An ecosystem is a complex collection of relationships between a given area's living resources, habitats, and residents. It encompasses animals, plants, trees, birds, fish, water, soil, micro-organisms, and humans. An ecosystem can be enormous, as it may consist of various plants and animals, or small in particularly harsh places around the world.

• This topic comes under the broader chapter of Ecology and Environment that carries a weightage of around 6% in NEET Biology Syllabus.
• A maximum of 3 questions can be asked from Ecosystems and a total of 5-6 questions can be asked from Ecology and Environment in NEET. 

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The syllabus contains other more important topics than Ecosystem and students are required to pay greater attention to them. Ecosystem, however, is perhaps the most simple chapter in the NEET 2020 Biology Syllabus. With thorough preparation, you can get a good score in this chapter and boost your overall score in NEET.

Must Read: 

• NEET Study Notes for Molecular Basis of Inheritance
• NEET Study Notes for Biological Classification
• NEET Study Notes for Reproduction

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Ecosystem and its Structure

A.G Tansley coined the word 'ecosystem' in 1935. Two components form the structure of any ecosystem. These are:

1. Abiotic Factors

An ecosystem's abiotic factors include surrounding non-living substances. Examples would be water, soil, air, light, minerals, climate, and pressure, to name a few. The ecosystem's biotic factors rely for their survival on the abiotic factors. 

2. Biotic Factors

The biotic factors include surrounding living organisms. Examples are plants, bacteria, and viruses. An ecosystem's biotic factors are classified into three principal groups, which are as follows:

Producers

• An ecosystem's producers are the organisms which perform photosynthesis. Examples include algae, bacteria, and plants. 
• The producers rely on the ecosystem's abiotic factors for the energy production. They come with chlorophyll. 
• Using abiotic factors such as light, CO2, water and minerals, chlorophyll is utilized in the synthesis of energy rich compounds. 
• A portion of the synthesized energy is used for growth and survival by producers and the residual energy is reserved for future use.

Consumers

• Consumers are beings that feed or devour other creatures. 
• Consumers are further separated into three types or more. 
• These are primary consumers, secondary consumers and tertiary consumers. 

Reducers or Decomposers:

• The decomposers are heterotrophic organisms which break down plants' dead bodies and their waste products. 
• These include fungi and some bacteria. They're involved in secreting enzymes. 
• The enzymes digest the debris and dead organic matter into smaller pieces or molecules. The reducers absorb these molecules. 
• The reducers discharge molecules into the environment as chemicals after they take energy to be used again by the producers.

Sample Question

Question: Which among the following ecosystems has the maximum biomass? 

(a) Grassland ecosystem

(b) Pond ecosystem

(c) Lake ecosystem

(d) Forest ecosystem

Ans: (d)

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Energy Flow 

• Transferring energy from one trophic to another trophic level is known as energy flow. 
• In an ecosystem, the energy flow is unidirectional. This means that it flows from the level of the producer to that of the consumer, and never in the opposite direction. Thus, energy can only be utilized in the ecosystem only once. 
• Within the ecosystem, however, the minerals circulate and recirculate several times. 
• At each trophic stage, a major amount of energy is lost. 
• It is calculated that when it is moved from one trophic level to another, 90 percent of the energy is lost.
• Just about 10 percent of the biomass is moved from one trophic level to another in a food chain. And at each trophic level, only about 10 percent of chemical energy is preserved. This is named as the 10% Law of Lindeman (1942). 

Sample Question 

Question: What is the rate at which food energy is integrated to the consumer trophic level called?

(a) Gross primary productivity

(b) Net primary productivity

(c) Secondary productivity

(d) None of the above

Ans: (c)

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Ecological Pyramid

• The number, biomass, and energy of organisms progressively decreases from the level of the producer to the level of the consumer. It can be depicted in the form of a pyramid known as the ecological pyramid. 
• An ecological pyramid is a graphical representation of the number, biomass, and energy of an ecosystem's successive trophic levels. 
• Charles Elton first mentioned the utility of an ecological pyramid in 1927. The producer serves as the base within the ecological pyramid, and the final consumer is at the top. 

There are three kinds of ecological pyramids as given below.

1. Pyramid of number

At the trophic level, the number of individuals declines from producer level to consumer level. This means that the number of producers in an ecosystem is very high. The number of herbivores is less than the number of producers. Likewise, carnivores are smaller in number than herbivores. 

• A cropland ecosystem: The crops are more abundant here. There is a lesser number of grasshoppers that feed on crop plants. The number of frogs feeding on grasshoppers is smaller still. The number of snakes feeding on frogs is even smaller. 

Crop -> Grasshopper -> Frogs -> Snakes -> Hawks

• A grassland ecosystem: The grasses are in large numbers in grasslands. In the given order, the consumers decrease in number. 

Grass -> Grasshopper -> Lizard -> Hawk

Grass -> Rabbit -> Fox -> Lion

• A pond ecosystem: In the order given, the numbers in a pond ecosystem are seen to decrease.

