CBSE Class 12 Biology Organisms And Populations Worksheet

Important Questions for NCERT Class 12 Biology Organisms and Populations

Question. At organismic level which type of ecology exists
(a) Synecology
(b) Physiological ecology
(c) Behavioural ecology
(d) Systematic ecology

Answer : B

Question. Which of the following match is incorrect for commensalism interaction?
(a) Epiphytes on trees
(b) Egrets with grazing cattles
(c) Hermit crab and sea anemone
(d) Sea anemone and clown fish

Answer : C

Question. Formation of different kind of biomes depends on
(a) Light
(b) Temperature
(c) Precipitation
(d) Both 2 and 3

Answer : D

Question. Regional and local variation within each biome lead to formation of -
(a) Climate
(b) Weather
(c) Habitat
(d) Niche

Answer : C

Question. During interaction between sea anemone and clown fish, which get benefitted :-
(a) Sea anemone only
(b) Clown fish only
(c) Both
(d) Neither 1 nor 2

Answer : B

Question. Which of the following is not an example of coevolution?
(a) Orchid and Bee
(b) Opuntia and Cactophagous moth
(c) Yucca and Pronuba
(d) Wasp and Fig

Answer : B

Question. What is / are key elements that leads to so much variation in the physical and chemical conditions of different habitats?
(a) Temperature
(b) Water and light
(c) Soil
(d) All above

Answer : D

Question. What is the salinity of hypersaline lagoons?
(a) 5 ppt
(b) 30-35 ppt
(c) More than 100 ppt
(d) Less than 50 ppt

Answer : C

Question. Which of the following forest plants controls the light conditions at the ground?
(a) Climbers
(b) Shrubs
(c) Tall trees
(d) Herbs 

Answer : C

Question. Ecological niche is
(a) the surface area of the ocean.
(b) an ecologically adapted zone.
(c) the physical position and functional role of a species within the community.
(d) formed of all plants and animals living at the bottom of a lake. 

Answer : B

Question. What will happen to a well growing herbaceous plant in the forest if it is transplanted outside the forest in a park?
(a) It will grow normally.
(b) It will grow well because it is planted in the same locality.
(c) It may not survive because of change in its microclimate.
(d) It grows very well because the plant gets more sunlight. 

Answer : C

Question. Autecology is the
(a) relation of a population to its environment.
(b) relation of an individual to its environment.
(c) relation of a community to its environment.
(d) relation of a biome to its environment. 

Answer : B

Question. What would be the percent growth or birth rate per individual per hour for the same population mentioned in the previous question
(Question 12)?
(a) 100
(b) 200
(c) 50
(d) 150 

Answer : B

Question. Amensalism is an association between two species where
(a) one species is harmed and other is benefitted.
(b) one species is harmed and other is unaffected.
(c) one species is benefitted and other is harmed.
(d) both the species are harmed.

Answer : B

Question. Which one of the following organisms reproduces sexually only once in its life time?
(a) Banana plant
(b) Mango
(c) Tomato
(d) Eucalyptus

Answer : A

Question. Among the following algae that inhabit the sea, which is likely to be found in the deepest water?
(a) Red algae
(b) Brown algae
(c) Green algae
(d) Golden brown algae

Answer : A

Question. Percolation and water holding capacity of soil does not depend on -
(a) Soil composition
(b) Biota
(c) Size of grains
(d) Aggregation

Answer : B

Question. Which of the following is main reason for non occurence of small size conformers ?
(a) Karyoplasmic index
(b) Area : volume ratio
(c) Basal metabolism
(d) All the above

Answer : B

Question. Shortening of ears, limbs and other extremeties of mammals so that heat loss can be minimise, is associated with  
(a) Allen's rule
(b) Bergeman's rule
(c) Jordan's rule
(d) Rensch's rule

Answer : B

Question. Which of the following mean was used by warblers to avoid competition and coexist
(a) Difference in foraging activities
(b) Habitat fragmentation
(c) Competitive release
(d) All of these

Answer : A

Question. Which of the following is not an adaptation of parasites for assurance of parasite host interaction
(a) Loss of sensory organs
(b) Presence of adhesive organs
(c) Loss of digestive system
(d) Low reproductive potential

Answer : D

Question. Altitude sickness can be seen at which specific height ?
(a) < 3500 m
(b) > 3500 m
(c) 5300 m
(d) < 530 m

Answer : B

Question. Which of the following is not an adaptation for altitude sickness ?
(a) Increase in red blood cell production
(b) Decrease in binding capacity of oxygen with haemoglobin
(c) Increased breathing rate
(d) Increased heart palpitations

Answer : D

Question. The sum total of the populations of the same kind of organisms constitute
(a) colony
(b) genus
(c) community
(d) species. 

Answer: D

Question. Fertility of soil is measured by its ability to
(a) retain nutrients
(b) hold organic materials
(c) hold water
(d) support life. 

Answer: D

Question. Soil particles determine its
(a) texture
(b) field capacity
(c) water holding capacity
(d) soil flora.

Answer: A

Question. Homeostasis is
(a) tendency of biological systems to change with change in environment
(b) tendency of biological systems to resist change
(c) disturbance of self regulatory system and natural controls
(d) biotic materials used in homeopathic medicines.

Answer: B

Question. Deep black soil is productive due to high proportion of
(a) sand and zinc
(b) gravel and calcium
(c) clay and humus
(d) silt and earthworm.

Answer: C

Question. Homeostasis is
(a) tendency to change with change in environment
(b) tendency to resist change
(c) disturbance in regulatory control
(d) plants and animal extracts used in homeopathy.

