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| Name of Board | AHSEC |
| Class: | Assam Board Class 12 |
| Subject: | AHSEC Class 12 Biology |
| Number of Chapter: | 11 |
| Chapter Name | Ecology Organisms and Populations |
| Content Type: | Text, Images and PDF Format |
| Academic Year: | 2024-25 |
| Medium: | English |
| Available Solution Link: | AHSEC Class Biology Notes |
AHSEC Class 12th Biology Chapter: 11 Ecology Organisms and Populations
[A] VERY SHORT ANSWER TYPE QUESTIONS(1 MARK)
1. What do you mean by population?
Ans. Population is an aggregation or grouping of individuals of the same species at the same time in a particular area or space that share or compete for similar resources.
2. What is organism?
Ans. Organisms are basic units of ecological organisation which have a distinct living entity carrying out all life processes, separate from those in other individuals.
3. Explain the meaning of community.
Ans. Community is an assemblage of populations of different species found in a particular area.
4. Name the two major factors that affect population.
Ans. Birth and death rate and Migration.
5. Name the biotic components of ecosystem.
Ans. The biotic components include pathogens, parasites, predators and competitors of the organism with which they interacts constantly.
6. What is xerophyte?
Ans. Xerophytes are plants that live in those habitats where availability of water from soil is low while transpiration from aerial parts is high usually because of high temperature.
7. What is mesophyte?
Ans. Mesophytes are plants that grow in soils having optimum moisture as well as aeration and shows luxuriant vegetative growths.
8. Mesophyte prefer what type of habitat?
Ans. Moist habitats.
9. What is hydrophyte?
Ans. Plants of aquatic habitats are called hydrophytes.
10. If the condition is dry what type of plant will develop in that region.
Ans. Xerophytic plants.
11. Explain the term carrying capacity of a habitat.
Ans. The maximum number of individuals of a population which can be provided with optimum resources for their healthy living is called carrying capacity.
12. What do you mean by migration?
Ans. Migration is two way movement of whole population from a stressful habitat to a congenial area for food shelter favourable climate and other reasons.
13. What is minicry of nature? Give one example of it.
Ans. Mimicry is when an animal or plant resembles another creature or inanimate object either for defence or to gain other advantage. eg. Vicreroy butterfly resembles highly distabteful and poisonous Monarch butteffly.
14. Define ecotone species of nature.
Ans. The species present in the four of transition presenting a situation of special ecological interest between two different types of communities is called ecotone species.
15. When and why do some animals like frogs hibernate?
Ans. Frogs hibernate during winter because to avoid stress dur- ing unfavourable condition they fail to migrate.
16. What is zero population growth?
Ans. When number of reproductive individuals is almost equal to prereproductive individuals there is no increase or decrease in its population size. Thus is called zero population growth.
(B) FILL UP THE BLANKS: (1 MARK)
1. A large biogeographic region consisting of a major vegetation type and associated animals in a specific climatic zone is called_____
Ans. Biome
2. Every individual of a species is called_____
Ans. Organism
3. Any place in the world, any specific location ranging from equator to polar region, hot spring, compost and even the human intestine are________of organisms.
Ans. Habitat
4. _______which live on dead organic matters are responsible for decomposition.
Ans. Saprophytes
5. Hydrophytes prefer_______habitat.
Ans. Aquatic
6. Soil is the source of all______compounds essential for life.
Ans. inorganic
7. ______and______regions receive more intense light and therefore temperature is more.
Ans. Equatorial and tropical
8. Altitude, slope, depression and other physical features of earth is calle______.
Ans. Topography.
9. _______is the phenomenon in which body temperature changes with the temperatures outside.
Ans. Conform
10. ______is a photosynthetic pathway in which the stomata remain open during night instead of day time.
Ans. CAM (Crassulacean acid metabolism)
11. _______shows association of both algae and fungi.
Ans. Lichen
12. _____bacteria fix atmospheric nitrogen in a leguminous plant.
Ans. Rhizobium
15. The aggregation of individuals of a species is called ______
Ans.: Population.
16. The relationship where one organism is benefited, while the other is neither benefitted nor harmed is referred as______
Ans. Commensalism
17. Desert plants can tolerate water stress by forming_______
Ans.: Spines.
