Ecosystem Class 12 Ncert Solutions PDF Notes & Important Questions

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ecosystem class 12 ncert solutions PDF Notes


Class12th 
Chapter No14
ProvidingQuestions And Answers, Notes & Numericals PDF
Chapter NameEcosystem
BoardCBSE
Book NCERT
SubjectBiology
Medium English
Study MaterialsFree VVI Study Materials are Available
Download PDF ecosystem class 12 ncert pdf

ecosystem class 12 notes


1. Ecosystem is a functional unit of nature, an association of organisms and their physical environment, interconnected by a continuous flow of energy and a cycling of nutrients.

Ecosystem Class 12 Ncert Solutions PDF Notes & Important Questions

2. An ecosystem varies greatly in size from a small pond to a large forest or a sea.

3. It is convenient to divide it into two categories 

(i) Terrestrial, e.g., forest, grassland and desert.

(ii) Aquatic, e.g., pond, lake. wetland, river and estuary ecosystems.

4. Crop fields and an aquarium are examples of man-made ecosystems.

5. Every ecosystem has inputs and outputs of energy and nutrients.

6. Energy cannot be recycled; therefore, a constant input of Energy from the sun is required to sustain all the ecosystems.

7. Energy flows through an ecosystem in one way beginning with photosynthetic autotrophs, known as primary producers of an ecosystem.

8. All other organisms are heterotrophs and extract energy from chemical compounds synthesised by autotrophs.

9. Structure of Ecosystem 

(i) Structure Interaction of biotic and abiotic components best results in a physical structure that is characteristic of each type of ecosystem.

(a) Biotic component of an ecosystem consists of producers, consumers, decomposers and detritivores. 

(b) Abiotic components consist of the physical environment. 

(ii) Species composition of an ecosystem is determined by the different plant and animal species occupying an ecosystem.

(iii) Stratification is the vertical distribution of species occupying different levels in an ecosystem, e.g., 

(a) Trees occupy the top vertical strata or layer of a forest.

(a) Shrubs the second. 

(c) Herbs and grasses occupy the bottom layers.

10. Function of Ecosystem

(a) Productivity 

(b) Decomposition

(c) Energy flow

(d) Nutrient cycling. 70gm

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(i) The abiotic and biotic materials are converted with the help of the radiant energy of the sun by autotrophs. 

(ii) Autotrophs are consumed by heterotrophs.

(iii) Decomposers decompose the organic matter to release them back for reuse by the autotrophs. 

(iv) The matter and minerals are recycled between biotic and abiotic components.

(v) The energy flow is unidirectional.

11. Productivity is defined as the amount of biomass or organic matter produced per unit area over a time period by plants during photosynthesis. 

(i) It is expressed in terms of weight (g) or energy (kcal-m²). 

(ii) The rate at which primary producers capture and store a given amount of energy in their tissues, in a given time

interval is the primary productivity of an ecosystem. 

(iii) Productivity can be divided into Gross Primary Productivity (GPP) and Net Primary Productivity (NPP).

(iv) Primary productivity factors influencing area,

(a) Plant species inhabiting a particular 

(b) Environmental factors.

(c) Availability of nutrients. 

(d) Photosynthetic capacity of plants.

Therefore, primary productivity varies in different types of ecosystems. The annual net primary productivity of the whole biosphere is approximately 170 billion tons (dry weight) of organic matter. 

(v) Secondary productivity is defined as the rate of formation of new organic matter by consumers.

12. Decomposition It is the breakdown of complex organic matter into inorganic substances like carbon dioxide, water and nutrients. The process is carried out by microbes and other organisms called decomposers.

(i) Detritus is the raw material for the decomposition. It constitutes dead plant remains such as leaves, bark, flowers and dead remains of animals, including faecal matter.

(ii) The important steps in the process of decomposition are 

(a) Fragmentation Detritivores (e.g., earthworms, sludge crabs and insects) break down detritus into smaller particles. This process is called fragmentation. 

(b) Leaching By the process of leaching, water-soluble inorganic nutrients go down into the soil horizon and get precipitated as unavailable salts.

(c) Catabolism It is the process by which bacterial and fungal enzymes degrade detritus into simple inorganic substances. 