Answer: B

Question. Which of the following is not an attribute of a population?
(a) Sex ratio
(b) Natality
(c) Mortality
(d) Species interaction

Answer: D

Question. Carnivorous animals lions and leopards, occupy the same niche but lions predate mostly larger animals and leopards take smaller ones. This mechanism of competition is referred to as
(a) character displacement
(b) altruism
(c) resource partitioning
(d) competitive exclusion. 

Answer: C

Question. Between which among the following, the relationship is not an example of commensalism?
(a) Orchid and the tree on which it grows
(b) Cattle Egret and grazing cattle
(c) Sea Anemone and Clown fish
(d) Female wasp and fig species

Answer: D

Question. Natality refers to
(a) death rate
(b) birth rate
(c) number of individuals leaving the habitat
(d) number of individuals entering a habitat.

Answer: B

Question. Which one of the following plants shows a very close relationship with a species of moth, where none of the two can complete its life cycle without the other?
(a) Hydrilla
(b) Yucca
(c) Banana
(d) Viola 

Answer: B

Question. In a growing population of a country,
(a) pre-reproductive individuals are more than the reproductive individuals
(b) reproductive individuals are less than the postreproductive individuals
(c) reproductive and pre-reproductive individuals are equal in number
(d) pre-reproductive individuals are less than the reproductive individuals. 

Answer: A

Question. Which one of the following population interactions is widely used in medical science for the production of antibiotics?
(a) Commensalism
(b) Mutualism
(c) Parasitism
(d) Amensalism

Answer: D

Question. Asymptote in a logistic growth curve is obtained when
(a) K = N
(b) K > N
(c) K < N
(d) the value of ‘r’ approaches zero. 

Answer: A

Question. Mycorrhizae are the example of
(a) amensalism
(b) antibiosis
(c) mutualism
(d) fungistasis.

Answer: C

Question. Which of the following is correct for r-selected species?
(a) Large number of progeny with small size
(b) Large number of progeny with large size
(c) Small number of progeny with small size
(d) Small number of progeny with large size

Answer: A

Question. If ‘+’ sign is assigned to beneficial interaction, ‘–’ sign to detrimental and ‘O’ sign to neutral interaction, then the population interaction represented by ‘+’ ‘–’ refers to
(a) mutualism
(b) amensalism
(c) commensalism
(d) parasitism.

Answer: D

Question. The principle of competitive exclusion was stated by
(a) C. Darwin
(b) G.F. Gause
(c) Mac Arthur
(d) Verhulst and Pearl.

Answer: B

Question. When does the growth rate of a population following the logistic model equal zero? The logistic model is given as dN/dt = rN(1–N/K)
(a) when N/K equals zero
(b) when death rate is greater than birth rate
(c) when N/K is exactly one
(d) when N nears the carrying capacity of the habitat.

Answer: C

Very Short Answer Questions

Question. What are ectotherms?
Answer. Ectotherms are those animals whose body temperature changes and matches with that of the environment in which they are living. They are also called cold-blooded animals.

Question. Species that tolerate wide range of salinity are called ____________. 
Answer. Euryhaline.

Question. Mention the effect of global warming on the geographical distribution of stenothermals like amphibiAnswer. 
Answer. Due to global warming, stenothermals would either migrate or die due to change in the temperature.

Question. Why are cattle and goats not seen browsing on Calotropis growing in the fields?
Answer. Calotropis produces highly poisonous cardiac glycosides. Therefore, cattle and goats do not browse on them.

Question. If 8 individuals in a laboratory population of 80 fruit flies died in a week, then what would be the death rate of population for the said period? 
Answer. Death rate = Number of individuals dead/Total number of individual 8/80 = 0.1
 The death rate will be 0.1 individuals per week.

Question. When and why do some animals go into hibernation?
Answer. When the animals are not able to tolerate the stressful conditions like low temperature, they hibernate to avoid the stress by escaping in time since they can not migrate.

Question. Why are mammals the most successful animals on earth?
Answer. The mammals are most successful animals on earth because they can maintain a constant body temperature with high range of tolerance whether in Antarctica or in Sahara desert.

Question. Why do people living in high altitude have more haemoglobin/high RBC count?
Answer. To acclimatise at high altitude, the people have more haemoglobin/high RBC count to compensate for the low oxygen availability.

Question. Name the type of association that the genus Glomus exhibits with higher plants.
Answer. Symbiosis/Mycorrhizae/Mutualism.

Question. Name a ‘photoperiod’ dependent process, one each in plants and in animals.
Answer. In plants, flowering and in animals, migration/foraging are photoperiod dependent processes.

Question. Between amphibians and birds, which will be able to cope with global warming? Give reasons. 
Answer. Birds being eurythermals can tolerate a wide range of temperature and thus will be able to cope with global warming more efficiently.

Question. Select the statement which explains parasitism best.
(a) One organism is benefited.
(b) Both the organisms are benefited.
(c) One organism is benefited, other is not affected.
(d) One organism is benefited, other is affected.
Answer. (d).

Question. An orchid plant is growing on the branch of mango tree. How do you describe this interaction between the orchid and the mango tree?
Answer. The interaction between an orchid and the mango tree is commensalism, because orchid is benefited by getting shelter from mango tree whereas the mango tree is neither harmed nor benefited.

Question. Why are some organisms called as eurythermals and some others as stenohaline?
Answer. Eurythermals are organisms that can tolerate and thrive in a wide range of temperature, whereasste nohalines can tolerate a narrow range of salinities.

Question. Which one of the two, stenothermals or eurythermals, shows wide range of distribution on earth and why?
Answer. Eurythermals show a wide range of distribution on earth, as they show tolerance for wide range of temperatures.