[C] SELECT TRUE OR FALSE STATEMENTS : (1 MARK)
1. Natality means birth rate.
Ans. True
2. All organisms other than green plants prepare their own food.
Ans. False
3. Mortality is increase in number of population.
Ans. False
4. The organism is required to generate heat to maintain temperature suitable for its body which is called thermo regulation.
Ans. True
5. Some organisms hibernate to escape extreme conditions of environment.
Ans. True
6. Population increase due to migration of individuals from an area is called emigration.
Ans. False
7. The potential to support a certain number of organisms in a habitat is called carrying capacity of the habitat.
Ans. True
8. Fungal association of roots of higher plant is called mycorrhiza.
Ans. True
9. Soil water available to plant is surface water.
Ans. False
10. Exponential population growth is d N/dt = rn.
Ans. True
11. Competition is most acute between individuals of same species.
Ans.: True
12. Symiosis is that when the interaction is useful to both the populations.
Ans.: True
13. An association of animals when both partners are ben- efitted called commensalisms.
Ans.: False
(II) SHORT TYPE QUESTIONS: (2 MARKS)
1. What is log and lag phase?
Ans. In the lag phase new population tries to acclimatise to the new habitat and population increases slowly. During the log phase population size rises rapidly.
2. What is commensalism? Give example of commensalism.
Ans. Commensalism is an interaction between the individuals of two different species in which one is benefited while the other is neither harmed nor benefited.
eg. Interaction between sea anemone that has stinging tentacles and the clown fish that lives among them. The fish gets protection from predators which stay away from the stinging tentacles. The anemone does not derive any benefit.
3. Define the phenomenon of antibiosis and allelopathy.
Ans. Antibiosis is a phenomenon by which one living being does not allow other living beings to flourish nearby.
Allelopathy is the phenomenon of inhibiting growth of other organisms through secretion of toxic chemicals.
4. What is amensalism? Give example of amensalism.
Ans. Amensalism is a phenomenon by which one living being does not allow other living beings to flourish nearby.
Eg.: Roots of walnut produces a toxic chemical which inhibits the growth of Apple, tomato etc.
5. What is meant by homeostasis.
Ans. Homeostasis is the phenomenon of maintaining a constant internal environment despite changes in external environment. Homeostasis gives the animals metabolic advantage and help them to remain active.
6. What is eurythermal and stenothermal? Give example of each type.
Ans. Organisms which can tolerate and thrive in a wide range of temperatures are called eurythermal. eg. Wall lizard. Organisms which are restricted to a narrow range of temperatures are called stenothermal. eg. Snakes.
7. What is Biome? Give examples of Biome.
Ans. Biome is a major category of regional ecological organisation which is delimited by a specific climatic zone. It is characterised by a major vegetation type and its associated fauna.
Eg. Desert, Rain forest and Tundra.
8. Define the Gause competitive exclusion principle.
Ans. Gause competitive exclusion principle states that two closely related species competing for the same resources cannot co-exist indefinitely and the competitively inferior one will be eliminated eventually.
9. Define the following terms.
(a) Predations
(b) Camouflage
(c) Amensalism.
Ans. (a) Predation: It is an interaction between members of two species in which members of one species capture, kill and eat up members of other species.
(b) Camouflage: It is the phenomenon of blending of an organism with the surrounding due to similar colour, marking and shape so as to avoid the predators.
(c) Amensalism: It is an interaction between two living individuals of different species in which an organism does not allow other organism to grow or live near it.
10. How light influences the growth and development in plants?
Ans. Light is essential for growth because it provides organic materials for it. Light is also essential for photomorphogenesis or differentiation of tissues and organs. UV radiations induce rosette habit, Blue light favours moderate but normal growth. High light intensity reduces growth but increases development of mechanical tissues.
11.What are the biotic and abiotic factors?
Ans. The biotic factors are pathogens, parasites, predators and competitors.
Abiotic factors are temperature, water, light and soil.
12. How does strong wind affect a plant?
Ans. Strong wind affects the rate of transpiration in plants. In high altitude and in coastalaregion where wind velocity is more the plants have strong roots & stems so that the tensile strength provided by the mechanical tissue of the stem is high and they can withstand the strong wind.