(d) Humification It leads to the accumulation of a dark-coloured amorphous substance called humus that is highly resistant to microbial action and undergoes decomposition at an extremely slow rate thereby serving as a reservoir of nutrients.

(e) Mineralisation The humus is further degraded by some microbes and the release of inorganic nutrients occurs through the process known as mineralisation.

13. Factors Affecting Decomposition 

(i) Decomposition is mainly an oxygen-requiring process The rate of decomposition is controlled by the chemical composition of detritus and climatic factors.

(ii) Decomposition rate is slower, if detritus is rich in lignin and chitin. It is faster if detritus is rich in nitrogen and water-soluble substances like sugars.

(iii) Temperature and soil moisture are the most important climatic factors that regulate decomposition through the effects on the activities of soil microbes. (iv) Warm and moist environments favour microbial growth and enzyme activity, thus enhancing the rate of decomposition. 

(v) Low temperature and anaerobic conditions inhibit decomposition resulting in a build-up of organic materials. 

14. Food Chain and Food Web A straight-line sequence of ‘who eats whom’ in an ecosystem is called a food chain. network of cross-connecting food chains involving producers consumers and decomposers is termed a food web.

(i) Food chains are of two types 

(A) Grazing Food Chains (GFC) in which energy flow from plants to herbivores and then through carnivores e.g., Grass Goat →→→→→ Man

(b) Detrital Food Chains (DFC) where energy flows go from photosynthetic organisms through detritivores and decomposers. e.g., Dead leaves Woodlouse→→→ Blackbird

(ii) Grazing food chain starts with green plants called producers at the first trophic level. (a) In this chain, a much less fraction of energy flows.

(b) Energy from the food chain comes from the sun.

(c) In an aquatic ecosystem, GFC is the major conduit for energy flow.

(iii) The detritus food chain begins with dead organic matter. It is made up of decomposers which are heterotrophic organisms, mainly fungi and bacteria.

(a) Decomposers secrete digestive enzymes that break down dead and waste materials into simple, inorganic materials, which are subsequently absorbed by them. These are also known as saprotrophs

(b) In a terrestrial ecosystem, a much larger fraction of energy flows through the detritus food chain than through the GFC.

(c) The Detritus food chain may be connected with the grazing food chain at some levels.

15. Trophic Level Organisms occupy a place in the natural surroundings according to their feeding relationship with other organisms. This forms a hierarchy in a food chain called trophic levels.

(i) Trees and other primary producers form the first trophic level. 

(ii) Herbivores (primary consumers) are in the second trophic level and carnivores (secondary consumers) form the third trophic level. 

(iii) The amount of energy decreases at successive trophic levels.

(iv) Organisms at each trophic level depend on those at the lower trophic level for their energy demands.

(v) The number of trophic levels in the grazing food chain is restricted, as the transfer of energy follows a 10% low. It means only 10% of the energy is transferred to each trophic level from the lower trophic level. 

(vi) When any organism dies, it is converted to detritus or dead biomass that serves as an energy source for decomposers.

(vii) Each trophic level has a certain mass of living material at a particular time called as the standing crop. The standing crop is measured as the mass of living organisms (biomass) or the number in a unit area.

16. Energy Flow

(i) Sun is the only source of energy for all ecosystems on Earth.

((ii) Out of the incident solar radiation less than 50% of it is Photosynthetically Active Radiation (PAR).

(iii) Plants and photosynthetic and chemosynthetic bacteria (autotrophs), use the sun’s radiant energy to produce food from simple inorganic materials.

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(iv) There is a unidirectional flow of energy from the sun to pid producers and then to consumers.

(v) Major producers in a terrestrial ecosystem are herbaceous and woody plants and in an aquatic ecosystem are various species like phytoplankton, algae and higher plants. (vi) All animals are called consumers or heterotrophs.

(a) Primary consumers feed on the producers. These are herbivores.

(b) Secondary consumers who feed on these herbivores are primary carnivores (though secondary consumers). 

(e) Tertiary consumers eat secondary consumers.

(i) The trophic structure of an ecosystem is represented in the form of ecological pyramids.

(ii) The base of each pyramid represents the producers or the first trophic level, while the apex represents tertiary or top-level consumers. 

