Ecosystem
EXERCISES
1. Fill in the blanks.
(a) Plants are called as_________because they fix carbon dioxide.
(b) In an ecosystem dominated by trees, the pyramid (of numbers) is_________type.
(c) In aquatic ecosystems, the limiting factor for the productivity is_________.
(d) Common detritivores in our ecosystem are_________.
(e) The major reservoir of carbon on earth is_________.
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Answer
(a) Plants are called as $\xrightarrow{\text { autotrophs }}$ because they fix carbon dioxide.
(b) In an ecosystem dominated by trees, the pyramid (of numbers) is of $\xrightarrow{\text { inverted }}$ type.
(c) In aquatic ecosystems, the limiting factor for productivity is $\xrightarrow{\text { light }}$.
(d) Common detritivores in our ecosystem are $\xrightarrow{\text { earthworms }}$
(e) A major reservoir of carbon on Earth is oceans
2. Which one of the following has the largest population in a food chain?
(a) Producers
(b) Primary consumers
(c) Secondary consumers
(d) Decomposers
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Answer
(d) Decomposers
Decomposers include micro-organisms such as bacteria and fungi. They form the largest population in a food chain and obtain nutrients by breaking down the remains of dead plants and animals.
3. The second trophic level in a lake is
(a) Phytoplankton
(b) Zooplankton
(c) Benthos
(d) Fishes
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Answer
(b) Zooplankton
Zooplankton are primary consumers in aquatic food chains that feed upon phytoplankton. Therefore, they are present at the second trophic level in a lake.
4. Secondary producers are
(a) Herbivores
(b) Producers
(c) Carnivores
(d) None of the above
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Answer
(d) None of the above
Plants are the only producers. Thus, they are called primary producers. There are no other producers in a food chain.
5. What is the percentage of photosynthetically active radiation (PAR) in the incident solar radiation?
(a) 100%
(b) 50 %
(c) 1-5%
(d) 2-10%
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Answer
(b) $50 %$
6. Distinguish between
(a) Grazing food chain and detritus food chain
(b) Production and decomposition
(c) Upright and inverted pyramid
(d) Food chain and Food web
(e) Litter and detritus
(f) Primary and secondary productivity
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Answer
(a) Grazing food chain and detritus food chain
| Grazing Food Chain | Detritus Food Chain |
|---|---|
| Energy is derived from the sun | Energy is derived from organic matter produced in trophic levels of the grazing food chain |
| It typically entails a large population | It is comparatively smaller |
| Starts with producers at the first trophic level. The plant biomass is then consumed by herbivores which in turn are consumed by different carnivores | Starts with detritus such as dead bodies of fallen leaves and animals that are consumed by detritivores or decomposers which in turn are consumed by predators. |
(b) Production and decomposition
| Production | Decomposition |
|---|---|
| Rate of producing food (organic matter) by producers is known as production | Disintegration of complex organic matter from the bodies of dead animals and plants with the help of decomposers into organic raw material, namely water, carbon dioxide, and other nutrients, is decomposition. |
| For primary production, sunlight is required | Decomposition does not require sunlight |
| Dependent on the photosynthetic capacity of producers | Takes place with the help of decomposers |
(c) Upright and inverted pyramid
| Upright pyramid | Inverted pyramid |
|---|---|
| Pyramid of energy is always upright | Pyramid of numbers and biomass can be inverted |
| At the producer level of an ecosystem, this pyramid has the highest number and biomass of organisms which declines at each trophic level in a food chain |
At the producer level of an ecosystem, this pyramid has the lowest number and biomass of organisms which rises at each increasing tropic level in a food chain |
(d) Food chain and Food web
| Food chain | Food web |
|---|---|
| Constitutes for a single linear sequence of entities |
Consists of a number of interconnected food chains |
| Members inhabiting higher trophic levels feed only on one type of entity |
Any given individual has alternate options for food sources |
(e) Litter and detritus
| Litter | Detritus |
|---|---|
| Comprises of all kinds of wastes above the ground level |
Comprises of residues of dead animals and plants |
| Consists of biodegradable and non- biodegradable substances |
Consists of biodegradable substances only |
(f) Primary and secondary productivity
| Primary productivity | Secondary productivity |
|---|---|
| It is the amount of organic matter generated by producers per unit area over a specific span of time |
It is the rate of generating organic matter by consumers over a span of time |
7. Describe the components of an ecosystem.
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Answer
An ecosystem is defined as an interacting unit that includes both the biological community as well as the non-living components of an area. The living and the non-living components of an ecosystem interact amongst themselves and function as a unit, which gets evident during the processes of nutrient cycling, energy flow, decomposition, and productivity. There are many ecosystems such as ponds, forests, grasslands, etc.