13. Distinguish between
(a) Hibernation and Aestivation
Ans. Hibernation
(i) It is winter sleep in which animal passes the winter period in dormant condition.
(ii) The animal rests in a warm place.
(iii) It is of longer duration and lasts for the whole duration of winter.
Aestivation
(i) It is summer sleep
(ii)The animal rest in a cool, shady and moist place.
(iii) It lasts for hot dry day time as nights are cooler.
(b) Ectotherms and endotherms
Ans. Ectotherms
(i) These are cold blooded (poikilothermal) animals.
(ii) They are unable to regulate their body temperature which changes with change of temperature of environment.
(iii) These animals show hibernation & aestivation
Endotherms
(i) These are warms blooded (homoiothermal) animals.
(ii) They maintain their body temperature in every condition.
(iii) These two activities rarely occur in these animals.
14. Distinguish between camouflage and mimicry. Give one example from each.
Ans. Camouflage
1. It is resemblance of animals as to their colour and marking with the surroundings.
2. It is beneficial to all types of animals.
3. E.g.. Chameleon changes its colour according to the sur- rounding
Mimicry
1. It is resemblance of an animals with another.
2. It is beneficial to only mimics.
3. Viceroy butterfly mimics highly dist. asteful Monarch butterfly.
15. Write the special characteristics feature of a parasitic organism.
Ans. Characteristic feature of parasitic organism on
(1) loss of unnecessary sense organs.
(2) Presence of adhesive organs or suckers to cling on the host.
(3) Loss of digestive system.
(4) High reproductive capacity.
16. What is symbiosis? Give one example each from plant and animal kingdom.
Ans. Symbiosis is the interaction in which both the interacting species are benefitted.
E.g. from plant- Mycorrhiza is association between fungi and roots of higher plants where fungi help the plant in the absorption of essential nutrients from the soil while the plant provides the fungi with food e.g. from animal- Fig and wasp relationship where the wasp uses the fruit for oviposition and developing seeds within the fruit for nourishing its larva. While searching for suitable egg laying site the wasp pollinates the fig.
(III) SHORT QUESTIONS TYPE: (3 MARKS)
1. What to you mean by competition? Define intraspecific and interspecific competition.
Ans. Competition is an interaction in which closely related species compete for the same resources that are limiting.
Competition between individuals of the same species is called intraspecific.
Competition between individuals of different species is called interspecific.
2. Define the process of migration. What is immigration and emigration?
Ans. Migration is two way movement of whole population from a stressful habitat to a congenial area for food, shelter favourable climate and other reasons.
Immigration is the number of individuals of the same species that have come into the habitat from elsewhere during the time period. Emigration is the number of individuals of the population who left the habitat and gone elsewhere during the time period.
3. What are the mechanism developed by the prey to avoid predators?
Ans. the mechanism developed by the prey to avoid predators are
(i) Some plants have thorns or spines for defence mechanism. eg. Cactus.
(ii) To avoid being detected easily by the predator some species of insect and frogs are cryptically coloured (camouflaged)
(iii) The Monarch butterfly is highly distasteful to its predator because of a special chemical present in its body which is acquired by feeding on a poisonous weed in caterpillar stage.
(iv) Some plants produced highly poisonous chemicals like cardiac glycosides, nicotine, quinine etc. as defences against grazers and browsers.
4. What is parasitism? Describe the endoparasite and ectoparasite with examples.
Ans. Parasitism is a relationship between two living organisms of different species in which one organism called parasite obtains its food directly from another living organism called host.
Parasites that feed on the external surface of the host organism are called ectoparasites eg lice on humans.
Parasites that live inside the host body at different sites are called endoparasites eg Malarial parasite
5. What role the predators play in the ecosystem?
Ans. The role played by predators are
(i) It is a conduit for energy transfer across tropic levels.
(ii) Predation keeps the prey population under control.
(iii) It maintains species diversity of a biotic community by reducing competition amongsh the prey species.