(iii) The three types of ecological pyramids are

(a) Pyramid of number

(b) Pyramid of biomass 

(c) Pyramid of energy.

(iv) Pyramid of number 

(v) Pyramid of biomass shows the relationship between producers and consumers in an ecosystem in terms of biomass. It can be

(a) Upright, e.g., in case of the grassland ecosystem. 

(b) Inverted, e.g., in the case of the pond ecosystem.

(vi) Pyramid of energy is the relationship between producers and consumers in an ecosystem in terms of the flow of energy. It is always upright because energy is always lost as heat at each step.

(vii) Limitations of ecological pyramids

(a) It includes, a simple food chain that never exists in 

(b) It never keeps an account of the same species’ nature. belonging to two or more trophic levels. 

(c) In spite of the vital role played by saprophytes/ decomposers, they are not given any position in ecological pyramids.

18. Ecological Succession

(i) The gradual and predictable change in the species composition of a given area is called ecological succession. During succession, some species colonise an area and their populations become more numerous, whereas populations of other species decline and even disappear.

(ii) The entire sequence of communities that successively change in a given area is called sere(s). 

(iii) The individual transitional communities are termed seral stages or seral communities.

(iv) In the successive seral stages, there is a change in the diversity of species of organisms, an increase in the number of species and organisms as well as an increase in the total biomass. 

(v) These changes lead finally to a community that is near equilibrium with the environment and called a climax community.

(vi) Ecological succession can be of two types:

(a) Primary succession begins in areas, where no living organisms ever existed, e.g. newly cooled lava, bare rock, newly created pond or reservoir. It is a very slow process, taking thousands of years for the climax to be reached.

(b) Secondary succession begins in areas, where natural biotic communities have been destroyed such as in abandoned farmlands, burned or cut forests, and lands that have been flooded. Since some soil or sediment is present, succession is faster than primary succession. 

(vii) Effect of ecological succession 

(a) It leads to changes in vegetation that affects food and shelter for various types of animals.

(b) As succession proceeds, the numbers and types of animals and decomposers also change.

(c) At any time during primary or secondary succession, natural or human-induced disturbances (fire, deforestation, etc.) can convert a particular seral stage of succession to an earlier stage. Also, such disturbances can create new conditions that encourage some species and discourage or eliminate other species.

19. Succession of Plants

(i) Based on the nature of the habitat, plant succession is of two types Dalen 

(a) Hydrarch succession takes place in wetter areas and is the successional series progress from hydric to mesic conditions (neither too dry nor too wet). 

(b) Xerarch succession takes place in dry areas and is the series progresses from xeric to mesic conditions. 

(ii) Primary succession on rocks 

(a) Pioneer species are the first ones to invade a bare area, i.e., on rocks.

(b) These are usually lichens that secrete acids to dissolve rock, helping in weathering and soil formation. 

(c) This paves the way for some very small plants like bryophytes, which are able to grow in a small amount of soil.

(d) Over time, they are succeeded by bigger plants and after several more stages, ultimately a stable climax forest community is formed. 

(e) The climax community remains stable if the environment remains unchanged.

(f) With time, the xerophytic habitat gets converted into a mesophytic one.

(iii) Primary succession in water

(a) Primary succession in water, the pioneers are the 

(b) They are replaced with time by free-floating small phytoplankton. brahmin angiosperms, then by rooted hydrophytes, sedges, grasses and finally the trees.

(e) The climax again would be a forest. 

(d) Lastly, the water body is converted into land.

20. Nutrient Cycle

(i) The movement of nutrient elements through the various components of an ecosystem is called nutrient cycling or biogeochemical cycles.

(ii) The amount of nutrients present in the soil at any given latovo time, is referred to as the standing state. It varies in different kinds of ecosystems and also on a seasonal basis. 

(iii) Nutrient cycles are of two types 

(a) Gaseous

(b) Sedimentary. 

(iv) Atmosphere is the reservoir for the gaseous type of cycle nutrient (e.g., nitrogen and carbon cycle). 

(v) Earth’s crust is the reservoir of the sedimentary cycle (eg, sulphur and phosphorus cycle). 

(vi) The function of the reservoir is to meet the deficit, which occurs due to imbalance in the rate of influx and efflux. 

