Biology Notes for Class 12
Chapter 14: Ecosystem
Chapter Summary
      An ecosystem is a structural and functional unit of nature and it comprises abiotic and biotic components. Abiotic components are inorganic materials- air, water and soil, whereas biotic components are producers, consumers and decomposers. Each ecosystem has characteristic physical structure resulting from interaction amongst abiotic and biotic components. Species composition and stratification are the two main structural features of an ecosystem. Based on source of nutrition every organism occupies a place in an ecosystem.
       Productivity, decomposition, energy flow, and nutrient cycling are the four important components of an ecosystem. Primary productivity is the rate of capture of solar energy or biomass production of the producers. It is divided into two types: gross primary productivity (GPP) and net primary productivity (NPP). Rate of capture of solar energy or total production of organic matter is called as GPP. NPP is the remaining biomass or the energy left after utilisation of producers. Secondary productivity is the rate of assimilation of food energy by the consumers. In decomposition, 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.
      Energy flow is unidirectional. First, plants capture solar energy and then, food is transferred from the producers to decomposers. Organisms of different trophic levels in nature are connected to each other for food or energy relationship forming a food chain. The storage and movement of nutrient elements through the various components of the ecosystem is called nutrient cycling; nutrients are repeatedly used through this process. Nutrient cycling is of two types—gaseous and sedimentary. Atmosphere or hydrosphere is the reservoir for the gaseous type of cycle (carbon), whereas Earth’s crust is the reservoir for sedimentary type (phosphorus). Products of ecosystem processes are named as ecosystem services, e.g., purification of air and water by forests.
      The biotic community is dynamic and undergoes changes with the passage of time. These changes are sequentially ordered and constitute ecological succession. Succession begins with invasion of a bare lifeless area by pioneers which later pave way for successors and ultimately a stable climax community is formed. The climax community remains stable as long as the environment remains unchanged.
ECOSYSTEMS
Ø The interaction between the living organism and the non-living environment is called ecosystem.
14.1 ECOSYSTEM – STRUCTURE AND FUNCTION:
Ø Interaction of biotic and abiotic components results in a physical structure that is characteristic of each type of ecosystem.
Ø Identification and description of plant and animal species of an ecosystem gives its species composition.
Ø Vertical distribution of different species occupying different levels is called stratification.
Ø The components of the ecosystem are seen to function as a unit:
- Productivity.
- Decomposition.
- Energy flow and
- Nutrient cycle.
Description of pond as an ecosystem:
Ø The abiotic components include all dissolved inorganic and organic substances and the rich soil deposit at the bottom of the pond.
Ø The solar input, cycle of temperature, day length, regulates the rater of function of the entire pond.
Ø The producer (autotrophic) includes phytoplankton, some algae and the floating, submerged and marginal plants found in edge of pond.
Ø The consumers are represented by zooplankton, free swimming and bottom dwelling animals.
Ø The decomposers are the fungi, bacteria especially abundant at the bottom of the pond.
Basic events (in terms of function) in an ecosystem:
14.2 PRODUCTIVITY:
Primary productivity:
Ø The amount of biomass or organic matter produced per unit area over a time period by plants during photosynthesis.
Ø It is expressed in terms of weight (g-2) or energy (kcal m-2)
Ø The rate of biomass production is called productivity.
Ø Gross primary productivity: (GPP) is the rate of production of organic matter during photosynthesis.
Net Primary productivity:
Ø A considerable amount of energy is utilized by plants in respiration.
Ø Gross primary productivity minus respiration losses (R) is the net primary productivity.
Ø GPP – R = NPP.
Ø Net primary productivity is the available biomass for the consumption to heterotrophs (herbivore and decomposers.
Ø Secondary productivity: is defined as the rate of formation of new organic matter by the consumer.
14.3 DECOMPOSITION:
Ø Earthworm is said to be ‘friends’ of farmer:
Ø Breakdown the complex organic matter.
Ø Loosening of the soil helps in aeration and entry of root.
Ø The decomposers break down complex organic matter into inorganic substances like carbon dioxide, water and nutrients, called decomposition.
Ø Dead plant remains such as leaves, bark, flowers and dead remains of animals, including fecal matter, constitute the detritus.
Ø The process of decomposition completed in following steps:
- Ø Fragmentation : Break down of detritus into smaller particles by detritivore (earthworm).
- Ø Leaching: Water soluble inorganic nutrients go down into the soil horizon and get precipitated as unavailable salts.
- Ø Catabolism : Bacterial and fungal enzymes degrade detritus into simple inorganic substances.
- Ø Humification: Accumulation of dark coloured amorphous substances called humus.
Importance of humus:
Ø Highly resistance to microbial action.
Ø Undergo decomposition at an extremely slow rate.
Ø Being colloidal in nature, it serves as reservoir for nutrients.