6. Describe the phenomenon of mutualism with examples.
Ans. Mutualism is an interaction between two organisms of different species where both the interacting species are benefited. For eg. Lichens which represent an intimate mutualistic relationship between a fungus and a photosynthesising algae. In this interaction the fungus helps in anchoring the lichen & absorption of water and minerals. The algal partner manufactures food not only for itself but also for the fungus.
7. What are the attributes of population growth?
Ans. The attributes of population growth are
(i) Natality or Birth rate: It is the number of births during a given period in the population that are added to the initial density
(ii) Mortality or death rate: It is the number of deaths in the population during a given period.
(iii) Migration It is a two way movement of the whole population from (emigration) or into (immigration) into an area.
(iv) Sex ratio: It is expressed as the number of females per 1000 males of a population in given time.
8. Explain the different types of adaptation adopted by plants to avoid predators?
Ans. 1) Some plants have thorns or spines as the most common morphological defence. eg. Cactus, Acacia.
2) Plants like Calotropis produce highly poisonous cardiac glycosides.
3) Many plants produce chemicals like nicotine, quinine, opium etc. as defence against grazers and browsers.
9. Describe the various type of positive and negative interaction between different species?
Ans. Negative interactions:
1) Predation: It is the act of capturing, killing and eating.
2) Competition It is a rivalry between two or more organisms for obtaining the same resources.
3) Parasitism: It is a relationship between two different species in which one organism obtains its food directly from another living organism.
4) Amensalism: It is a phenomenon by which one living being does not allow other living beings to flourish nearby.
Positive interaction:
1) Commensalism: It is an interaction between two different species in which one is benefitted while the other remains unaffected.
2) Mutualism: It is an interaction in which both the interating species gets benefitted.
10. What are the levels of organisation?
Ans. The levels of organisation are
(i) Organism: Living component of the environment at individual level is called organism.
(ii) Population It is the sum total of all individuals of a species in a specific geographical area.
(iii) Biotic community: It is the assemblage of all the populations of different species present in an area that interact among themselves.
(iv) Ecosystem: The sum total of the biotic and abiotic components of a particular geographical area.
(v) Biome: The large unit of environment consisting of a major vegetation type and its associated fauna in a specific climatic zone.
(vi) Biosphere: All the ecosystems of the world are collectively called biosphere.
11.What is adaptation? Describe the adaptation in organisms to extreme environmental condition.
Ans. Adaptation is any attribute of the organisms (morphological, physiological, behavioural) that enables the organism to survive and reproduce in its habitat. Organisms adapt to extreme conditions in the following ways
(i) In the absence of an external source of water, the Kangaroo rat in North American deserts is capable of meeting its water requirements through internal fat oxidation and concentrates its urine for minimum loss of water.
(ii) Desert plants have thick, waxy coating on leaves called cuticle to prevent transpiration, they have special photosynthetic pathway (CAM) that enables their stomata to remain close during day time and also in some plants like opuntia leaves are reduced to spines.
12. In an aquarium two herbaceous species of fish are living together and feeds on phytoplankton. As per Gause theorem one of the species is to be eliminated in course of time. But it is found that both of the species are surviving in the aquarium. Give possible reasons.
Ans. Both the species are surving in the aquarium because of resource partitioning. It is a phenomenon in which species facing competition might evolve mechanisms that promote co- existence rather than exclusion. If two species complete for the same resource they could avoid competition by choosing different times for feeding. In this way they can avoid competition and co- exist.
13. In what ways are prey species benefited from the predators? Do scavengers which feed on the same species have the same effect? Explain.
Ans. Predators reduces the intensity of competition among com- peting prey species and prevents them from extinction. It does not overexploit the prey and maintains species diversity.
No scavengers which feed on the same species do not have the same effect because scavengers eat the dead organisms. So they cannot maintain species diversity and also cannot reduce the inten- sity of competition among competing prey species.
14. What is ecological niche of an organism? Why it is believed that no two species can occupy exactlythe same niche?
Ans. Niche is specific part of habitat occupied by individuals of a species which is circumscribed by its range of tolerance, the resources it utilises and its functional role in an ecosystem.