(vii) Environmental factors, e.g., soil, moisture, pH, temperature, etc., regulate the rate of release of nutrients into the atmosphere.

Differences between Gaseous and Sedimentary Cycle ban

image 10

21. Carbon Cycle

(i) In living organisms, carbon constitutes 49% of the weight. also represent a reservoir of carbon. their dry

(ii) Out of the total quantity of global carbon, 71% of carbon is found dissolved in oceans. This oceanic reservoir regulates the amount of carbon dioxide in the atmosphere. Fossil fuels

(iii) Carbon cycling occurs through the atmosphere, ocean and through living and dead organisms. 

(iv) It is estimated that about 4x 1013 kg of carbon is fixed in the biosphere through photosynthesis annually.

(v) A large amount of carbon returns to the atmosphere as CO, through

(a) Respiratory activities of the producers and consumers.

(b) Breakdown activities of decomposers.

(c) Forest fire and combustion of organic matter.

(d) Volcanic activity.

(e) Rapid deforestation.

(f) Burning of wood and fossil fuel.

(vi) Human activities have influenced the carbon cycle by significantly increasing the rate of release of carbon dioxide into the atmosphere, leading to many harmful effects on the environment.

22. Phosphorus Cycle day 

(i) Phosphorus is a major constituent of biological membranes, nucleic acids, cellular energy transfer systems (ATP) and also of shells, bones and teeth. 

(ii) The natural reservoir of phosphorus is rock, which contains phosphorus in the form of phosphates. 

(iii) When rocks are weathered, minute amounts of these Ho phosphates dissolve in soil solution and are absorbed by the roots of the plants.

(iv) Herbivores and other animals obtain this element from plants.

(v) The waste products and the dead organisms are decomposed by phosphate-solubilising bacteria releasing phosphorus,

23. Differences between Carbon and Phosphorus Cycle 

(1) There is no respiratory release of phosphorus into the atmosphere.

(ii) Atmospheric inputs of phosphorus through rainfall are much smaller than carbon inputs. 

(iii) Gaseous exchange of phosphorus between organism and environment is negligible.

24. Ecosystem Services 

(i) The products of ecosystem processes are termed ecosystem services. 

(ii) Forests are the major sources of ecological services. These are useful in the following ways: 

(a) Purification of air and water.

(b) Mitigating droughts and floods. 

(c) Cycling nutrients.

(d) Generating fertile soils. 

(e) Providing wildlife habitat.

(f) Maintenance of biodiversity. 

(g) Pollination of crops.

(h) Providing storage site for carbon. 

(i) Providing aesthetic, cultural and spiritual values.

(iii) Robert Constanza and his colleagues tried to put price tags on nature’s life support services, i.e., about US $ 33 trillion a year.

(iv) It is very important, therefore, to understand how much nature is providing us for free and if we overuse or misuse its resources, we’ll pay a heavy price for it.


ecosystem class 12 ncert solutions-Exercises


Question 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 productivity is……..

(d) Common detritivores in our ecosystem are………. 

(e) The major reservoir of carbon on Earth is……..

Answer

(a) Plants are called producers because they fix carbon dioxide. 

(b) In an ecosystem dominated by trees, the pyramid (of numbers) is upright type.

(c) In aquatic ecosystems, sunlight is the limiting factor for productivity.

(d) Common detritivores in our ecosystem are earthworms.

(e) The major reservoir of carbon on Earth is the oceans. 

Question 2. Which one of the following has the largest population in a food chain? 

(a) Producers

(b) Primary consumers 

(c) Secondary consumers

(d) Decomposers

Answer (a) Producers have the largest population in a food chain.

Question 3. The second trophic level in a lake is

(a) phytoplankton

(b) zooplankton

(c) benthos

(d) fishes Answer

(b) Zooplankton is the second trophic level in a lake.

Question 4. Secondary producers are

(a) herbivores

(b) producers

(c) carnivores

(d) None of these

Answer (d) None of these

Question 5. What is the percentage of Photosynthetically Active Radiation (PAR) in the incident solar radiation

(a) 100% TIG

(b) 50 %

(c) 1-5%

(d) 2-10%

Answer (b) 50% of the incident solar radiation is Photosynthetically Active Radiation (PAR).