Question. In a pond there were 20 Hydrilla plants. Through reproduction 10 new Hydrilla plants were added in a year. Calculate the birth rate of the population. 
Answer. Birth rate = Number of individuals born /Total number of individuals = 10/20 = 0.5 Birth rate is 0.5 plants per year.

Question. Name important defence mechanisms in plants against herbivory.
Answer. 

Question. Mention the effect of global warming on the geographical distribution of stenothermals like amphibians. 
Answer. Due to global warming, stenothermals would either migrate or die due to change in the temperature.

Question. Give an example for:
(a) An endothermic animal (b) Ectothermal animal
(c) An organism of benthic zone.
Answer. (a) Monkey (b) Snake (c) Angler fish.

Question. When and why do some animals go into hibernation?
Answer. When the animals are not able to tolerate the stressful conditions like low temperature, they hibernate to avoid the stress by escaping in time since they can not migrate.

Short Answer Questions

Question. Many fresh water animals cannot survive in marine environment. Explain.
Answer. Marine environment has high salt concentration. These fresh water animals in such hypertonic surroundings suffer from osmotic problems. Their bodies start losing water by exosmosis.

Question. How do seals adapt to their natural habitat? Explain.
Answer. Seals adapt to the cold climate by developing a thick layer of fat (blubber) below their skin that acts as an insulator and reduce excess loss of body heat.

Question. Some organisms suspend their metabolic activities to survive in unfavourable conditions.
Explain with the help of any four examples.
Answer. (i) Polar bear: They hibernate during winter to escape the cold weather.
(ii) Snails/fishes: They go into aestivation during summer to avoid heat related problems and desiccation.
(iii) Seeds of higher plants/spores of bacteria/fungi: They become dormant in unfavourable conditions and in case of Amoeba cyst formation takes place.
(iv) Some species of zooplankton: They undergo diapause.

Question. Plants that inhabit a rain-forest are not found in a wetland. Explain. 
Answer. Plants in wetland are adapted differently. The soil in wetland lacks oxygen. So, for respiration the roots have to grow above the soil. These roots are called breathing roots or pneumatophores. This feature is not present in roots of plants growing in rainforest.

Question. How does our body adapt to low oxygen availability at high altitudes? 
Answer. Our body adapts to low oxygen availability by increasing red blood cell production, decreasing the binding capacity of haemoglobin and by increasing breathing rate.

Question. How does a desert plant adapt to the dry, warmer environmental conditions? 
Answer. Adaptations of a desert plant:
(i) Leaf surface has a thick cuticle.
(ii) Stomata are situated in deep pits.
(iii) Stem is flattened and performs photosynthesis.
(iv) Leaves are modified into spines as in Opuntia. 

Question. The density of a population in a habitat per unit area is measured in different units. Write the unit of measurement against the following:
(a) Bacteria, (b) Grass, (c) Banyan, (d) Deer, (e) Fish 
Answer. (a) Numbers/volume
(b) Coverage/area
(c) Biomass/area
(d) Numbers/area
(e) Weight/area or Number/area

Question. Explain relationship between biotic potential and environmental resistance.
Answer. Biotic potential is defined as the maximum inherent capacity of an organism to reproduce or increase the number of individuals. Whereas the environmental resistance is the biotic and abiotic factors of the environment, that do not allow the population of organisms to grow unlimited and keeps the population size in control.

Question. “Snow leopards are not found in Kerala forests and tuna fish are rarely found beyond tropical latitude in the ocean”. Study the above two cases and state the possible reasons for the same
Answer. Change in temperature from their established habitats affects the kinetics of the enzymes and through it, the basal metabolism, activity and other physiological functions of the organism.

Question. (a) Explain “birth rate” in a population by taking a suitable example.
(b) Write the other two characteristics which only a population shows but an individual cannot. 
Answer. (a) Birth rate is expressed as the number of births per 1,000 individuals of a population per year.
For example, in a pond there were 200 frogs and 40 more were born in a year. Then, the birth
rate of the population will be 40/200 = 0.5 frogs per year.
(b) Sex ratio, age distribution, population density, population growth. 

Question. Categorise the following plants into hydrophytes, xerophytes, halophytes and mesophytes. Write the type of plant against the following examples.
(a) Salvinia (b) Opuntia (c) Rhizophora (d) Mangifera 
Answer. (a) Hydrophyte (b) Xerophyte (c) Halophyte (d) Mesophyte

Question. In a pond, we see plants which are free-floating, rooted-submerged, rooted emergent, rooted with floating leaves. Write the type of plants against each of them.
(a) Hydrilla, (b) Typha, (c) Nymphaea, (d) Lemna, (e) Vallisnaria 
Answer. (a) Submerged (b) Rooted emergent (c) Rooted with floating leave (d) free-floating (e) Rooted Submerged

Question. The density of a population in a habitat per unit area is measured in different units. Write the unit of measurement against the following:
(a) Bacteria, (b) Grass, (c) Banyan, (d) Deer, (e) Fish 
Answer. (a) Numbers/volume (b) Coverage/area (c) Biomass/area (d) Numbers/area (e) Weight/area or Number/area

Question. Explain relationship between biotic potential and environmental resistance.
Answer. Biotic potential is defined as the maximum inherent capacity of an organism to reproduce or increase the number of individuals. Whereas the environmental resistance is the biotic and abiotic factors of the environment, that do not allow the population of organisms to grow unlimited and keeps the population size in control.