Ø Mineralization: The humus is further degraded by some microbes and release of inorganic nutrients occur.
Factor affects rate of decomposition:
Ø Decomposition is largely an oxygen-requiring process.
Ø Detritus rich in chitin and lignin has slow rate of decomposition.
Ø Detritus rich in nitrogen and water-soluble substance like sugar has faster decomposition.
Ø Temperature and soil moisture are most important climatic factor that regulate decomposition
Ø Warm and moist environment favor decomposition.
Ø Low temperature, dryness and anerobiosis inhibit decomposition.
14.4 ENERGY FLOW IN ECOSYSTEM:
Ø Except for deep sea hydrothermal ecosystem, sun is the only source of energy for all ecosystems on earth.
Ø Less than 50% of incident solar radiation is photosynthetically active radiations (PAR).
Ø Plants capture 2-10 % of PAR and used in photosynthesis.
Ø All organisms depend on the producers, either directly or indirectly.
Ø Energy flow in the ecosystem is unidirectional i.e. energy transferred from producer to consumers.
Ø Energy transfer is not absolute, and spontaneous, unless energy is degraded it can not be transfer. When energy transferred from one trophic level to another, lot of energy lost in the form of heat to the environment.
Ø Only 10% of energy transferred from one trophic level to other.
Food chain:
Grazing food chain:
Ø Detritus food chain: Begins with dead organic matter (detritus) and pass through detritus feeding organism in soil to organisms feeding on detritus-feeders.
Ø In aquatic ecosystem GFC is the major conduit for energy flow.
Ø In terrestrial ecosystems a much larger fraction of energy flows through the detritus food chain than through GFC
Ø Different food chains are naturally interconnected e.g. a specific herbivore of one food chain may serve as food of carnivores of other food chains. Such interconnected matrix of food chains is called food web.
Ø Trophic level: A group of organism irrespective of their size having same source of energy or similar food habit constitute a trophic level.
Ø Standing crop: each trophic level has a certain mass of living material at a particular time called as the standing crop.
Ø The standing crop is measured as the mass of living organisms (biomass) or the number in a unit area.
Ø The number of trophic levels in a food chain is restricted by 10 % flow of energy, less amount of energy available to the last trophic level.
14.5 ECOLOGICAL PYRAMID:
Ø The base of the pyramid is broad and it narrows down at the apex. The similar shape is obtained when food or energy relationship between organisms at different trophic level.
Ø The relationship can be expressed in terms of number, energy or biomass.
Ø The base of the pyramid represented by producer and apex is the top consumer; other trophic levels are in between.
Ø In most ecosystems, all the pyramids, of number, of energy and biomass are upright.
Ø The pyramid of number in a tree ecosystem is inverted.
Ø The pyramid of biomass in sea also inverted because the biomass of fishes is far exceeds that of phytoplankton.
Ø Pyramid of energy is always upright, can never be inverted, because when energy flows from a particular trophic level to the next, some energy is always lost as heat at each step.
Limitations of ecological pyramids:
Ø It does not take into account the same species belonging to two or more trophic levels.
Ø It assumes a simple food chain, it never exits in nature.
Ø It dose not accommodate food web.
Ø Saprophytes are not given place in ecological pyramids.
14.6 ECOLOGICAL SUCCESSION:
Ø The gradual and fairly predictable change in the species composition of a given area is called ecological succession.
Ø Composition and structure of the community constantly change in response to changing environmental condition.
Ø This change is orderly and sequential, parallel with the changes in the physical environment.
Ø All the changes lead finally to a community that is in near equilibrium with the environment and that is called climax community.
Ø During succession some species colonize and area and their populations become more numerous, whereas populations of other species decline and even disappear.
Ø The entire sequences of communities that successively change in a given area are called sere.
Ø The individual transitional communities are termed as seral stages.
Ø In the successive seral stages there is a change in the diversity of species of organisms, in crease in number of species and total biomass.
Ø Primary succession: succession that starts where no living organisms are there- these could be areas where no living organism ever existed may be a bare rock or new water body.
Ø Secondary succession: succession that starts in areas that somehow, lost all the living organisms that existed there.
Ø Primary succession occurs in:-
- newly cooled lava,
- bare rock,
- Newly created pond or reservoir.
- Secondary succession begins in areas where natural biotic communities have been destroyed such as
- In abandoned farm lands.
- Burned or cut forest,
- land that have been flooded
- Since some soil or sediment is present, secondary succession is faster than primary succession.
14.6.1 Succession in plants:
Ø Based on the nature of habitat – whether it is water or it is on very dry areas- succession of plants is called hydrarch or xerarch.
Ø Hydrarch      succession takes    place   in    water    areas   and    the   successional   series   progress from hydric to mesiccondition.
Ø Xerarch succession takes place in dry areas and the series progress from xeric to mesic conditions.
