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BSC 3rd Year Zoology Important Questions

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Q1. Define food chain. Illustrate it with reference to a freshwater lake. Explain why the number of steps or links in food chain sequence is limited? Give an overview of ‘Detritus’ food chain.


What is food chain? Describe food chain in a terrestrial community.

Ans. Definition- There is passage of materials from producers through primary, secondary, tertiary or quartenary consumers. Individuals connected in this manner constitute a food chain. Thus a food chain can be defined as a group of organisms in which there is a transfer of food energy through a series of repeated eating and being eaten.

Composition- A food chain begins with producers i.e., animals of varying orders and is completed by decomposers or reducers, namely bacteria and fungi. But the reducers are often omitted since they operate at all the levels of a food chain. Thus, a food chain always begins with plants and ends with a larger animals which is not preyed upon by any one. In a generalized form, a food chain may be constitute as under:

Producers Herbivores → Carnivores

The primary source of energy is sun. Green plants alone are able to trap solar energy, which they use to reduce carbon dioxide. This carbon form carbon dioxide. Carbohydrates, lipids, and proteins—the sources of life’s energy—are made of this carbon. The plants store the energy trapped in these molecules, which serves as the main energy source for all other living things. Hence, plants are the ecosystem’s producers.

In animal community, the plant-eating animals or herbivores are the primary consumers. The herbivores in turn, are fed upon by the secondary consumers which may include carnivores (flesh eating animals) or omnivores (feeding on mixed diet).

Limited Number Of Steps In Food Chain

Number of steps in a food chain is limited to four or five and at each step, there is transfer of energy in the chain, a large portion of the potential energy is lost as heat. Eventually, all the the solar energy that entered the living system through the producers goes back into the living world. This means that the shorter the food chain greater will be the biomass that can be supported with a given basic source of potential food energy. Hence, the food chain with lesser number of links or transfers will prove to be the most systematic.

Types Of EcosystemProducersHerbivoresPrimary CarnivoresSecondary CarnivoresTertiary Carnivores
A.Pond/Lake EcosystemPhytoplanktonZooplanktonSmall FishesLarge FishesPredatory Birds
B.Forest EcosystemTreesPhytophagous
Lizard, wolf, birds, lions, etc.
C.Grassland Ecosystem1. Grasses
2. Grasses
3. Grasses
Rats & mice
Predatory Birds
Predatory Birds

Food Chains In Different Types Of Ecosystems

Food Chain in Aquatic Ecosystem (Pond or lake as an example of food chain) In a pond or lake, phytoplankton are the producers. The zooplankton such as copepods are the primary consumers which feed on phytoplankton. The zooplankton in turn are fed upon by aquatic insects and are the primary carnivores or secondary consumers in the food chain. The aquatic insects and other smaller animals are consumed by fishes. The small sized fishes are eaten up by the large sized fishes and represent secondary and tertiary carnivores. Thus the sequence of food chain in an aquatic ecosystem may be represented as shown in above table.

Food Chain in a Terrestrial Ecosystem – There are innumerable examples of food chains in the Terrestrial Ecosystem and all of them represent the same trophic levels as found in the food chain of aquatic organisms. The basic trophic level is occupied by autotrophic plants, i.e., the producers.

The producers of Terrestrial Ecosystem are chiefly dominated by large-rooted green plants called the trees, herbs, shrubs and grasses. Correlated with the large number of niches provided by producers, the consumers exhibit extreme variations. The primary consumers include not only small organism like insects but also very large herbivores, like hoofed mammals.

Ecological Principles

Following main ecological principles emerge from the study of food chains:

  1. There is unidirectional flow of energy from sun to producers and then to a series of consumers in a food chain. Thus a food chain always begins with photosynthesis and ends with decay.
  2. The shorter a food chain, the more efficient it is. The more steps it has, the greater is the wastage of energy.
  3. The size of any population s determined by the number of trophic levels in a food chain. With the reduce in useful energy at each step, there is a reduction in the population size. Thus the size of a population of quarternary consumers is lower than that of tertiary consumers & that of tertiary consumers is very smaller than secondary consumers.