Question. “Snow leopards are not found in Kerala forests and tuna fish are rarely found beyond tropical latitude in the ocean”. Study the above two cases and state the possible reasons for the same.
Answer. Change in temperature from their established habitats affects the kinetics of the enzymes and through it, the basal metabolism, activity and other physiological functions of the organism.

Question. Why are coral reefs not found from West Bengal to Andhra Pradesh but found in Tamil Nadu on the east coast of India?
Answer. High salinity, optimal temperature and less siltation are essential to colonise corals. If siltation and fresh water inflow are very high, the corals don’t colonise. In contrast when the siltation and fresh water in flow by the rivers are very less, the corals do colonise.

Question. In a sea shore, the benthic animals live in sandy, muddy and rocky substrata and accordingly developed the following adaptations. Find the suitable substratum against each adaptation.
(a) Burrowing ______________________
(b) Building cubes ______________________
(c) Holdfasts/peduncle ______________________ 
Answer. (a) Sandy, (b) Muddy, (c) Rocky.

Question. Name two basic types of competition found amongst organisms. Which one of them is more intense and why?
Answer. The two basic types of competitions are:
(i) Interspecific competition
(ii) Intraspecific competition
The intraspecific competition is more intense because the requirement of the individual of the species are similar.

Question. Mention four adaptive features that help cacti survive in xeric environment. 
Answer. Adaptation in desert plants:
(i) Desert plants have thick waxy coating on leaf called cuticle for minimum loss of water, through transpiration.
(ii) They have special photosynthetic pathway (CAM) that enables minimum loss of water during day time because stomata remain closed.
(iii) Some desert plants develop spines instead of leaf and photosynthetic function is carried out by the flattened stem.
(iv) Stomata are arranged in deep pits to minimise loss, through transpiration.

Question. List the attributes that populations, but not individuals possess.
Answer. The attributes that populations but not individuals possess are:
(i) Population density (ii) Population growth
(iii) Mortality or death rate (iv) Natality or birth rate
(v) Sex ratio (vi) Age distribution

Long Answer Questions

Question. Comment on the following diagrams: A, B, C, D, G, P, Q, R, S are species.

Answer. Fig. I: It is a single population and all individuals are of the same species, i.e., A individuals interact among themselves and their environment.
Fig. II: It is a community and it contains three populations of species A, B and C. They interact with each other and their environment.
Fig. III: It is a biome. It contains three communities of which one is in climax and other two are in different stage of development. All three communities are in the same environment and they interact with each other and their environment.

Question. (a) Explain giving reasons why the tourists visiting Rohtang Pass or Mansarovar are advised to resume normal ‘active life’ only after a few days of reaching there.
(b) It is impossible to find small animals in the polar regions. Give reasons. 
Answer. (a) Initially the person suffers from altitude sickness/nausea, fatigue and heart palpitation because of low oxygen availability and low atmospheric pressure. Gradually the body increases RBC production, decreasing binding capacity of Hb and increases the breathing rate to get acclimatised.
(b) Small animals have larger surface area relative to their volume, so they lose heat much faster, & have to spend more energy to generate body heat.

Question. Draw and explain a logistic curve for a population of density (N) at time (t) whose intrinsic rate of natural increase is (r) and carrying capacity is (K).
Answer. 

Question. (a) Explain with the help of a graph the population growth curve when resources are
(i) limiting and (ii) not limiting.
(b) “Nature has a carrying capacity for a species.” Explain. 
Answer. (a) QU There are two models of population growth:
(i) The exponential growth
(ii) Logistic growth
(i) Exponential Growth
QU The exponential or geometric growth is common where the resources (food + space) are unlimited.
QU Each species has the ability to realise fully its innate potential to grow in number.
QU The equation for exponential growth can be derived as follows: to
(b) (i) The resources become limited at certain point of time, so no population can grow exponentially.
(ii) Every ecosystem or environment or habitat has limited resources to support a particular maximum number of individuals called its carrying capacity (K).

Question. How do kangaroo rats and desert plants adapt themselves to survive in their extreme habitat? Explain.
Answer. Kangaroo rats are capable of meeting its water requirements through its internal fat oxidation in which water is a by product. It also has the ability to concentrate its urine so that minimal volume of water is used to remove excretory products. Desert plants have a thick cuticle on their leaf surface and have their stomata arranged in deep pits to minimise water loss. They also have leaves reduced to spines and deep roots to absorb more water. They have a special photosynthetic pathway (CAM).

Question. (a) Name the two growth models that represent population growth and draw the respective growth curves they represent.
(b) State the basis for the difference in the shape of these curves.
(c) Which one of the curves represent the human population growth at present? Do you think such a curve is sustainable? Give reason in support of your answer.
Answer. (a) Exponential growth curve and logistics growth curve For graph refer (b) The difference in the shape of the curve is due to the amount of resources available for the given population. When resources are unlimited, each species realises its innate potential to grow in number and result in a J-shaped curve in exponential growth while in logistics growth no population has unlimited resources leading to competition for resources and show S-shaped curve.
(c) Logistic growth represents human population growth at present. Such a curve is not sustainable because with growing population natural resources are getting depleted and its availability is not increasing enough.

Question. (a) Represent diagrammatically three kinds of age-pyramids for human populations.
(b) How does an age pyramid for human population at given point of time helps the policy-makers in planning for future.
Answer. (a) 
(b) Age pyramid helps in planning the healthcare programmes, the education policies and the infrastructure of the area Analysis of age pyramid of a population can give the correct information about the status of the people in the area and their requirements.

