-

o   type of plant species inhabiting a particular area

o   photosynthetic capacity of plants

o   nutrient availability

➔  DECOMPOSITION

✔     Decomposition: Break down complex organic matter into inorganic substances like carbon dioxide, water and nutrients by the decomposers.

✔     Detritus: Dead plant remains such as leaves, bark, flowers and dead remains of animals, including fecal matter.

✔     Detritus acts as the raw materials for the decomposition.

✔     The important steps in the process of decomposition are fragmentation, leaching, catabolism, humification and mineralisation.

o   Fragmentation: Breakdown of detritus into smaller particles by detritivores (earthworm)

o   Leaching: Water- soluble inorganic nutrients go down into the soil horizon and get precipitated as unavailable salts.

o   Catabolism: Bacterial and fungal enzymes degrade detritus into simpler inorganic substances.

o   Humification: Accumulation of humus (a dark coloured amorphous substance).

▪    Humus is highly resistant to microbial action.

▪    It undergoes decomposition at an extremely slow rate.

▪    It serves as a reservoir of nutrients.

o   Mineralisation: Degradation of humus to release inorganic nutrients.

✔     Decomposition is largely an oxygen-requiring process.

✔     Rate of decomposition is controlled by:

o   chemical composition of detritus

▪    decomposition rate is slower if detritus is rich in lignin and chitin.

▪    quicker, if detritus is rich in nitrogen and water-soluble substances like sugars.

o   climatic factors

▪    Warm and moist environments favour decomposition.

▪    Low temperature and anaerobiosis inhibit decomposition.

➔  ENERGY FLOW

✔     Photosynthetically active radiation (PAR) :Of the incident solar radiation less than 50 % of solar radiation; that can be used by autotrophs to make food from simple inorganic materials.

✔     Plants capture only 2-10 percent of the PAR.

✔     Producers: The green plant in the ecosystem that produces the food.

o   In terrestrial ecosystem: herbaceous and woody plants

o   In the aquatic ecosystem: various species like phytoplankton, algae and higher plants.

✔     Consumers: All animals depend on plants (directly or indirectly) for their food needs.

✔     Food Chain:

✔     Grazing food chain (GFC): A food chain that begins with producers.

✔     Detritus food chain (DFC): A food chain that starts with dead organic matter.

o   It is made up of decomposers which are heterotrophic organisms, mainly fungi and bacteria.

o   They meet their energy and nutrient requirements by degrading dead organic matter or detritus.

o   Secrete digestive enzymes that break down dead and waste materials into simple, inorganic materials, which are subsequently absorbed by them. (saprophytes)

✔     Food Web: Interlinked food chains.

✔     Trophic level: Every organism occupies a specific level in their food chain known as the trophic level.

o   Producers – first trophic level

o   Herbivores (primary consumer) – second trophic level

o   Carnivores (secondary consumer) – third trophic level

✔     Standing crop: The mass of living material (biomass) that is present in a trophic level at a particular time.

✔     10% law: Only 10 percent of the energy is transferred to each trophic level from the lower trophic level;

o   As a result of which the number of trophic levels in the grazing food chain is restricted.

➢  ECOLOGICAL PYRAMIDS:

✔     An ecological pyramid is a graphical representation of the food or energy relationship between organisms at different trophic levels.

✔     The relationship is expressed in terms of number, biomass or energy.

✔     The base of each pyramid represents the producers or the first trophic level while the apex represents tertiary or top level consumers.

➔  Pyramid of numbers in a grassland ecosystem:

✔     Only three top-carnivores are supported in an ecosystem based on production of nearly 6 millions plants.

➔  Pyramid of biomass:

✔     Sharp decrease in biomass at higher trophic levels.

➔  Inverted pyramid of biomass:

✔     Small standing crop of phytoplankton supports a large standing crop of zooplankton.

✔     E.g.-      Number of insects feeding on a big tree.

o   Pyramid of biomass in sea – biomass of fishes far exceeds that of phytoplankton.

➔  An ideal pyramid of energy:

✔     Only 1% of the energy in the sunlight available to them into NPP

✔     Pyramid of energy is always upright, can never be inverted-

o   Because when energy flows from a particular trophic level to the next trophic level, some energy is always lost as heat at each step.

✔     A trophic level represents a functional level and not a single species as such. Also, a single species may become a part of more than one trophic level in the same ecosystem at the same time depending upon the role it plays in the ecosystem.