Phytoplankton -> Zooplankton -> Fishes -> Snakes

2. Pyramid of biomass

The biomass corresponds to the total weight per unit area of living matter. In an ecosystem, the biomass reduces from level of the producer to level of the consumer. 

3. Pyramid of energy

In an ecosystem, energy flows from the level of the producer to the level of the consumer. 80 to 90 percent of energy is lost at each trophic level. Therefore, the amount of energy is reduced from the level of the producer to that of the consumer. It can be reflected at the consumer level in an energy-level pyramid.

Sample Question 

Question: All these claims contain drawbacks of ecological pyramids, with the exception of:

(a) They take into consideration a simple food chain and not food webs 

(b) Saprotrophs are not given any place in the ecological pyramids 

(c) They do not consider the same species which belong to two or more trophic levels 

(d) They do not reflect relationships at different trophic levels between organisms;

Ans: (d)

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Succession and its ways

• Each community endures a set of changes before a group of organisms is formed who can most successfully live and reproduce in that environment. This is known as biotic succession. 
• Hult (1885) coined the term 'succession'. 
• Normally, a biotic community goes through continual changes. In general, definite and orderly sequences of communities emerge slowly over a period of time in a region. 
• A particular sequence of a community's growth refers to a particular collection of physical and chemical conditions. This is called sere. 
• In a sere, the last succession is named climatic climax or simply climax.

Succession occurs in two ways: 

• Primary succession: This involves changes that occur when living things are formed in an environment that was previously uninhabited, like a newly uncovered sea floor, lake sediments, or dunes. 
• Secondary succession: Early ecosystems have been destroyed, leaving a few species and significant organic matter. Along with some new, these remaining species regenerate a new community. 

Life forms: Raunkiaer (1934) differentiated plants by size, shape, branching, crown, life span, and perenniality into five forms. 

1. Therophytes: Annual plants that perennate as seeds.
2. Cryptophytes: Buds arise deep in the soil. For example, bulbs, rhizomes, tubers, corms. 
3. Hemicryptophytes: Perennating structures that exist at the ground level. In the beginning of winter, the aerial shoots die. Rosette plants are a good example. 
4. Chamaephytes: Small plants of cold areas lying at or above ground level with perennial buds or shoot apices. 
5. Phanerophytes: Perennial herbs, shrubs and trees, epiphytes, succulents, lianas, to name a few, where perennial buds grow at a height above ground level of 10 cm or more.

Sample Question 

Question: In the process of ecological succession:

(a) the animal types and numbers remain constant 

(b) the creation of a new biotic community is rapid in its primary stages 

(c) predictable and gradual variations in the composition of species occur in a particular region 

(d) changes result in a community that is in close balance with the environment and is referred to as a pioneering community

Ans: (c)

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Nutrient Cycles 

• Carbon Cycle: Carbon cycling between the biotic and the abiotic systems is known as the carbon cycle. It is a gaseous cycle. 
• Carbon dioxide (CO2) is the principal source of carbon. 
• CO2 resides in the air and in water. Wind is the primary reservoir. Air CO2 content is 0.03 percent. This volume remains constant. 

Carbon flow into the biotic system: Carbon flows through the biotic system in two ways: 

i) Photosynthesis

Carbon comes in from photosynthesis into the biotic system. Green plants use CO2 in photosynthesis, which convert CO2 carbon into glucose. Glucose gets used for synthesis of carbohydrates, proteins, and lipids of certain forms. These carbon-containing compounds are preserved in the tissues of plants. When herbivores chew up plants, the carbon flows through the food chain into the body of herbivorous animals. When carnivores eat herbivores, the carbon reaches the carnivorous animals' bodies. 

6CO2 + 6H2O ® C6H12O6 + 6O2

ii) Shell formation

Marine animals, such as protozoans, corals, molluscs, and algae use the CO2 dissolved in seawater for shell construction. In these animals, CO2 gets converted into calcium carbonate ( CaCO3) that is used for shell construction. 

CO2 + H2O -> H2CO3 (Carbonic acid)

H2CO3 -> H+ + HCO3 (Bicarbonate)

HCO3 + Ca+ -> H+ + CaCO3 (Calcium carbonate)

Carbon into the abiotic system: The biotic system's carbon flow into the abiotic system happens in five ways: 

i) Respiration

By respiration, plants and animals emit CO2 (biological oxidation). 

ii) Decomposition

As plants and animals perish, decomposers such as bacteria, algae, and so on decompose the dead bodies into CO2. 

iii) Shells 

After the demise of marine organisms, CaCO3 retained in the shells is either stored as sedimentary rocks or dispersed in water to release CO2 by the reversal of the reactions described above. 

iv) Coal

Any part of plant carbon is stored as coal. Coal carbon returns to air through combustion and weathering, in the form of CO2. 

v) Forest fire

In the forest wood, combustion absorbs carbon from plants in the form of CO2.

Sample Question

Question: Which among the following factors contributes to the carbon cycle?

