CBSE Class 12 Biology Ecosystem Notes Set B

ECOSYSTEM – STRUCTURE & FUNCTION

Types of ecosystems
• Terrestrial ecosystem: Forest, grassland, desert etc.
• Aquatic ecosystem: Pond, lake, wetland, river & estuary.
• Man-made ecosystem: Crop fields and aquarium.
• Entire biosphere is regarded as global ecosystem.
• In an ecosystem, biotic and abiotic components interact and function as a unit.
• Vertical distribution of different species occupying different levels is called stratification. E.g. in a forest, trees occupy top strata (layer), shrubs the second and herbs & grasses the bottom layers.

Pond (Aquatic ecosystem)
A pond is a shallow, simple, self-sustainable water body that exhibits all basic components of an ecosystem.
• Abiotic components: Water and soil deposit.
• Climatic conditions: Solar input, cycle of temperature, day-length etc.
• Autotrophic components: Phytoplankton, some algae and the floating, submerged and marginal plants.
• Consumers (heterotrophs): Zooplankton, free swimming and bottom dwelling forms.
• Decomposers: Fungi, bacteria and flagellates.

Pond performs all the functions of an ecosystem. E.g.
o Autotrophs convert inorganic into organic material using solar radiant energy.
o Heterotrophs consume the autotrophs.
o Decomposition and mineralization of the dead matter to release them back for reuse by the autotrophs.
4 basic components of functioning of an ecosystem:
1) Productivity 2) Decomposition
3) Energy flow 4) Nutrient cycling

1. PRODUCTIVITY
• Solar energy is the basic requirement for an ecosystem to function and sustain.
• Amount of biomass (organic matter) produced per unit area over a time period by plants during photosynthesis is called primary production. It is expressed in weight (g^–2) or energy (kcal m^–2).
• The rate of biomass production is called productivity. It is expressed in g^–2 yr^–1 or (kcal m^–2) yr^–1.
• It is divided into gross primary productivity (GPP) and net primary productivity (NPP).
• Gross primary productivity (GPP): It is the rate of production of organic matter during photosynthesis. A considerable amount of GPP is used by plants in respiration.
• Net primary productivity (NPP): It is the available biomass for the consumption to heterotrophs (herbivores & decomposers). i.e., NPP is the Gross primary productivity minus respiration losses (R).

NPP = GPP – R
• Secondary productivity: It is the rate of formation of new organic matter by consumers.
• Primary productivity varies in different ecosystems because it depends on
o The plant species inhabiting an area.
o Environmental factors.
o Availability of nutrients.
o Photosynthetic capacity of plants.
• Annual net primary productivity of whole biosphere is about 170 billion tons (dry weight) of organic matter. Of this, despite occupying about 70 % of the surface, the productivity of the oceans is only 55 billion tons.

2. DECOMPOSITION
• It is the breakdown of complex organic matter by decomposers into inorganic substances like CO₂, water and nutrients. It is largely an oxygen-requiring process.
• Raw material for decomposition is called Detritus. E.g. dead plant remains (leaves, bark, flowers etc.), dead remains of animals, fecal matter etc.

Steps of decomposition
a. Fragmentation: It is the breakdown of detritus into smaller particles by detritivores (e.g. earthworm).
b. Leaching: Water soluble inorganic nutrients go down into soil horizon and precipitate as unavailable salts.
c. Catabolism: Degradation of detritus into simpler inorganic substances by bacterial and fungal enzymes.
The above three processes occur simultaneously.
d. Humification: Accumulation of humus (dark amorphous substance) in soil. Humus is resistant to microbial action and so decomposes very slowly. Being colloidal, it serves as a reservoir of nutrients.
e. Mineralization: It is the release of inorganic nutrients due to the degradation of humus by some microbes.

Factors influencing decomposition
• Chemical composition of detritus:
o Decomposition is slow in detritus rich in lignin & chitin.
o It is quicker in detritus rich in nitrogen and water-soluble substances like sugars.
• Climatic factors (temperature & soil moisture):
o Warm and moist environment favour decomposition.
o Low temperature & anaerobiosis inhibit decomposition resulting in buildup of organic materials.

3. ENERGY FLOW
• Sun is the only source of energy for all ecosystems (except deep sea hydro-thermal ecosystem).
• Of the incident solar radiation, less than 50% is photosynthetically active radiation (PAR).
• Plants and photosynthetic bacteria (autotrophs), fix solar radiant energy to make food.
• Plants capture only 2-10% of the PAR. This energy sustains the entire living world.
• Ecosystems obey 2nd Law of thermodynamics. They need a constant supply of energy to synthesize the molecules. It helps to counteract the entropy.