Question. Predation is usually referred to as a detrimental association. State any three positive roles that a predator plays in an ecosystem. 
Answer. (i) They predators act as conduits for energy transfer across trophic levels.
(ii) They keep prey populations under control.
(iii) They help in maintaining species diversity in a community by reducing the intensity of competition among prey species.

Question. Write a short note on:
(a) Adaptations of desert plants and animals
(b) Adaptations of plants to water scarcity
(c) Behavioural adaptations in animals
(d) Importance of light to plants
(e) Effect of temperature or water scarcity and the adaptations of animals.
Answer. (a) Adaptations of desert plants are as follows:
(i) Desert plants have cuticles to minimise transpiration.
(ii) In some desert plants, leaves are modified into spines to minimise loss of water.
(iii) They have long roots and adaptations to reduce transpiration, e.g., Acacia.
(iv) Stomata are present in deeppits.
Adaptations of desert animals are as follows:
(i) Desert animals have concentrated their urine for minimum loss of water, e.g., Kangaroo rat.
(ii) Desert animals absorb heat from the sun, when the body temperature drops below the comfort zone.
(iii) They live in burrows during hot season and have little water requirement, e.g., camel.
(iv) Meet water requirement by interval fat oxidation.
(b) Adaptations of plants to water scarcity
(i) Some desert plants develop special photosynthetic pathway (CAM) to minimise the loss of water and close stomata during day.
(ii) Some desert plants have sunken stomata to minimise the loss of water.
(iii) Epidermis is thick walled with thick cuticles and often possess wax, thus, reducing thesurf ace transpiration.
(iv) Roots are deep-seated, almost reaching the water table, e.g., Prosopis.
(v) These xerophytes possess hard and pointed spines (modified leaves) to reduce transpiration.
(c) Behavioural adaptations in animals
(i) Desert lizards bask in the sun and absorb heat when their body temperature drops below the comfort zone, but move into shade when the ambient temperature starts increasing.
(ii) Some species are capable of burrowing into the soil to hide and escape from ground heat.
(iii) Hibernation and aestivation are quite common in ectothermal animals.
(d) Importance of light to plants
(i) Light is important for manufacturing food by the process of photosynthesis.
(ii) Duration of light determines flowering and fruit formation.
(iii) Light also determines the temperature which is associated with functioning of enzymes.
(iv) Light is essential for growth and development of plant because it provides organic materials.
(e) Effect of temperature or water scarcity and the adaptations of animals.
(i) Animals living in arid areas reduce water loss to minimum. For example, Kangaroo Rat feeds on dry seeds and seldom drink water.
(ii) The requirement of water is often compensated by food and metabolic water. Water loss is prevented by burrowing into the soil to hide and escape from the above ground heat, concentration of urine and solid faeces. Camel stops producing urine when water is not available and can remain without water for many days.
(iii) Animals protect themselves from excessive cold by deposition of fat, fur, etc. Bears undergo hibernation during winters.

Question. (a) List any three ways of measuring population density of a habitat.
(b) Mention the essential information that can be obtained by studying the population density of an organism. 
Answer. (a) By physical counting, percent cover or total biomass, from relative density, counting pugmarks, counting faecal pellets. (Any three)
(b) Status of habitat, whether competition for survival exists or not, whether population is increasing or declining, natality, mortality, emigration, immigration.

Question. (a) “Organisms may be conformers or regulators.” Explain this statement and give one example of each.
(b) Why are there more conformers than regulators in the animal world?
Answer. (a) Conformers are organisms which cannot maintain a constant internal environment under varying external environmental conditions. They change body temperature and osmotic concentration with change in external environment. For example, all plants, fishes, amphibians etc.
Regulators are organisms which can maintain homoeostasis (by physiological means or behavioural means) i.e., they maintain constant body temperature and osmotic concentration.
For example, birds and mammals.
(b) Thermoregulation is energetically expensive for animals. Therefore, more conformers are found.

Question. Distinguish between the following:
(a) Hibernation and aestivation
(b) Ectotherms and endotherms
Answer. (a) Differences between hibernation and aestivation

(b) Table 13.5 Differences between ectotherms and endotherms 

Important Notes for Class 12 Biology Chapter 13 Organisms And Populations

- A single organism cannot live alone, due to which there is always seen an inter-relationship between organisms and their surrounding. The branch of biology which deals with different principles that control this relationship is known as ecology.
- Ecology It is the study of the interactions among organisms and between organism and its physical (abiotic) environment. It consists of two branches, Autecology, i.e. study of ecology at the level of species and Synecology, i.e. study of ecology at the level of communities.

Organisational Levels of Ecology

Ecology is basically concerned with four levels of organisation
- Organism It refers to the living component of the environment at individual level. It forms the basic unit of the study of ecology.
- Population It refers to the sum total of all organisms having similar features and potential to interbreed among themselves and produce fertile offsprings.
- Communities Assemblage of all the populations of different species in a specific geographical area.
- Biome It is a large unit which consists of a major vegetation type and associated fauna in a particular climatic zone. Tropical rainforest, deciduous forest, sea coast, deserts, etc., are the major biomes of India.