The two components of an ecosystem are:
(a) Biotic component: It is the living component of an ecosystem that includes biotic factors such as producers, consumers, decomposers, etc. Producers include plants and algae. They contain chlorophyll pigment, which helps them carry out the process of photosynthesis in the presence of light. Thus, they are also called converters or transducers. Consumers or heterotrophs are organisms that are directly (primary consumers) or indirectly (secondary and tertiary consumers) dependent on producers for their food.
Decomposers include micro-organisms such as bacteria and fungi. They form the largest population in a food chain and obtain nutrients by breaking down the remains of dead plants and animals.
(b) Abiotic component: They are the non-living component of an ecosystem such as light, temperature, water, soil, air, inorganic nutrients, etc.
8. Define ecological pyramids and describe with examples, pyramids of number and biomass.
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Answer
An ecological pyramid is a graphical representation of various ecological parameters such as the number of individuals present at each trophic level, the amount of energy, or the biomass present at each trophic level.
Ecological pyramids represent producers at the base, while the apex represents the top level consumers present in the ecosystem. There are three types of pyramids:
(a) Pyramid of numbers
(b) Pyramid of energy
(c) Pyramid of biomass
Pyramid of numbers:It is a graphical representation of the number of individuals present at each trophic level in a food chain of an ecosystem. The pyramid of numbers can be upright or inverted depending on the number of producers. For example, in a grassland ecosystem, the pyramid of numbers is upright. In this type of a food chain, the number of producers (plants) is followed by the number of herbivores (mice), which in turn is followed by the number of secondary consumers (snakes) and tertiary carnivores (eagles). Hence, the number of individuals at the producer level will be the maximum, while the number of individuals present at top carnivores will be least.
On the other hand, in a parasitic food chain, the pyramid of numbers is inverted. In this type of a food chain, a single tree (producer) provides food to several fruit eating birds, which in turn support several insect species.
Pyramid of biomass
A pyramid of biomass is a graphical representation of the total amount of living matter present at each trophic level of an ecosystem. It can be upright or inverted. It is upright in grasslands and forest ecosystems as the amount of biomass present at the producer level is higher than at the top carnivore level. The pyramid of biomass is inverted in a pond ecosystem as the biomass of fishes far exceeds the biomass of zooplankton (upon which they feed).
9. What is primary productivity? Give brief description of factors that affect primary productivity.
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Answer
It is defined as the amount of organic matter or biomass produced by producers per unit area over a period of time.
Primary productivity of an ecosystem depends on the variety of environmental factors such as light, temperature, water, precipitation, etc. It also depends on the availability of nutrients and the availability of plants to carry out photosynthesis.
10. Define decomposition and describe the processes and products of decomposition.
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Answer
Decomposition is the process that involves the breakdown of complex organic matter or biomass from the body of dead plants and animals with the help of decomposers into inorganic raw materials such as carbon dioxide, water, and other nutrients. The various processes involved in decomposition are as follows:
(1) Fragmentation: It is the first step in the process of decomposition. It involves the breakdown of detritus into smaller pieces by the action of detritivores such as earthworms.
(2) Leaching: It is a process where the water soluble nutrients go down into the soil layers and get locked as unavailable salts.
(3) Catabolism: It is a process in which bacteria and fungi degrade detritus through various enzymes into smaller pieces.
(4) Humification: The next step is humification which leads to the formation of a dark-coloured colloidal substance called humus, which acts as reservoir of nutrients for plants.
(5) Mineralization: The humus is further degraded by the action of microbes, which finally leads to the release of inorganic nutrients into the soil. This process of releasing inorganic nutrients from the humus is known as mineralization.
Decomposition produces a dark coloured, nutrient-rich substance called humus. Humus finally degrades and releases inorganic raw materials such as $\mathrm{CO}_{2}$, water, and other nutrient in the soil.
11. Give an account of energy flow in an ecosystem.
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Answer
Energy enters an ecosystem from the Sun. Solar radiations pass through the atmosphere and are absorbed by the Earth’s surface. These radiations help plants in carrying out the process of photosynthesis. Also, they help maintain the Earth’s temperature for the survival of living organisms. Some solar radiations are reflected by the Earth’s surface. Only 2-10 percent of solar energy is captured by green plants (producers) during photosynthesis to be converted into food. The rate at which the biomass is produced by plants during photosynthesis is termed as ‘gross primary productivity’. When these green plants are consumed by herbivores, only $10 %$ of the stored energy from producers is transferred to herbivores. The remaining $90 %$ of this energy is used by plants for various processes such as respiration, growth, and reproduction. Similarly, only $10 %$ of the energy of herbivores is transferred to carnivores. This is known as ten percent law of energy flow.
