Notes-Class 9-Science-Chapter-7-Energy Flow In An Ecosystem-Maharashtra Board

Topics to be learn :

Food chain and food web
The energy pyramid
Bio-geo-chemical cycle : Carbon, Oxygen and Nitrogen cycles.

Ecosystem : The biotic and abiotic factors and their mutual interactions form an ecosystem.

Types of ecosystem: There are two types of ecosystem, viz., aquatic ecosystem and terrestrial ecosystem.

Interactions between biotic and abiotic factors of an ecosystem :

Every living organism requires speciﬁc abiotic factors for its survival.
The type of soil, quality of light, temperature, humidity, etc. are some of the abiotic factors that affect the biotic factors.
Living organisms (biotic factors) constantly interact with the abiotic factors. They affect the quality of abiotic factors surrounding them. This indirectly affects the quality of other biotic factors sharing the same ecosystem.
In any ecosystem, every living organism plays a speciﬁc role with respect to other factors.

Energy flow in an ecosystem :

Primary consumers (herbivores) : They are directly dependent on autotrophs (producers). Ex. Grasshopper, squirrel, elephant, etc.

Secondary consumers (carnivores) : These consumers use herbivores as their food. Ex. Frog, owl, fox, etc.

Apex or top consumers (carnivores) : These consumers use herbivores and carnivores as their food. No other animals feed on top consumers. Ex. Tiger, lion, etc.

Omnivores (mixed consumers) : Feed on herbivores and carnivores. Ex. Humans, bear, etc.

Food chain and Food web :

Food chain: Feeding relationships among producers, consumers and saprophytes in an ecosystem is called food chain. Each chain consists of four, ﬁve or more links.

Food web : Food webs are interconnected food chains in an ecosystem.

Food webs are intricate and complex intertwining of different food chain
In nature, food chains are never simple. One organism can feed on a variety of other organisms and in turn can be eaten up by another set of predators. This forms a web-like relationships between different organisms.
An insect may eat leaves, seeds or fruits. This insect has many predators such as frog, lizard, bird or a mammal like rat. The bird may eat a rat too.
Frog eats insects but can be consumed by a snake. Snake in turn can be picked up by a bird. Snake can eat a rat too.
So the feeding relations are rarely simple.

This intricate complex network, called food web Operates in every ecosystem.

Producer & Consumer :

Producer :

Producer is autotrophic.
Producers form the ﬁrst trophic level of food chain and food web.
Producers trap the solar energy and convert it into chemical energy.
Most plants in the world are producers.

Consumer :

Consumer is heterotrophic.
Consumers form the second, third and apex trophic level of food chain and food web.
Consumers, make use of chemical energy of food and use it for its nutrition.
Animals in the world are consumers.

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Energy Pyramid :

Energy Pyramid : Energy Pyramid is the diagrammatic representation showing how the energy travels up in a food chain through different trophic levels.

Figure shows the energy transfer that takes place at each trophic level. There are different levels of energy exchange in the food chain. The initial quantity of energy goes on decreasing at every level of energy exchange. Similarly, the number of organisms also decreases from the lowest level to the highest level. This pattern of energy exchange in an ecosystem is called a ‘Pyramid of energy’.

Trophic level : Each level of the food chain is called a trophic level. A trophic level is the step at which the organism obtains its food in the chain.
First trophic level is formed of producers while second trophic level is formed of herbivores. The third trophic level is formed of carnivores and the apex carnivores form the fourth trophic level.
The amount of matter and energy gradually decrease from producers to top consumers.
Lindman in the year 1942 studied the food chain and ﬂow of energy through it.
Ecological pyramid or Eltonian pyramid: Charles Elton in the year 1927 ﬁrst proposed the concept of the Ecological pyramid after his study of the Tundra Ecosystem of the Beer islands in England. Therefore, this pyramid is also known as Eltonian pyramid.
The ultimate source of energy in the ecosystem is the sun.
The ﬂow of energy in an ecosystem is always unidirectional.

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Energy ﬂow and ﬂow of nutrients in an ecosystem :

Distinguish between Energy ﬂow and ﬂow of nutrients in an ecosystem :

Energy ﬂow in an ecosystem

Fow of nutrients in an ecosystem

1-Flow of energy in an ecosystem is unidirectional.

2-The energy from producers to consumers is passed from one trophic level to the other trophic level.

3-Some part of energy which is not utilized is thrown into the atmosphere.

1-Flow of nutrients in an ecosystem is cyclic.

2-Nutrients from producers is passed from primary consumers and from primary consumers to secondary consumers. Only decomposers make the nutrients available in a cyclic way.

3-Nutrients are not destroyed into the ecosystem.

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Bio-geochemical cycle :

The cyclic ﬂow of nutrients within an ecosystem is called bio-geochemical cycle.
The bio-geochemical cycles provide different types of nutrients to all living organism for their growth and development.
Nutrients that are necessary for the growth and development of organisms are continuously transferred from abiotic to biotic factors and vice versa within an ecosystem.
The bio-geochemical cycle operates through the medium of the biosphere which is formed by the lithosphere, hydrosphere and atmosphere.
The recycling of chemical, geological and biological sources of nutrients is a complex process and is completely dependent upon the transfer of energy level in the ecosystem.