Organism and its Environment

- An environment is termed as the sum total of all external conditions (biotic and abiotic) which influence the organisms in terms of survival and reproduction.
- Ecology at the organismic level is essentially physiological ecology, which studies the adaptations of organisms essential for survival and reproduction in any given environment.
- We know that the rotation of our planet around the sun and the tilt of its axis cause annual variations in the intensity and duration of temperature, resulting in distinct seasons

- These variations together with annual variation in precipitation account for the formation of major biomes such as desert, rainforest and tundra.
- Regional and local variations within each biome lead to the formation of a wide variety of habitats.
- Habitat is a place, where an organism lives and represents a particular set of environmental conditions suitable for its successful growth.
- Each organism has an invariably defined range of conditions (evolved through natural selection) that it can tolerate, diversity in the resources it utilises and a distinct functional role in the ecological system all these together comprise its niche.
- Ecological equivalents are the organisms which occupy a part of the same niche, but have different habitats.
- Life on earth exists not just in a few favourable habitats but even in extreme and harsh habitats like scorching Rajasthan desert, rain-soaked Meghalaya forests, deep ocean trenches, torrential streams, permafrost polar regions, high mountain tops, boiling thermal springs, stinking compost pits and even our intestine is an unique habitat for hundreds of microbial species.

Note Ramdeo Misra is known as the Father of Ecology in India. Due to his efforts, the Government of India established the Ministry of Environment and Forests in 1984.
- The habitat of an organism is completely characterised by two components or factors, i.e. abiotic and biotic factors. The major abiotic physico-chemical factors include temperature, water, light and soil, biotic factors include pathogens, parasites, predators, competitors, etc., with which the organism interacts.

Adaptation

Any attribute of an organism (morphological, physiological or behavioural) that enables it to survive and reproduce in its habitat can be referred to as adaptation.

Adaptations in Plants
Different plants show different anatomical or physiological adaptations. A few examples of adaptations in plants are as follows

Adaptations in Desert Plants
(Xerophytic Plants)
- Roots grow very deep to explore any possibility of available underground water.
- Many desert plants have a thick cuticle on their leaf surfaces and have their stomata arranged in deep pits to minimise water loss through transpiration. They also have a special photosynthetic pathway known as Crassulacean Acid Metabolism (CAM) that enables their stomata to remain closed during day time so as to minimise transpiration.
- Some desert plants like Opuntia, have no leaves. Their leaves are reduced to spines and photosynthesis occurs in flattened stems.

Adaptations in Aquatic Habitats
(Hydrophytic Plants)
Aquatic plants or hydrophytes have evolved aerenchyma for buoyancy and floating. They have covering of wax to avoid damage through water. Roots are generally absent in plants like Hydrilla and Nymphaea.

Adaptations to Saline Environments
(Halophytic Plants)
- The plants of saline habitats which not only have the ability to tolerate high concentration of salts in their rooting medium but are also able to obtain their water supply from the same, are called halophytes.
- These are found in tidal marshes, coastal dunes, mangroves and saline soils. Certain green algae are also found in these areas, e.g. Dunaliella.
- Mangroves are the areas that not only have excess salt, but also have excess water and anaerobic conditions besides difficulty in anchoring and seed germination.
- A number of plants possess small negatively geotropic vertical roots called pneumatophores (have lenticels for gaseous exchange), e.g. Avicennia, Aegialitis.
- Another adaptation of mangrove plants is vivipary or seed germination while the fruit is still attached to plants, e.g. Rhizophora, Aegiceras, Ceriops.
Note Oxygen release by plants This acts as the most significant long term adaptation of wetland species to soil anaerobiosis, e.g. Eichhornia, Pistia, etc.

Adaptations in Animals
Animals have different physiological and behavioural adaptations to environmental stresses, etc. Some examples are given below

Adaptations in Kangaroo Rat
- The kangaroo rat in North American deserts is capable of meeting all its water requirement by internal oxidation of its body fat (water is a byproduct).
- It can also concentrate its urine, so that minimal volume of water is used to expel excretory products.

Adaptations in Desert Lizards
- They absorb heat from sun when the body temperature drops below the comfort zone and move into shade when the ambient temperature starts increasing.
- Some species burrow into the soil and escape from the above ground heat. These are behavioural responses.

Adaptations in Mammals
- Mammals from colder climates generally have shorter ears and limbs to minimise heat loss. This is called Allen’s rule.
- In polar regions, aquatic mammals like seals have a thick layer of fat (blubber) below their skin that acts as an insulator and reduces the loss of body heat.

Adaptations at High Altitudes in Humans
- At high altitude places like Rohtang Pass near Manali (> 3500 m) and Mansarovar (in China occupied Tibet) people suffer from altitude sickness.
- The common symptoms include nausea, fatigue and heart palpitations. This is because at low atmospheric pressure of high altitudes, body does not get enough oxygen. The relief occurs gradually due to acclimatisation.
- The body copes up with this low oxygen stress by
(i) increasing red blood cells production.
(ii) decreasing the binding affinity of haemoglobin.
(iii) increasing the breathing rate.
Note l Tribal people living at high altitudes of Himalayas have higher RBC count than people living in plains.

Adaptations at High and Low Temperature
- Certain bacteria (e.g. Archaebacteria) can flourish in places having temperature exceeding 100°C such as, deep sea hydrothermal vents, hot springs, etc., with the help of certain enzymes they possess.
These enzymes can withstand high temperatures.
- Some invertebrates and fishes can tolerate temperatures below 0°C by extra solutes like glycerol and anti-freeze proteins that lower the

Populations

- Population is a set of individuals of a particular species, which are found in a particular geographical area and can interbreed.
- The population that occupies a very small area and is smaller in size, is called local population.
- A group of such closely related local populations is called metapopulation.
- Population ecology is an important area of ecology because it links ecology to population genetics and evolution.

Population Attributes
A population has certain attributes that an individual organism does not have. Some of them are given below

1. Population size or density It is the number of individuals of a species per unit area or volume.
Population Density (PD)
Number of individuals in a region (N)/Size of unit area in the region (S); PD=  N/S

2. Birth rate or Natality It is the number of births of new individuals per unit of population per unit time, e.g. if in a pond, there are 20 lotus plants last year and through reproduction, 8 new plants are added, taking the current population to 28.
Then, birth rate =8/20 =0.4 offspring per lotus per year.