Producers (Autotrophs):
• These are organisms that synthesize food.
• In a terrestrial ecosystem, major producers are herbaceous and woody plants. Primary producers in an aquatic ecosystem are phytoplankton, algae and higher plants.
• The energy trapped by the producer is passed on to a consumer or the organism dies.

Consumers (heterotrophs):
• These are animals that directly or indirectly depend on plants for food. They include:
o Primary consumers (herbivores): Feed on plants. E.g. insects, birds, mammals, molluscs etc.
o Secondary consumers (primary carnivores): Feed on herbivores. E.g. frog, fox, man etc.
o Tertiary consumers (secondary carnivores): Feed on primary carnivores. E.g. tiger, lion etc.
• The chain of feeding relationship between different organisms is called a food chain. It is 2 types:
• Grazing Food Chain (GFC): Here, primary consumer feeds on living plants (producer). E.g.

• Detritus Food Chain (DFC): Here, primary consumer feeds on dead organic matter (detritus). Death of organism is the beginning of the DFC.
• Detritus is made up of decomposers (saprotrophs) such as fungi & bacteria. They secrete digestive enzymes that breakdown detritus into simple, inorganic materials, which are absorbed by them. Thus, they get energy & nutrients.
• In an aquatic ecosystem, GFC is the major conduit for energy flow.
• In a terrestrial ecosystem, a much amount of energy flows through the DFC than through the GFC.
• DFC may be connected with GFC at some levels. Some organisms of DFC are prey to the GFC animals. Some animals (cockroaches, crows, human etc.) are omnivores.
Such interconnections of food chains are called food web.
• A specific place of organisms in the food chain is known as their trophic level.

• The amount of energy decreases at successive trophic levels. When an organism dies it becomes dead biomass (detritus). It is an energy source for decomposers.
• Organisms at each trophic level depend on those at the lower trophic level for their energy.
• The amount of living material in a trophic level at a given time is called standing crop. It is measured as the biomass (mass of living organisms) or the number in a unit area.
• Biomass of a species is measured in terms of fresh or dry weight. Dry weight is more accurate because it is the exact mass of body which remains constant.
• Number of trophic levels in GFC is restricted as it follows 10% law (only 10% of energy is transferred to each trophic level from the lower trophic level).

ECOLOGICAL PYRAMIDS

• The representation of a food chain in the form of a pyramid is called ecological pyramid.
• The base of a pyramid represents producers (first trophic level). The apex represents tertiary or top-level consumer.
• Ecological pyramids are 3 types: Pyramid of number, Pyramid of biomass and Pyramid of energy.

• Any calculations of energy content, biomass, or numbers has to include all organisms at that trophic level.
• A trophic level represents a functional level, not a species as such. A species may occupy more than one trophic level in the same ecosystem at the same time. E.g. A sparrow is a primary consumer when it eats seeds, fruits, peas. It is a secondary consumer when it eats insects & worms.
• In most ecosystems, all the pyramids are upright, i.e., producers are higher in number, biomass and energy than the herbivores, and herbivores are higher in number, biomass and energy than the carnivores.
• But in some cases, inverted pyramids for number and biomass are present.
• Inverted pyramid of number: E.g. Insects feeding on a tree.
• Inverted pyramid of biomass: E.g.
o Small standing crop of phytoplankton supports large standing crop of zooplankton.
o Pyramid of biomass in sea is inverted because the biomass of fishes far exceeds that of phytoplankton.

• Pyramid of energy is always upright because some energy is always lost as heat at each trophic level. So energy at a lower trophic level is always more than at a higher level.

Limitations of ecological pyramids
o It does not consider the same species belonging to two or more trophic levels.
o It assumes a simple food chain that never exists in nature.
It does not accommodate a food web.
o Saprophytes are not included.