No two species can occupy exactly the same niches because of competition from other species if two species happen to occupy the same niche one is always excluded. Thus only one species can exist in one niche
(IV) LONG QUESTIONS (5 MARKS)
1. Describe the logistic population growth curve with suitable examples.
Ans. Under normal situation a population of microorganisms, plants or animals show ideal S-shaped growth curve or sigmoid curve. In this S-shaped growth curve there are three phases. The early phase is called the lag phase. In this phase a new population tries to aclimatise to the new habitat and therefore population increases slowly. Once the population is stabilised it multiplies rapidly if food is available and the environment is favourable. It shows exponential form of growth in the second phase called log phase. Ultimately when food is exhausted or the environment becomes unsuitable for further growth the number of individuals produced becomes equal to the number of individuals die. In this phase called steady phase there is no net increase in population. This limit beyond which no further growth is possible is called nature’s carrying capacity. This type of population growth is called Verhulst Pearl Logistic growth and is described by the following equation.
2. What is age-pyramid? Describe the pre-reproductive, reproductive and post-reproductive age pyramids with suitable diagrams.
Ans. An age pyramid is a graphic representation of proportio of various age groups of a population with pre-reproductive at the base, reproductive in the middle and post reproductive at the top. There are three types of age pyramids
(i) Triangular age pyramid (Expanding population): The number of pre-reproductive individuals is very large, number of reproductive individuals is moderate while post reproductive individuals are fewer. Because of the very large number of pre reproductive individuals, more and more of them enter the reproductive phase and rapidly increase the size of population.
(ii) Bell shaped (Stable population): The number of pre reproductive and reproductive individuals is almost equal. Post reproductive individuals are comparatively fewer. The population size remains stable, neiher growing nor diminishing.
(iii) Urn Shaped )Declining population): In this type a smaller base is represented by small number of pre-reproductive individuals, a larger belly represented by larger number of reproductive individuals and a small sized tip formed of post reproductive individuals. As there are fewer individuals in pre- reproductive age group, the population size will decline with time.
3. How does population interacts? Give the different types of interactions found in population.
Ans. Animals, plants and microbes cannot live in isolation but interacts in various ways to form a biological community. Interactions can be interspecific or intraspecific. Interspecific interactions arise from the interaction of populations of two different species. Whereas intraspecific interaction takes place between populations of the same species.
The different types of interactions found in population are
(i) Predation: It is an interaction between members of two species in which members of one species capture, kill and eat up members of other species.
(ii) Parasitism: It is a relationship between two living organisms of different species in which one organism çalled parasite obtains its food directly from another living organisms called host.
(iii) Competition: It is a process in which the fitness of one species is significantly lower in the presence of another species.
(iv) Commensalism: It is the interaction in which one species benefits and the other is neither harmed nor benefited.
(v) Mutualism: It is an interaction in which both the interacting species confers benefits.
(vi) Amensalism: It is an interaction in which one organism is harmed and the other remains unaffected.
4. Name and define the attributes of an individual and a population.
Ans. The attributes are
(i) Natality or birth rate: It refers to the number of births during a given period in the population that are added to the initial density.
(ii) Mortality or death rate: It is the number of deaths in a population during a given period.
(iii) Emigration It is the number of individuals of the population who left the habitat and gone elsewhere during the time period.
(iv) Immigration It is the number of individuals of the same species that have come into the habitat from elsewhere during the time period.
(v) Sex ratio: It is expressed as the number of females per 1000 males of a population in given time.
(vi) Populaton density: Population density is the number of individuals of a population found in a unit area or space at a given time.
5. How does abiotic factor influences the growth and development in plants?
Ans. The abiotic factors are
(i) Water: Water is the natural solvent with which necessary elements are absorbed by the plants from soil for growth. It is a medium of transport and a source of cell turgedity. Water is a reagent in photo-synthesis and a product in respiration. Plants show wetting of softer parts during day time due to higher rate of transpiration than absorption. Rate of photosynthesis and other metabolic activities are reduced.
(ii) Light: It is essential for growth because it provides the organic material. In presence of organic food growth can occur even in the absence of light. However light is essential for differentiation of tissues and organs. Blue light favours normal growth while red light supports increased growth. More mechanical tissues develop at high light intensity.