Q2. Describe various types of food chains.

What are food chains? Describe various food chains and their importance in an ecosystem.

What do you mean by food chain? What are their types? Give a brief overview of each.

Ans. Food Chain

Hint- Refer Q. 1.

Types Of Food Chain

Basically two types of food chain are recognized : Grazing food chain & Detritus food chain.

1. Grazing food chain– Grazing food chain starts from green plant and ends at carnivores by passing through herbivores. In herbivores the assimilated food can be stored as carbohydrates, proteins or fat. The ultimate disposition of energy in herbivores occurs by three routes: respiration, decay of organic matter by bacteria and other decomposing organisms and consumption by carnivores.

Primary carnivores or secondary consumers eat herbivores or primary consumer of the ecosystems. Similarly, secondary carnivores or tertiary consumers eat primary carnivores. The total energy assimilated by primary carnivores is derived entirely from the tissues of herbivores. Its disposition into respiration, decay and further consumption by other carnivores is analogous with that of herbivores. The energy flow through grazing food chain can be described in terms of trophic levels as follows:

Autotrophs → Herbivores → Primary Carnivores → Second Carnivores

The food chain demonstrates the amount of energy found at any tropic level, the transfer of energy from one trophic level to the next, and the amount of energy lost from the grazing food chain. Further, the producer herbivores carnivores chain is a predator chain.

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Although food energy is passed down the food chain from larger to smaller animals, the parasitic food chain also begins with herbivores. Therefore, the large animals are the host and the small animals which fulfil their nutritional requirements from the host are described as parasites.

2. Detritus Food Chain– The food chain that passes through dead organic matter is known as detritus food chain. In some ecosystems, considerably more energy flows through the detritus food chain as compared to the grazing food chain. In the detritus food chain, the energy flow is a continuous passage rather than a stepwise flow between different trophic levels. Detritus organisms ingest partially decomposed organic matter, digest them partially, after utilizing some of the energy in the food to run their metabolism, excrete the remains in the form of simpler organic molecules. The waste from one organism are immediately utilized by the second one which repeats the process.

Detritus food chains operate in the decomposing, accumulated litter in temperate forests and mangroves. In a mangrove, leave of Rhizophora fall into the warm, shallow waters. The fallen leaves are acted upon by saprotrophic fungi, bacteria and protozoa, etc., and are eaten by a group of small animals, e.g., crabs, copepods, insect lavae, shrimps, amphipods, etc., All these animals are called detritus consumers. These consumers are eaten by some minnows, game fish, etc., which in turn are eaten by large game fish and fish eating birds. Such animals are referred as top carnivores. Thus, a detritus food chains up like a grazing food chain.

Importance Of Food Chain

Hint- Refer Q.1.

Q3. What is a food web? Give it’s main characteristics in the ecosystem.

Ans. Definition- The food chains, as a matter of fact, do not occur in isolation. Since many animals eat more than one kind of food, different kinds of food chains exist in an ecosystem. This network of interconnected food chains is called food web. Thus food web can be classified as the network of a number of food chains existing in an ecosystem.

Unlike a food chain, food web has various alternative pathways for the flow of energy. In the food web there are several food chains operating in a food web. The food web starts from the plants which are producers and ends with the top carnivore.

Characteristics Of A Food Web- A food web has following characteristics:

  1. In a food web, no food chain is independent and no linear arrangement of food chains occurs.
  2. It is formed by the interlinking of 3 types of food chains: Predatory chains, parasitic chains and saprophytic chains.
  3. Food web provides alternative pathways of food availability, i.e., if a particular crop fails the herbivores graze on other types of crops. Thus, greater the number of alternate pathways, more stable the ecosystem.
  4. Food web helps in checking the overpopulation of highly fecundive species of animals and plants.
  5. The position of an animal in a food web is determined by the age and size of the species and availability of food source.

Q4. What are trophic levels? Give example. Why there is a limited number of trophic levels in an ecosystem?

Or What are trophic levels? Give an account of ecological pyramids.