Question 6. Distinguish between to

(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 Primary and secondary productivity

Answer

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Question 7. Describe the components of an ecosystem.

Answer: An ecosystem consists of two components-abiotic and biotic. Abiotic components include the physical environmental factors, e.g., water, soil, wind, sunlight, etc. Biotic component includes

(1) Primary producers or autotrophs, e.g., plants, phytoplankton, some algae, etc., that can use sunlight to make food.

(ii) Primary consumers feed on the producers and the plants. The primary consumers are all herbivores.

Some common herbivores are insects, birds and mammals in terrestrial ecosystems and molluscs in aquatic ecosystems. 

(i) Secondary consumers are animals that eat plant-eating animals. The consumers that feed on these herbivores are primary carnivores (though secondary consumers), e.g., spiders, beetles and birds.

(iv) Tertiary consumers eat secondary consumers. Animals that depend on the primary carnivores for food are labelled as secondary carnivores, e.g., owls, eagles and foxes. 

(v) The decomposers are the fungi, bacteria and other small organisms that break down the complex organic matter into inorganic substances like carbon dioxide, water and nutrients. 

(vi) Detritivores eg, earthworms, slugs, crabs and insects break down detritus such as leaves, bark, flowers and dead remains of animals, including faecal matter into smaller particles. 

Question 13. Outline salient features of carbon Answer Carbon cycle is a gaseous nutrient cycle.

A major part of carbon is found dissolved in oceans. This oceanic reservoir regulates the amount of carbon dioxide in the atmosphere. Fossil fuels also represent a reservoir of carbon. Carbon cycling occurs through the atmosphere, ocean and through living and dead organisms.

Carbon is fixed in the biosphere through photosynthesis by autotrophs and is released back into the atmosphere as CO₂ through

(i) respiratory activities of the producers and consumers 

(ii) breakdown activities of decomposers.

(iii) forest fire and combustion of organic matter. 

(iv) volcanic activity.

(v) rapid deforestation.

(vi) Burning of wood and fossil fuel.

Human activities have influenced the carbon cycle by significantly increasing the rate of release of carbon dioxide into the atmosphere.


ecosystem class 12 notes 


Question 1. In the North-East region of India, during the process of hum cultivation, forests are cleared by burning and left for regrowth after a year of cultivation. How would you explain the regrowth of forests in ecological terms?

Answer Forests that are cleared by burning and left for regrowth will show secondary succession. Since soil is already present, the buried seeds will germinate. With wind dispersal and other natural forces, some new seeds will be brought into the area and new species will colonise.

Question 2. What is the ultimate source of energy for ecosystems?

Answer: The sun is the ultimate source of energy for ecosystems. 

Question 3. Is the common edible mushroom an autotroph or a heterotroph?

Answer: The common edible mushroom is a heterotroph.

Question 4. Why are oceans the least productive? 

Answer Oceans are the least productive because

(i) There is insufficient radiation as sunlight decreases with the increasing depth of the ocean. 

(ii) oceans are nitrogen deficient which is an important nutrient for plants. 

(IiI) conditions of high salinity which is not favourable for all plants.

(iv) There is no substratum to support plants.

Question 5. Why are nutrients in nature called biogeochemical cycles?

Ans:- Nutrient cycles are called biogeochemical cycles because ions/molecules of a nutrient are transferred from the environment (rocks, air and water) to organisms (life) and then back to the environment in a cyclical fashion. The literal meaning of biogeochemical is bio-living organisms and geo-rocks, air and water. 

Question 6. Give any two examples of xerarch succession.

Answer Xerarch succession of ecological communities originates in extremely dry conditions such as sand deserts and rock deserts.


class 12 biology ecosystem ncert solutions 


Question 1. Organisms at a higher trophic level have less energy available. Comment.

Answer Energy flow in the ecosystem follows the 10% energy flow low. According to this, only 10% of the energy available at one trophic level gets transferred to the next trophic level, the rest is lost to the environment as heat. As we move to higher trophic levels, the energy available to organisms keeps on decreasing.

Question 2. Is an aquarium a complete ecosystem?” 

Answer Yes. Since it has all the biotic and abiotic components required for fish to survive. 