(a) fossil fuel combustion

(b) respiration

(c) photosynthesis

(d) all of these

Ans: (d)

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Phosphorus Cycle 

• Phosphorus cycling between the biotic and abiotic systems is known as the phosphorus cycle. This is a sedimentary cycle. 
• Phosphorus is a vital nutrient in a mineral. 
• The rocks are the primary source of phosphorus. The phosphorus is made available in the soil through erosion and weathering. 
• Plants take up ionic phosphate by their roots. This is integrated in plants into the protoplasmic components such as DNA, RNA, AMP, ADP, ATP, GDP, GTP, NADP, phospholipids, etc. from plants, passes to herbivores and animals, decomposes the organic phosphate-containing molecules and releases phosphate as inorganic ion phosphate. It is utilised again by plants.
• The excessive phosphate in animal bodies is excreted through faces. A significant quantity of phosphate is in the bird guano (excreta). 
• Phosphate is also released into the soil by burning of woodland trees and grasses. 
• By sedimentation, a significant amount of phosphate is deposited in the sea. Some amount of phosphorus becomes locked in bones and teeth. 

Sample Question 

Question: Which of the following describes phosphorus cycle the best?

(a) Gaseous cycle 

(b) Perfect cycle 

(c) Imperfect cycle 

(d) Partly gaseous and partly sedimentary

Ans: (c)

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Important Topics in Ecosystems 

Under Ecosystems, certain subtopics bear more weightage than others. Even though it is advised that a student studies all the subtopics, the more important topics given below may be allocated more time.

1. Structure of Ecosystem
2. Ecological Pyramid
3. Energy Flow in Ecosystem
4. Ecological Succession
5. Nutrient Cycles: Carbon and Phosphorus 

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Previous Year Solved Sample Questions on Ecosystems 

Question 1: Which of the following entities in an ecosystem occupies more than one trophic level? 

(a) Frog

(b) Phytoplankton

(c) Zooplankton

(d) Fish

Ans: (d)

Question 2: Which among the following will be the first to colonize a bare rock?

(a) Herbs and shrubs

(b) Annual plants

(c) Lichens

(d) Perennial plants

Ans: (b)

Question 3: The deep-sea hydrothermal vent ecosystem's primary producers are ___. 

(a) blue-green algae

(b) coral reefs

(c) chemosynthetic bacteria

(d) green algae

Ans: (c)

Question​: Which among the following is a characteristic feature of the cropland ecosystem? 

(a) Absence of weeds

(b) Ecological succession

(c) least genetic diversity

(d) absence of soil entities

Ans: (c)

Question​: When 20J of energy is trapped at the producer level, how much energy will be available in the subsequent chain to a peacock as food?

Plant -> Mice -> Snake -> Peacock

(a) 0.2 J

(b) 0.02 J

(c) 0.002 J

(d) 0.0002 J

Ans: (b)

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Some Tips to Solve Questions 

• Use the mental note method

When confronted with answering questions, candidates are often faced with a lot of irrelevant details. Many of the specifics given are meaningless for the solution. In these situations, candidates must mentally take note of the aspects of the question related to problem-solving to prevent silly mistakes. Aspirants must pay special attention to words like 'all', 'some', etc. 

• Eliminate options cautiously 

Even when the question is new to the aspirant, it is possible to realize through the elimination process that about 50 percent of the 4 probable answers are not valid. Afterward, all candidates have to evaluate and understand which of the 2 possible responses is the correct choice.

• Try to Avoid negative marking

If you do not know the answer, and are sure you will not remember it, do not even attempt it. This is to guarantee that you do not get any negative markings. Considering your total score, getting a zero is better than getting a negative marking in NEET 2020.

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Study Plan for NEET Biology 

Especially when it comes to studying for NEET Biology, keeping the whole syllabus in perspective for a successful preparation plan goes a long way. The available time for the syllabus of NEET and each of its subjects must be allocated according to the weightage of the topic. It is also important that one complies with the study plan and implements modifications as one progresses. Since Ecosystems in Biology is considered an easy-to-score section, follow the indicators below to design an efficient plan for the topic and the subject.

1. Incorporation of Smart Study: You need to learn smartly for effective preparation. Make a note of the areas in which you are strong and weak. Prioritize your strong areas in such a way that they become stronger and allow enough time to focus on your weak areas. Let no topic remain untouched. 
2. Usage of Charts, Tables, or Maps: Rather than simply cramming details into your mind through continuous revision, it is important that you create diagrams such as charts, tables or maps for logical comprehension and inculcation of data. 
3. Creation of mnemonic phrases for series and classifications: Candidates could create mnemonic phrases for series and classifications such as kingdom classifications using initials from the key elements in the categories. 
4. Answer mock tests and previous years' papers: The National Testing Agency is setting up practice centers in different locations throughout the country to allow aspirants to practice online testing. Until then, answer as many previous year question papers and mock tests as you can. Know More NEET Mock Test
5. Selection of the best books: One's score will reveal the standard of the books that one used to study. In the view of several experts and toppers, the NCERT will form the foundation for one's comprehension of concepts. Nevertheless, aspirants must seek to include the books mentioned below in their preparation for NEET Biology, in order to improve practice. Read More