Types of Bio-geochemical cycle :

Types of Bio-geochemical cycle : There are two main types of bio-geochemical cycle, viz. gaseous cycle and sedimentary cycle.

Gaseous cycle :

The gaseous cycle is found in the earth’s atmosphere.
It includes nitrogen, carbon, oxygen and water vapour cycles.

Sedimentary cycle :

The sedimentary cycle is found in soil, sediment and sedimentary rocks of the earth.
It includes soil components like iron, calcium and phosphorous.

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Carbon cycle, oxygen cycle and nitrogen cycle are the most important bio-geochemical cycles.

Carbon cycle :

Carbon cycle : The circulation and recycling of carbon from the atmosphere to living organisms and after their death back to the atmosphere is called the carbon cycle.

From the atmosphere, carbon in the form of carbon dioxide reaches the green plants.
From green plants carbon in the form of food is carried to plants and from plants to animals.
From both plants and animals, carbon reaches the atmosphere following a cyclic path. This cyclic path of carbon from plants to animals and from both these to the atmosphere is called carbon cycle.
In the atmosphere and water bodies, carbon dioxide is the main source of carbon. Green plants synthesize carbohydrates from carbon dioxide of the atmosphere as shown below :

6CO₂ + 12H₂O \(\frac{sunlight}{Chlorophyll}\)> C₆H₁₂O₆ + 6H₂O + 6O₂

The carbohydrates (glucose) in the form of food through food chain reaches the plants, from plants to herbivorous animals and from herbivorous animals to carnivorous animals. In this way, the carbon moves from one living being to another living being.
During the process of respiration, both plants and animals oxidize carbohydrates and liberate energy along with the evolution of CO₂.

C₆H₁₂O₆ + 6 O₂ ---> 6CO₂ + 6 H₂O + Energy

During the process of fermentation and anaerobic respiration microorganisms produce carbon dioxide.
Carbon compounds undergo decomposition through micro-organisms. During this process also carbon dioxide is produced which mixes with the atmosphere.
During combustion, the organic carbon undergoes oxidation and produces energy and carbon dioxide.
In this way, carbon in the form of carbon dioxide from the atmosphere reached the food chain through plants. Processes like respiration, combustion, fermentation, decomposition, etc. release carbon dioxide which reaches the atmosphere again and in this way there is a cyclic movement of carbon.

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Carbon dioxide is released into the atmosphere through abiotic processes like burning of fossil fuels and wood, forest fires and volcanic activity.
Oxygen is released into the atmosphere by the biotic process of photosynthesis and CO2 through respiration.
The equilibrium of oxygen and carbon dioxide gases in the atmosphere is maintained by plants.

Oxygen cycle :

Oxygen cycle :

The air (atmosphere) contains about 21% of oxygen. The atmospheric oxygen is slightly soluble in water.
Terrestrial plants and animals obtain oxygen from the air while aquatic plants and animals use oxygen dissolved in water for respiration.
During the process of respiration carbohydrates are oxidized and carbon dioxide is released along with the release of energy.

Coal, wood, petrol, etc. need oxygen during combustion. During the process of combustion also along with carbon dioxide, energy is also released.
Both respiration and combustion take oxygen from the atmosphere and release carbon dioxide to the atmosphere.
Green plants during the process of photosynthesis use carbon dioxide of the atmosphere and release oxygen to the atmosphere. Oxygen mixes with the air and the cycle is completed.

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Nitrogen cycle : The circulation and recycling of nitrogen gas into the form of different compounds through various biotic and abiotic processes in nature is called the nitrogen cycle.

The atmospheric nitrogen moves from atmosphere to the soil,
from the soil to the plants,
from plants to the animals and
from all the living beings to the soil and
from the soil back to atmosphere.

This cyclic journey of the nitrogen is called nitrogen cycle.

Important processes of the nitrogen cycle :

Important processes of the nitrogen cycle

1) Nitrogen fixation: Conversion of nitrogen into nitrates and nitrites through atmospheric, industrial and biological processes.

There are two processes of nitrogen fixation. viz., physical ﬁxation : Further divided into atmospheric ﬁxation and industrial fixation and biological ﬁxation.

(a) Physical ﬁxation : The lightning in the sky brings about oxidation of atmospheric nitrogen and converts it into nitrogen oxide (NO₂). Nitrogen oxide dissolves in rain water and forms nitrous and nitric acid. These acids seep in the soil with the rain water. They undergo chemical reactions with other elements in soil and form different compounds of nitrogen.

(b) Biological ﬁxation :

The symbiotic bacteria, Rhizobium living in the root nodules of the leguminous plants carry out the function of biological nitrogen fixation.
Azotobacter and Clostridium are free living bacteria in the soil. They also bring about biological nitrogen fixation.
The blue-green aquatic algae Nostoc and Anabaena also perform biological nitrogen fixation.

2) Ammonification: Release of ammonia through decomposition of dead bodies and excretory wastes of organisms.

3) Nitrification: Conversion of ammonia into a nitrite and then nitrate.

4) Denitrification: Conversion of nitrogen compounds into gaseous nitrogen.

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Source of Nitrogen : Plants cannot take the atmospheric nitrogen as it is. Therefore they take up various compounds of the nitrogen that are available in the soil. For the plants nitrogen compounds in the soil is the only source of nitrogen.

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