Ø Both hydrarch and xerarch successions lead to medium water conditions (mesic) – neither too dry (xeric) nor too wet (hydric).
Xerarch succession: Succession in bare rock:
Ø The species that invades bare area are called pioneer species.
Ø In primary succession on bare rock the pioneer species is the lichen.
Ø Lichen secretes acid to dissolve rock, helping in weathering and soil formation.
Ø The little soil, leads to growth of bryophytes (mosses).
Ø The mosses speed up the process of soil accumulation by trapping wind-blown particles.
Ø Lichen moss carpet provides suitable substratum for the germination of seeds of herbaceous plants.
Ø Gradually more soil is accumulated and herbaceous species make way for the invasion of shrubs followed by trees.
Ø The climax community is generally dominated by trees.
Ø Hydrarch (succession in aquatic environment)
Ø In primary succession in water, the pioneer species are phytoplankton.
Ø Zooplanktons.
Ø Sub merged plant stage. (rooted hydrophytes)
Ø Sub merged and free-floating plant stage.
Ø Reed-swamp stage.
Ø Marsh-meadow stage.
Ø Shrub stage
Ø Trees
Ø The climax again would be the forest
Ø All the succession whether taking place in water or on land, proceeds to a similar climax community – the mesic.
14.7 NUTRIENT CYCLING:
Ø Organism needs constant supply of nutrients to grow, reproduce, and regulate various body functions.
Ø Standing state: the amount nutrients such as carbon, nitrogen, phosphorus, calcium etc. present in soil at any given time.
Ø Nutrient cycling: The movement of nutrient elements through the various component of an ecosystem is called nutrient cycling.
Ø Another name of nutrient cycling is biogeochemical cycle.
Ø Nutrient cycles are of two types:
- Gaseous cycle
- Sedimentary cycle.
Ø The reservoir for gaseous type of nutrient cycle (nitrogen, carbon) exists in the atmosphere.
Ø The reservoir for sedimentary cycle (sulphur, phosphorus) is Earth’s crust.
Ø Environmental factors like soil, moisture, pH temperature regulate the rate of release of nutrient into the atmosphere.
Ø The function of the reservoir is to meet the deficit which occurs due to imbalance in the rate of influx and efflux.
14.7.1 Ecosystem – Carbon cycle:
Ø Carbon constitutes 49 percent of dry weight of organism.
Ø Out of total global carbon:
- 71 percent carbon found dissolved in ocean.
- About 1 percent in the atmosphere.
Ø 4 X 1013 kg of carbon is fixed in the biosphere by photosynthesis, annually.
Ø Large amount of carbon returned to the atmosphere as CO2 through respiration of producers and consumers.
Ø Decomposers also return CO2 to reservoir during decomposition process.
Ø Some amount of Carbon is lost to sediments and removed from circulation.
Ø Burning wood, forest fire, combustion of organic matter, fossil fuel, volcanic activities are additional sources for releasing CO2 to atmosphere.
Ø Influence of human activity on Carbon cycling.
Ø Rapid deforestation.
Ø Massive burning of fossil fuel for energy and transport
Ø Increased the rate of release of CO2 into the atmosphere.
14.7.2 Ecosystem Phosphorus cycle:
Ø Phosphorus is a major constituent of biological membranes, nucleic acids and cellular energy transfer system(ATP)
Ø Animals need phosphorus to make shell, bones and teeth.
Ø Reservoir pool of phosphorus is the rock, which contain phosphorus in the form of phosphates.
Ø During weathering of rock small amount of phosphates dissolved in soil solution and are absorbed by the roots of the plants.
Ø Herbivore and other animals obtain organic form of phosphorus from plants.
Ø The waste product and dead organisms are decomposed by phosphate-solubilising bacteria releasing phosphorus.
How phosphorus cycle differs from carbon cycle?
Ø There is no respiratory release of phosphorus into atmosphere.
Ø Atmospheric inputs of phosphorus through rainfall are much smaller.
Ø Gaseous exchange of phosphorus between organism and environment are negligible.
14.8 ECOSYSTEM SERVICES:
Ø The products of ecosystem processes are named as ecosystem services.
Ø Healthy forest ecosystems purify air and water.
Ø Mitigate droughts and flood.
Ø Cycle nutrients.
Ø Generates fertile soil.
Ø Provide wildlife habitat.
Ø Maintain biodiversity.
Ø Pollinate crops.
Ø Provide storage site for carbon
Ø Provides aesthetic, cultural and spiritual values
Abbreviation:
Ø PAR:      Photosynthetically Active Radiation
Ø GAP:      Gross Primary Productivity
Ø NPP:      NetPrimaryProductivity
Ø DFC:      Detritus Food Chain
Ø GFC:      Grazing Food chain
Disclaimer: All contents are originally prepared by Shri K C Meena Ji, Principal, KVS.Â
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