ECOLOGICAL SUCCESSION
• It is a gradual, slow and predictable change in the species composition of an area leading to a climax community(community that is in equilibrium with the environment).
• In this, some species colonize an area and increase in number, whereas other species decline and disappear.
• The entire sequences of communities that successively change in an area are called sere. Individual transitional communities are termed seral stages (seral communities).
• The species invading a bare area are called pioneer species.
• During succession, there is a change in species diversity, increase in number of species and organisms and an increase in total biomass.
• Present-day communities are due to succession of millions of years. Succession and evolution were parallel processes.
• Succession is 2 types:
o Primary: The succession taking place in areas where no living organisms ever existed. E.g. newly cooled lava, bare rock, newly created pond or reservoir.
To establish a biotic community, fertile soil must be formed. So primary succession is a very slow process.
o Secondary: The succession taking place in an area after the existed organisms are lost. E.g. abandoned farm lands, burned or cut forests, lands that are flooded.
Since some soil or sediment is present, succession is faster than primary succession.
The species that invade depend on the nature of the soil, availability of water etc.
• In succession, changes in vegetation affect food & shelter of animals. Thus, succession leads to change in number and types of animals & decomposers.
• Natural or human induced disturbances (deforestation, fire etc.) convert a particular seral stage to an earlier stage.
They create new conditions that encourage some species and discourage or eliminate other species.

Succession of Plants
• Based on the nature of the habitat, succession of plants is 2 types: hydrarch and xerarch.
o Hydrarch succession: It takes place in wetter areas. It progresses from hydric to mesic conditions.
o Xerarch succession: It takes place in dry areas. It progresses from xeric to mesic conditions.
• Hence, both hydrarch & xerarch successions lead to medium water conditions (mesic, the climax community).

Primary succession on rocks (xerophytic habitat):
Lichens (pioneer species. They secrete acids to dissolve rock, helping in weathering & soil formation) → small plants like bryophytes (they need only small amount of soil) → bigger plants → forest (mesophytic).
The climax community (forest) remains stable if the environment remains unchanged.
• Primary succession in water:
Phytoplankton (pioneers) → rooted-submerged plants → rooted-floating angiosperms → free-floating plants → reed-swamp → marsh-meadow → scrub → trees (climax community is a forest).
With time, the water body is converted into land.

4. NUTRIENT CYCLING
• Amount of nutrients (C, N, P, Ca etc.) present in the soil in a given time is called the standing state. It varies in different
kinds of ecosystems and also on a seasonal basis.
• Nutrients are never lost from the ecosystems. They are recycled again and again.
• The movement of nutrient elements through various components of an ecosystem is called nutrient cycling (biogeochemical cycles).
• Nutrient cycles are 2 types:

a. Gaseous cycle: For this, the reservoir exists in the atmosphere. E.g. Nitrogen & Carbon cycles.
b. Sedimentary cycle: For this, the reservoir is located in Earth’s crust. E.g. Sulphur & Phosphorus cycles.
• Environmental factors (soil, moisture, pH, temperature, etc.) regulate the rate of release of nutrients into the atmosphere. The reservoir meets with the deficit of nutrients due to imbalance in the rate of influx and efflux.

• Reservoir of carbon: Atmosphere (about 1%), organisms (49% of dry weight), oceans (71% dissolved carbon. It regulates the amount of atmospheric CO₂), fossil fuel etc.
• Carbon cycling occurs through atmosphere, ocean and through living and dead organisms.
• 4×10¹³ kg of carbon is fixed in the biosphere through photosynthesis annually.
• A major amount of carbon returns to the atmosphere as CO₂ through respiration.
• Processing of wastes & dead organic matter by decomposers also release CO₂.
• 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 and volcanic activity are other sources for releasing CO₂ in the atmosphere.
• Role of human activities in carbon cycle: Deforestation, burning of fossil fuel etc. has increased the rate of release of CO₂ into the atmosphere.

Phosphorus Cycle
• Phosphorus is a constituent of biological membranes, nucleic acids & cellular energy transfer systems. Many animals use phosphorus to make shells, bones and teeth.
• The natural reservoir of phosphorus is rock (in the form of phosphates).
• When rocks are weathered, minute amounts of phosphates dissolve in soil solution and are absorbed by the plants.
Herbivores and other animals obtain this from plants. The waste products and the dead organisms are decomposed by phosphate-solubilising bacteria releasing phosphorus.

ECOSYSTEM SERVICES
• The products of ecosystem processes are called ecosystem services.
• E.g. 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 provide aesthetic, cultural & spiritual values.
• Robert Costanza and his colleagues have tried to put price tags on nature’s life-support services.
• Researchers have put an average price tag of US $ 33 trillion a year on fundamental ecosystems services. This is nearly twice the value of the global gross national product GNP (US $ 18 trillion).
• Out of this total cost, soil formation accounts for about 50%.
• Contributions of other services like recreation & nutrient cycling are less than 10% each.
• The cost of climate regulation and habitat for wildlife are about 6 % each.