Question 3. Human activities interfere with the carbon cycle. List any two such activities.

Answer Two human activities that interfere with carbon cycles are (1) rapid deforestation and (ii) the massive burning of fossil fuels for energy and transport.

Question 4. Define ecological pyramids and describe with examples, pyramids of number and biomass.

Ans:- Ecological pyramids are graphical representations of the relationship between organisms of different trophic levels that can be expressed in terms of number, biomass or energy.

In most ecosystems, the pyramid of numbers is upright, i.e., producers are more in number than herbivores and herbivores are more in number than carnivores. But, the pyramid may be inverted as in a forest ecosystem, where the number of insects (primary consumers) is greater than the number of trees (producers).

The pyramid of biomass is also upright, generally, as the biomass of producers is more than the biomass of herbivores and that of herbivores is more than the biomass of carnivores. But, it is inverted in many ecosystems like in sea ecosystems, where the biomass of fishes (primary consumers) far exceeds that of phytoplankton (producers).

Question 5. What is primary productivity? Give a brief description of factors that affect primary productivity.

Answer: The rate of biomass production is called productivity.

It is expressed in terms of g2-yr-1 or (kcal-m2) yr to compare the productivity of ecosystems. It can be divided into Gross Primary Productivity (GPP) and Net Primary Productivity (NPP).

The gross primary productivity of an ecosystem is the rate of production of organic matter during photosynthesis. A considerable amount of GPP is utilised by plants in respiration. Gross primary productivity minus respiration losses (R), is the net primary productivity (NPP), GPP-R = NPP Primary productivity depends on

(i) the plant species inhabiting a particular area. 

(ii) the environmental factors.

(iii) availability of nutrients. 

(iv) photosynthetic capacity of plants,

Question 6. Define decomposition and describe the processes and products of decomposition.

Answer Decomposition is the process of breakdown of complex organic matter into inorganic substances so that they can be reused, Products of decomposition are carbon dioxide, water and nutrients, e.g., nitrogen, phosphorus, calcium, potassium, etc.

The important steps in the process of decomposition are 

(1) Fragmentation Breakdown of detritus (dead plant remains and dead remains of animals) into smaller particles by detritivores (e.g….. earthworm).

(Ii) Leaching Water soluble inorganic nutrients go down into the soil horizon and get precipitated as unavailable salts.

(iii) Catabolism Bacterial and fungal enzymes degrade detritus into simpler inorganic substances. 

(iv) Humification Accumulation of a dark-coloured amorphous substance.

(v) Mineralisation Humus is further degraded by some microbes and the release of inorganic nutrients.

Question 7. Give an account of energy flow in an ecosystem. 

Answer in an ecosystem, a constant supply of energy is required.

(1) Sun is the only source of energy for all ecosystems on Earth.

(Ii) Plants and photosynthetic and chemosynthetic bacteria (producers), fix energy from solar radiation and store it in their tissues. 

(iii) All organisms are dependent for their food on plants, either directly or indirectly. It means there is a unidirectional flow of energy from the sun to producers and then to consumers.

(iv) The energy trapped by the producer, is either passed on to a consumer or to the decomposer (in case the plant dies) through two types of food webs-grazing food web and detrital food web that cross-connect many times. 

Question 8. Write important features of a sedimentary cycle in an ecosystem.

The answer Phosphorus cycle is an example of a sedimentary nutrient cycle since it moves from land to the sediments at the bottom of the seas, then back to land again. The natural reservoir of phosphorus is Earth’s crust. The rock contains

phosphorus in the form of phosphates. By weathering and soil erosion, phosphates enter streams, rivers and then oceans. With great movements of the crustal plates, the sea floor is uplifted and phosphates become exposed on the drained land surfaces. From here, weathering over long periods of time releases phosphate From rocks, a minute amount of these phosphates dissolve in soil and are absorbed by the roots of the plants.

Herbivores and other animals obtain this element from plants. The waste products and the dead organisms are decomposed by phosphate-solubilising bacteria releasing phosphorus.

1. First trophic level (producers) – Plants 

2. Second trophic level (primary consumers) – Herbivores 

3. Third trophic level (secondary consumers) – Carnivores

4. Fourth trophic level (tertiary consumers) – Top carnivores.

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