➔  Limitations of ecological pyramids:

o   The ecological pyramids do not take into account the same species belonging to more than one trophic level.

o   It assumes a simple food chain that almost never exists in nature. It does not explain food webs.

o   Saprophytes are not given a place in ecological pyramids even though they play a vital role in the ecosystem.

➢  ECOLOGICAL SUCCESSION

✔     The gradual and fairly predictable change in the species composition of a given area.

✔     The composition and structure of a community constantly change in response to the changing environmental conditions.

✔     These changes lead finally to a climax community.

✔     Climax community: The community that is in near equilibrium with the environment.

✔     Sere: The entire sequence of communities that successively change in a given area.

✔     Seral stages / seralcommunities : The individual transitional communities.

✔     Primary succession: The succession that happens in areas where no life forms ever existed as in bare rocks, cool lava, etc.

o   It takes hundreds to thousands of years as developing soil on bare rocks is a slow process.

✔     Secondary succession: The succession that happens in areas which have lost all life forms due to destructions and floods, etc.

o   Since some soil or sediment is present, succession is faster than primary succession.

➔  Successions of plants

✔     Hydrarch succession: It takes place in wetter areas and the successional series progress from hydric to the mesic conditions.

✔     Xerarch succession: It takes place in dry areas and the series progress from xeric to mesic conditions.

✔     Both hydrarch and xerarch succession lead to medium water conditions (mesic) – neither too dry (xeric) nor too wet (hydric).

✔     Pioneer species: The first species that invade a bare area.

Primary succession on rocks:

o   Lichens are the pioneer species.

o   They secrete acids to dissolve rock, helping in weathering and soil formation.

o   This later helps the plants like bryophytes to grow there.

o   The bryophytes with time gets succeeded by bigger plants.

o   After several more stages, ultimately a stable climax forest community is formed.

o   The xerophytic habitat gets converted into a mesophytic one.

✔     The climax community remains stable as long as the environment remains unchanged.

Primary succession in water :

o   Pioneer species are the small phytoplanktons.

o   They are replaced with time by rooted-submerged plants.

o   Rooted-floating angiosperms are replaced by free-floating plants

o   The free-floating plants are replaced by reed-swamp, marsh-meadow, scrub and finally the trees.

o   The climax community here is forest.

o   With time the water body is converted into land.

Secondary Succession:

✔     The pioneer species depends on the following factors:

o   Condition of the soil

o   Availability of water

o   The environment

o   The seeds or other propagules present

✔     As the soil is present at the beginning the climax community is reached much quickly.

➢  NUTRIENT CYCLING

✔     The amount of nutrients present in the soil at a given time is known as the standing state.

✔     Nutrients are never lost from the ecosystem. They are only recycled from one state to another.

✔     The movement of nutrients through the various components of the ecosystem is called nutrient cycling or biogeochemical cycles. They are of two types:

o   Gaseous Reservoirs for these types of cycles exist in the atmosphere.

o   Sedimentary − Reservoir for these types of cycles exist in the earth’s crust.

➔  Ecosystem-Carbon Cycle

✔     About 49% of the dry weight of living organisms is made up of carbon.

✔     The ocean reserves and fossil fuels regulate the amount of CO₂ in the atmosphere.

✔     Plants absorb CO₂ from the atmosphere for photosynthesis, of which a certain amount is released back through respiratory activities.

✔     A major amount of CO₂ is contributed by the decomposers who contribute to the CO₂ pool by processing dead and decaying matter.

✔     The amount of CO₂ in the atmosphere has been increased considerably by human activities such as burning of fossil fuels, deforestation.

➔  Ecosystem-Phosphorous Cycle

✔     Phosphorus is a major constituent of biological membranes, nucleic acids and cellular energy transfer systems.

✔     Many animals also need large quantities of this element to make shells, bones and teeth.

✔     The natural reservoir of phosphorus is rock, which contains phosphorus in the form of phosphates.

✔     When rocks are weathered, some of the phosphate gets dissolved in the soil solution and is absorbed by plants.

✔     The consumers get their phosphorus from the plants.

✔     Phosphorus returns back to the soil by the action of phosphate- solubilising bacteria on dead organisms.

➢  Ecosystem Services

✔     The products of ecosystem processes are named as ecosystem services:

o   healthy forest ecosystems purify air and water, mitigate droughts and floods, cycle nutrients, generate fertile soils, provide wildlife habitat, maintain biodiversity, pollinate crops, etc.

✔     Robert Constanza and his colleagues tried to put price tags on nature’s life-support services.

✔     The Average price tag was calculated to be of US $ 33 trillion a year on these fundamental ecosystems services.

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