Ans. Definition- Trophic levels may be defined as the steps in a food chain at which transfer of food energy takes place. The number of trophic levels is uniform to the number of steps in a food chain. The several trophic levels in a food chain are:

  1. Producers or plants which constitute the first trophic level.
  2. Herbivores that constitute the second trophic level.
  3. Carnivores that feed on herbivores constitute the third trophic level.
  4. Top carnivores that eat the small carnivores constitute the fourth trophic level.
  5. Decomposers from the last or detritus trophic level.

Examples of trophic levels: The trophic levels in various food chains can be understood by following examples:

1) Grass → Deer → Lion

This food chain involves three trophic levels. Grass represents the first trophic level. Deer represents the second trophic level and lion represent the third tropic level.

2) Grass → Insects → Frog → Hawk (Birds)

In this food chain there are 4 trophic levels. Grass forms the first trophic level and the hawk forms the last or fourth trophic level. Frog and snake form the intermediate trophic levels.

3) Plants → Goat → Man

In this food chain involving man, there are three trophic levels. Here plants represent the first trophic level, goat the second trophic level and man forms the third trophic level.

4.Top Carnivores
( Fourth Trophic Level)
(Third Trophic Level)
(Second Trophic Level)
(First Trophic Level)
Diagram showing various trophic levels in a food chain.

Finite Number of Trophic Levels in a Food Chain

A large portion of energy is lost at each trophic level in a food chain. This results in lesser amount of energy being passed on by the organisms at one trophic level to the next trophic level. This is why the no. of trophic levels in a food chain is little. As the number of trophic levels in a food chain increases, the organisms at the extreme right side of the food chain receive least amount of energy. This limits the number of trophic levels or steps in a food chain to maximum five or six.

Organism of one trophic level have the same food habit but may have several food resources like leaves, seeds, fleshy fruits, grasses, etc., Thus, a group of species belonging to a trophic level which consumes a common resources base is known as guild, eg., nectar feeding birds, grazing animals and frugivorous animals.

Thus an ecosystem has a limited number of trophic levels because:

  1. There is loss of food energy at each transfer.
  2. Food is not completely utilized by the organisms of a trophic level. Some part of the food goes waste.
  3. A large amount of energy is used in respiration.

Q5. Describe energy flow in an ecosystem.

What is the path of biological energy flow in an ecosystem? Describe rules governing it.

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Describe the significance of trophic levels during energy flow in an ecosystem. What is an ecosystem? How does the energy flow in an ecosystem? Write an overview of flow of energy.

Explain the tropic level of the ecosystem with the help of ecosystem. Also explain how energy flows through the ecosystem.

Ans. Trophic Levels

Hint- Refer Q.4.

Laws Governing Energy Flow– Energy circuit in an ecosystem starts with solar energy that is absorbed and fixed by green plants. Its flow through different levels and the final loss as heat into space are governed by the two laws of thermodynamics. As such, sun is the ultimate source of energy which meets the need of our ecosystem.

The energy captured by autotrophs will never revert back to the sun. Similarly, the energy which passes to the herbivore does not go back to autotrophs or plants and so on. Thus the flow of energy is Uni-directional.

The 2nd crucial fact is that at each trophic level energy content decreases progressively. This is because that some of the trapped solar energy is used up in respiration and some is lost as heat. This can be explained by the 1st & 2nd laws of thermodynamics.

1. The first law of thermodynamics states that energy can neither be created nor destroyed but can change from one form to another.

2. The second law of thermodynamics states that some useful energy is converted into unusable waste heat during every energy transformation.

Flow of Energy– The food chain and food web are used to transfer energy. Plants, who are the primary energy providers in the ecosystem, use their chloroplasts to collect sunlight, which is then partially converted into chemical energy during photosynthesis.

Q6. Give various steps of transfer of energy in an ecosystem. What conclusions you can derive from the transfer of energy?

Ans. First Step: Green plants absorb about 1 percent of the total energy of the sun reaching the earth and by the process of photosynthesis convert it into potential chemical energy. This energy is stored in the plants as carbohydrates. A part of this energy is used by the plants in performing their metabolic activities like growth, respiration, etc., Some of the energy is also released as heat in the environment.