3. Death rate or Mortality It is the number of loss of individuals per unit of population per unit time due to death or due to the different environmental changes, competition, predation, etc.
For example, if in a laboratory population of 40 individuals, 4 fruit flies died during a specified time interval, then the death rate = 4/40 = 0.1 individuals per fruit fly per week.

4. Sex ratio An individual is either a male or a female but a population has sex ratio. It is the number of females and males per 1000 individuals of a population in a given time.

Age Pyramid

- Population at any given time is composed of individuals of different ages. When the age distribution (per cent individuals of a given age or age group) is plotted for the population, this is called age pyramid.
- The age pyramids of human population generally show the age distribution of males and females in a combined diagram.
- The growth status of the population is reflected by the shape of the pyramids.
- The three types of age pyramids are as follows
(i) Expanding (ii) Stable (iii) Declining

Population Growth

The size of a population for any species is not a static parameter, it keeps changing with time. It depends on factors such as food availability, predation pressure and adverse weather. The density of a population in a given habitat during a given period fluctuates due to the four basic processes

1. Natality It refers to the number of births during a given period in the population that are added to the initial density.
2. Mortality It is the number of deaths in the population during a given period.
3. Immigration It is the number of individuals of the same species that have come into the habitat from elsewhere during the time period under consideration.
4. Emigration It is the number of individuals of the population who left the habitat and moved elsewhere during a given time period.

Out of these four, natality and immigration contribute to an increase in population density, while mortality and emigration contribute to the decrease in population density.
So, if N is the population density at time t, then its density at time t +1 is
N_t+1= [N_t + (B+ I) - (D+ E)]

where, N = Population density, t = Time, B = Birth rate,I = Immigration, D= Death rate and E = Emigration
From the above equation, we can see that population density will increase, if (B + I ) is more than (D + E).

Growth Models
To study the behaviour and pattern of different populations, the following two models of population growth are used.

1. Exponential Growth
Availability of resources (food and space) is essential for the growth of population. Unlimited availability of such resources results in exponential growth of population. The increase or decrease in population density during a unit time period (t) is calculated as
dN/dt =(b -d)N
Let (b - d) = r, then, dN/dt = rN
Where, N is population size, b is birth per capita,d is death per capita, t is time period and r is intrinsic rate of natural increase.
r is an important parameter that assesses the effects of biotic and abiotic factors on population growth. It is different for different organisms, e.g. its value is 0.015 for Norway rat and 0.12 for flour beetle.
The above equation results in a J-shaped curve as shown in graph.

Integral form of exponential growth equation is
N_t = N₀e^rt

Where, N_t = Population density after time t
N₀ = Population density at time zero
r = Intrinsic rate of natural increase
e = Base of natural logarithms (2.71828).
Any species growing exponentially under unlimited resource conditions without any check, can reach enormous population densities in a short time.

2. Logistic Growth
Practically, no population of any species in nature has unlimited resources at its disposal. This leads to competition among the individuals and the survival of the ‘fittest’.
Therefore, a given habitat has enough resources to support a maximum possible number, beyond which no further growth is possible. This is called the carrying capacity (K) for that
species in that habitat.
When N is plotted in relation to time t , the logistic growth shows sigmoid curve and this type of growth is called Verhulst-Pearl Logistic Growth. It is calculated as dN/dt = rN(K-N/K)
Where, N is population density at time t , K is carrying capacity and r is intrinsic rate of natural increase.

A population growing in a habitat with limited resources shows initially a lag phase followed by phases of acceleration, deceleration and finally an asymptote phase, when the population density reaches the carrying capacity (K).

The integral form of logistic growth equation is

This model is more realistic in nature because no population growth can sustain exponential growth indefinitely as there will be competition for the basic needs due to finite resources.

Population Interactions

In nature, living organisms such as animals, plants and microbes, cannot live in isolation and therefore, interact in various ways to form a biological community.
Interspecific interactions occur between populations of two different species. These interactions could be beneficial (+), detrimental (-) or neutral (0) as shown in given table. Population interactions and their effects are as follows

Names of Interaction    Effects on       Effects on
                                       Species A       Species B
Mutualism                         +                         +
Competition                       -                          -
Predation                          +                          -
Parasitism                        +                          -
Commensalism                +                          0
Amensalism                      -                          0
Various population interactions are as follows

1. Predation

- It is an interspecific interaction, where an animal called predator kills and consumes the other weaker animal called prey.
- This is a biological control method. It is the nature’s way of transferring energy to higher trophic levels, which is fixed by plants at the first trophic level, e.g. tiger (predator) and deer (prey).
- Important roles of predators are as follows
(i) In the absence of predators, prey species could achieve very high population densities and cause instability. So, besides acting as ‘conduits’ for energy transfer across trophic levels, predators play very important role in providing population stability.
(ii) They help in maintaining species diversity in a community, by reducing the intensity of competition among competing prey species, e.g. predator starfish Pisaster in the rocky intertidal communities of American Pacific Coast. In a field experiment, when all the starfish were removed from the area, more than 10 species of invertebrates became extinct within a year, because of interspecific competition.
- If a predator is too efficient and over exploits its prey, then the prey might become extinct. Following it, the predator will also become extinct because of the lack of food. This is why predators in nature are prudent. Prey species have evolved various defence mechanisms to lessen the impact of predation. These are as follows

(a) Some species of insects and frogs are cryptically coloured (camouflaged) to avoid being detected easily by the predator. Some are poisonous and therefore, avoided by the predators. Monarch butterfly is highly distasteful to its predators (birds) because of a special chemical present in its body. The butterfly acquires this chemical during its caterpillar stage by feeding on a poisonous weed.