(iii) Soil: Soil is the source of all inorganic compounds essential for life. Plants absorb all such compounds and build up food essential for growth. It provides anchorage and water also to plants. Nature and properties of soil are dependent upon climate, weathering process, particle size nutrients and pH. These characteristics determine the type of vegetation and their growth pattern.
(iv) Temperature: Activity in plants are confined between the maximum and the minimum required temperature. Each plant requires a certain number of heat units before it can produce flowers. Different types of plants therefore have different vegetative and flowering season. Temperature also plays an important role in breaking dormancy of buds & seeds.
6. Parasitism is a specific interactions in which one partner derives benefits at the expens of other. What are the evidents or facts observed due to parasitism.
Ans. Parasitism is an interaction in which the parasite obtains nourishment from the host and spends a part or whole of its life on or inside the body of the host. Parasite robs the host of its nutrition, causes injury, weakens the host, shortens its life cycle, resuces its reproduction to the extent of causing sterility and therefore alters the population growth of the host. Both host and parasite tend to co-evolve. If the host evolves a mechanism to resist the parasite the parasitse also evolves mechanism to counter and neutralise the same. Parasites have high biotic potential but poor means of dispersal requiring specialised structures to reach or invade the hosts.
To survive within the host or to obtain maximum benefits fromthe host the parasite undergoes certain modifications such as
(i) Resistants eggs and cysts which are not harmed by advers environment and even digestive enzymes of the host.
(ii) Presence of adhesive organs such as suckers and hooks.
(iii) Less of certain organs such as locomotory organs.
(iv) Higher rate of multiplication.
(v) Involvement of one or two intermediate hosts to complete the life cycle.
7. ‘Nature has a carrying capacity for a species’. Explain the statement with suitable examples.
Ans. Every population receives resources from its environment They include food, water and space. The maximum number of individuals of a population which can be provided with optimum resources for their healthy living is called carrying capacity of the environment. Beyond carrying capacity the resources become de- ficient and the individuals are starved of food and shelter. Absence of shelter exposes the individuals to vagaries of weather and preda- tors. Semistarved conditions increase mortality in young individuals. Pathogen infestation also increases. There will be decline in natality as well. As a result population size decreases. Scarcity of food affects both herbivores and cannivores. However, omnivores are able to survive due to switching over to alternate food items.
8. Does light factor affect the distribution of organisms? Give a brief description with some suitable examples of liv- ing organisms.
Ans. Light affects the distribution of animals by affecting the vegetation. For eg. Tundra biome is characterized by insufficient light, low temperature, less vegetation so less animals life. While tropical area is characterized by high rainfall, strong light, sufficient vegetation and abundant animals life some plants grow under higher amount of sunlight called as heliophytes other grow will under less amount of sunlight called sciophytes.
There is also light zonation in deep lakes and occans. The vari- ous gones are (i) Littoral zone: It is shallow coastal region where light is able to pass and reach the bottom. Therefore, produces occur through-out from surface to bottom.
(ii) Limnetic zone: In this zone water is very deep and light decreases with depth. It has three parts- photic, aphotic & benthic
(a) Photic: It is the upper part where light can penetrate. In sea the green algae remain near the surface, brown algae at inter- mediate depth which red algae flourish at the deepest part.
(b) Aphotic: Light does not penetrate to this zone so producers do not occur, only consumers and found.
(c) Benthic zone: It is the bottom zone of perpetual darkness where only consumers occur. Benthic fishes of sea are commonly luminescent with protuding eyes.
9. Name the five key tools for accomplishing the tasks of recombinant DNA technology. Mention the function of each tool.
Ans. The five key tools accomplishing the task of recombinant DNA technology are-
(i) Restriction endonucleases for cutting the desired DNA at desired places.
(ii) Gel electrophoresis for superheating the desired DNA frag- ments.
(iii) Ligase enzyme for joining the desired DNA into the vector.
(iv) Cloning vector is required to carry the desired DNA into a host cell.
(v) Competent host to take up to accept the foreign DNA/ recombinant DNA.
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Last Words on AHSEC Class 12 Biology Chapter: 11 Ecology Organisms and Populations
The Chapter 11 of AHSEC Class 12 Biology deals with sexual reproduction in Human for a brief understanding. You can Download this HS 2nd Year Biology Notes in PDF 2025.