Second Step: When plants are eaten by the herbivorous animals, the chemical energy stored in plant food is transferred to the herbivorous animals. Herbivorous animals utilise this energy for their metabolic activities and growth. Here again some of the energy is released as heat into the environment.

Third Step: The primary consumers or herbivorous animals are eaten by the carnivorous animals. The chemical energy stored in the flesh of herbivorous animals is transferred to the carnivorous animal. Here also some of the energy is lost to the environment as heat. This process is repeated again at every step of the food chain.

This transfer of energy from one trophic level to the other is called transfer of energy in a food chain of an ecosystem.

Some of the energy from producers and consumers is also utilized by the decomposers in their life processes. The decomposer release the unutilized energy as heat into the environment. The heat energy which remains unutilized by the producers, consumers & decomposers is lost into the environment as heat. This is called community heat.

From the energy flow in a food chain, we can draw the following conclusions:

  1. While energy can be transformed from one form to another, it cannot be produced. In actuality, plants transform the light energy from the sun into chemical energy. Plants are referred to as producers. However, plants do not generate energy. In reality, all they do is turn light energy into chemical energy. Be a result, plants are referred to as energy converters or producers.
  2. There is a continuous transfer of energy from one trophic level to the next trophic level in the food chain. The transfer of energy takes place in the form of chemical energy of food. Thus, energy is transferred from the plants to the herbivores and from herbivores to carnivores.

Plants → Herbivores → Carnivores

3. At each trophic level, some energy is utilized by the organisms for metabolic activities. Some energy is passed on the next trophic level and some energy is lost as heat.

4. The amount of energy available is less at each trophic level than the energy available at the previous level.

5. The energy lost as heat at each trophic level, if taken together, is quite substantial.

Thus, there is a gradual decrease in the amount of energy available from the 1st trophic level(plants) to the 2nd trophic level(herbivorous animals) to the 3rd trophic level(carnivorous animals).

Q7. Define the terms Habitat and Niche. Give an account of the modern concept of niche.

Ans. Habitat- Habitat of an organism or community is the place or physical location where it actually lives. It represent some physical area at some specific part of the earth’s surface. The habitat may be as large as the ocean or a prairie or as small as the underside of a rotten log or the intestine of a termite. An organism may be specific to its habitat or may show wide adaptations.


  1. Larvae of insect Chironomous are adapted to live in water with little oxygen.
  2. Annelid Tubifex lives in flowing freshwater but is also adapted to live in water with organic matter.
  3. Hilsa, a fish lives in both brackish & freshwater water.

Adaptations found in organisms are specific to the habitat which are not found in organisms of other habitats. Aquatic organisms have some specific traits not found in terrestrial animals.


Microhabitat is a part of habitat having a specific property with its own distinct types of organisms, e.g., muddy bottom of a pond, surface water, forest floor, tree canopy, tree trunk and edge of a pond.

Ecological Niche

Niche or ecological niche is the physical position or status that a given species occupies within the community or ecosystem on account of its adaptations, physiological role and specific behaviour(N.M. Jessop, 1970). Ecological niche is an abstraction that includes all the physical, chemical, physiological and biotic factors that an organism faces during its existence. It depends not only on where the animal lives but also what it eats, what is its range of movement and what is its effect on other organisms. According to E.P. Odum, habitat of an organisms is its address and ecological niche is its profession or lifestyle.

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  1. Seven species of Millipedes occurring on floor of oak forest were found to feed in different layers of decaying litter.
  2. Different stages of an organism often occupy different niches, e.g., Tadpole is herbivore and Frog is carnivore.
  3. Two species of Sparrows, Ploceus melanocephalus and P. collar is occur in the same nesting area but occupy different niches as one is insectivorous and the second is carnivorous.

The different between habitat and ecological niche can be made clear by the following examples:

  1. In shallow waters at the edge of a lake different types of water bugs are found, all of which have the same habitat.
  2. Some species occupy a very broad ecological niche and feed on many kinds of food.
  3. The spatial niche can be illustrated with the example of seven species of Millipedes, occupying the same general habitat and belong to the same trophic level that is they are all detritus feeders. Each species utilizes a different source of energy and hence each species predominates in a different microhabitat.