(b) Nearly 25% of all insects are known to be phytophagous (feeding on plant sap and other parts of plants) apart from other herbivores. So, plants have evolved various defences against them, e.g. thorns of Acacia and cactus are the most common morphological means of defence.

- Some plants produce highly poisonous chemicals like cardiac glycosides, e.g. weed Calotropis that makes the herbivore sick, etc.
- Chemicals like nicotine, caffeine, quinine,strychnine, opium, etc., are actually defence mechanisms against grazers and browsers.

2. Competition

- It is the interaction in which closely related species compete for the same resources which are limited.
However, this is not always true.
- Some totally unrelated species could also compete for the same resources, e.g. in some shallow South American lakes, visiting flamingos and resident fishes compete for their common food, i.e. zooplanktons.
- Resources need not be limiting for competition to occur. In interference competition, the feeding efficiency of one species might be reduced due to the interfering and inhibitory presence of the other species, although the resources are plenty, e.g. when goats were introduced in Galapagos Islands, the Abingdon tortoise became extinct within a decade due to greater browsing efficiency of the goats.
- Therefore, competition can be best defined as a process in which the fitness of one species (measured in terms of its r, the intrinsic rate of increase) is significantly lower in the presence of another species.
- Competitive release provides another evidence of competition in nature. It is a phenomenon, in which a species whose distribution is restricted to a small geographical area because of the presence of a competitively superior species, is found to expand its distributional range dramatically, when the competing species is experimentally removed.
- Connel’s elegant field experiments showed that on the rocky sea coasts of Scotland, the larger and competitively superior barnacle Balanus dominates the intertidal area and excludes the smaller barnacle Chathamalus from that zone.
- Gause’s competitive exclusion principle states that two closely related species competing for the same resources cannot coexist indefinitely and the competitively inferior one will be eliminated eventually. This may hold true in case of limited resources.
- Resource partitioning is a mechanism evolved by competing species to ensure their co-existence. This refers to competitive co-existence.

3. Parasitism

It is the mode of interaction between two species in which one species (parasite) depends on the other species (host) for food and shelter and damages the host. In this process, one organism is benefitted (parasite), while the other is being harmed (host).

(i) Adaptation methods of a parasite are Parasite is host-specific in a way that both host and parasite tend to co-evolve. According to its lifestyle, a parasite evolved special adaptations as
(a) Loss of unnecessary sense organs.
(b) Presence of adhesive organs or suckers for clinging on to host.
(c) Loss of digestive system.
(d) High reproductive capacity.

(ii) The life cycles of parasites are often complex, involving one or two intermediate hosts or vectors to facilitate parasitisation of its primary host. For example,
(a) Human liver fluke (a trematode parasite) depends on two intermediate hosts (a snail and a fish) to complete its life cycle.
(b) Malarial parasite (Plasmodium) needs a vector (mosquito) to spread to other hosts.

(iii) Majority of parasites harm the host. The harm is done in the following ways
- They reduce the survival, growth and reproductive ability of the host.
- They reduce its population density.
- They might render the host more vulnerable to predation by making it physically weak.

Types of Parasites

Parasites are broadly divided into following main types

1. Ectoparasites depend on the external surface of the host organism for food and shelter, e.g. lice on humans, ticks on dogs, copepods in marine fishes and Cuscuta, a parasitic plant that grows on hedge plants.

2. Endoparasites live inside the host’s body at different sites like liver, kidney, lungs, etc., for food and shelter, e.g. tapeworm, liver fluke, Plasmodium, etc. The life cycles of endoparasites are more complex because of their extreme specialisation.

Brood Parasitism
It is an example of parasitism in which one organism (parasite) lays its eggs in the nest of another organism (host) for the later to incubate them. The eggs of parasitic birds have evolved to resemble the host’s egg to reduce the chances of host bird from detecting and ejecting the parasitic eggs from nest, e.g. cuckoo (koel, parasite) and crow (host) during breeding season (spring to summer).

4. Mutualism

It is an interaction that confers benefits to both the interacting species.

Some examples of mutualism are
- Lichens represent an intimate mutualistic relationship between a fungus and photosynthesising algae or cyanobacteria. Here, the fungus helps in the absorption of nutrients and provides protection, while algae prepare the food.
l Mycorrhizae show close mutual association between fungi and the roots of higher plants. Fungi help the plant in absorption of nutrients, while the plant provides food for the fungus, e.g. many members of genus–Glomus.
- Plants need help from animals for pollination and dispersal of seeds. In return, plants provide nectar, pollens and fruits to them.
- To safeguard the mutually beneficial system, plant-animal interactions involve co-evolution of the mutualists, i.e. the evolution of the flower and its pollinator species are tightly linked with one another. For example,

(i) Fig and its partner wasp species, the female wasp uses the fruit not only as an oviposition (egg-laying) site but uses the developing seeds within the fruit for nourishing its larvae. In return, the wasp pollinates the fig inflorescence, while searching for suitable egg-laying sites.
(ii) Mediterranean orchid Ophrys employs ‘sexual deceit’ to get pollinated by a species of bee. One petal of its flower bears an uncanny resemblance to the female of the bee in size, colour and markings. The male bee is attracted to what it perceives as a female and ‘pseudocopulates’ with the flower.

During this process, pollens are dusted from the flower on to the male bee. When the same bee pseudocopulates with another flower, it transfers pollens to it and thus, pollinates the flower.