Advantages of Niche Segregation

  1. Animals occupying different niches escape from continuous competition.
  2. Segregation into niches avoids confusion of activities between organisms in the community and permits an orderly and efficient life cycle.
  3. The segregation of species into different niches permits large number of species to occupy that area.

Q8. Write an account of ecological equivalents and ecotypes.

Ans. Different species of organisms living under similar environment conditions in different geographical regions are known as ecological equivalents.

  1. The mountain goat, Oreamanos americanus, found at high altitudes in mountain areas of North America occupies the same niche, on the basis of its habitat, food and relationship to other organisms as does the Ibex (Capre ibex) which at one time was common in the Alps of Southern Europe.
  2. The mountain lion of North America feeds on deer whereas the African lion feeds on antelopes and wild beasts.


Most species of plants and animals in nature are composed of many biotypes, that is types of individuals that grow and react differently because of different genetic constitutions. Some biotype groupings establish themselves in various ecological zones of each specie’s range due to varied environmental factors. These ecological subdivision of the species are known as ecotypes, and are genetically different races. Each ecotype is the result of selection by its environment and has become especially adapted for a particular set of conditions. These wide ranging species are represented in different parts of their range by different ecotypes.

Q9. Write an account of ecological indicators.

Ans. Every organism is a measure of the environment because it is more or less a product of the conditions in which it grows. The plants are utilized as better ecological indicators. Further, the dominant species are the most main indicators because they receive the full impact of the habitat. Organisms are frequently employed by ecologists as indicators in exploring new situations and exploring large areas.

Following important considerations must be kept in mind while dealing with ecological indicators:

  1. Larger species are better indicators than the smaller species.
  2. In general ‘steno’ specie are better indicators than ‘eury’ species.
  3. Sufficient field evidence is essential before relying on single species or groups of species.

Many conditions of the environment are judged by the indicator species communities. Some of them are as follows:

1. Indicators of agriculture– The native vegetation of a region is the most reliable indicator for the agricultural prospects of that region. Therefore, the forests and grasslands have been found of use as indicators by the agriculturists. The potential productivity of the land or particular habitat is measured with the help of forests and grasslands.

2. Indicators of soil type- Many plant species are helpful in indicating the soil type. The sandy loam soil is indicated by the deeply rotted and taller species like Andropogon Scoparium and Ipomea Leptophylla.

3. Indicators of soil erosion- Plants like Carissa spinarum and Capparis sepiaria are indicators of soil erosion while Zizyphus rotundifolia is an indicator of soil formation.

4. Indicators of soil water- Certain species of plants like Enicostemma littorale indicates well drained soil. In Africa, Acacia glandulefera is used as indicator of underground water.

5. Indicators of former forests- When the forests disappear due to fire , pests, overgrazing or cutting, the area becomes free from the competition. Now some seral stages of succession make their appearance towards reforestation.

Q10. Write a note on ecotone and edge effect.

Ans. Ecotone may be defined as the place or area, where two major communities meet and blend together, or the broad transitional belt between the two biomes. The transitional zone between forest and grassland, between tundra and coniferous forest, between soft bottom and hard bottom marine community, and the estuarine region between freshwater and marine habitat are the examples of various ecotones.

Therefore, ecotone is a junction zone or tension belt which is narrower than the adjoining areas between which it lies. It contains some organisms from each of the communities and in addition some species which are restricted to the ecotone. As a result, ecotone has more species and a higher population density than either of the other biomes. Edge effect is a term used to describe this ecotone tendency. The tendency of ecotones to have more species and a higher population density is known as the edge effect. Edge species are defined as organisms that are mostly or frequently found in the ecotone or junction area between the two groups.


One of the most common ecotones as far as man is concerned is forest edge. A forest edge is the ecotone separating forest and grassland communities. Whenever man settles down in a particular area, he tries to maintain forest edge communities in his vicinity. If he settles down in the forest he cuts and removes trees to reduce forest to small scattered areas mixed with grassland or croplands. Some of the forest animals are able to survive in man made forest edge. Several species of insects, birds and mammals increase in this newly formed forest edge area.

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