CBSE Class 12 Biology Organisms And Populations Worksheet Set B

Very Short Answer Questions

Question: Why has life history of variation evolved? 
Answer. (a) Life history of variation has evolved.
(b) In order to maximise reproductive fitness of individual.

Question: When and why do some animals like snails go into aestivation?
Answer. Snails undergo aestivation if they are unable to migrate in order to avoid stressful condition of high temperature.

Question: Name the interaction between sea anemone and the hermit crab that grows on it.
Answer. Commensalism

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: State Gause’s Competitive Exclusion principle. 
Answer. Gause’s Competitive Exclusion Principle states that two closely related species competing for same resources, cannot coexist indefinitely, (the inferior will be eliminated) by the superior one.

Question: Give an example of an organism that enters ‘diapause’ and why. 
Answer. Many species of Zooplankton under unfavourable conditions enters diapause which delay overall development and hence they can pass unfavourable conditions.

Question: What is a tree line? 
Answer. When we go up the altitude, beyond a particular height no tree are found and the vegetation comprise only of shrubs and herbs. The altitude beyond which no tree is seen is known as tree line.

Question: Why are green algae not likely to be found in the deepest strata of the ocean?
Answer. The wavelength of light at the deepest strata is unsuitable for growth of green algae.

Question: What is the ecological principle behind the biological control method of managing with pest insects?
Answer. The ecological principle operating in the biological control method of managing with pest insect is through their natural enemies, i.e., predators and parasites.

Question: What is mycorrhiza?
Answer. Mycorrhiza is a symbiotic association between a fungus and the roots of higher plants.

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.

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: Explain Verhulst-Pearl Logistic Growth of a population. 
Answer. According to Verhulst–Pearl Logistic growth, a population growing in a habitat with limited resources initially shows a lag phase, followed by phases of acceleration and deceleration and finally an asymptote when the population density reaches the carrying capacity. It is given by the following equation:

dN /dt = rN [K-N/K]

where, N = population density at time t, r = intrinsic rate of natural increase, K = carrying capacity.

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: Why are small animals rarely found in the polar regions? Explain.
OR 
Why are small birds like humming birds not found in polar regions? Explain. 
Answer. Small animals like humming birds have a large surface area relative to their volume. So they tend to lose body heat very fast when it is cold outside. Then, these animals have to use their energy (generated by metabolic reactions) to generate body heat. That is the reason why small sized animals are rarely found in the polar regions.

Question: Co-evolution is a spectacular example of mutualism between an animal and a plant. Describe co-evolution with the help of an example. 
OR
Describe the mutual relationship between fig tree and wasp and comment on the phenomenon that operates in their relationship. 
Answer. Co-evolution can be observed in Fig (plant) and wasp (animal). The female wasp uses the fruit for oviposition or egg laying. It also uses developing seeds within the fruit for nourishing its larvae.
The wasp in turn pollinates the fig inflorescence. The given Fig species can be pollinated by its ‘partner’ wasp species and no other species.

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
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. (Any two)

Question: In certain seasons we sweat profusely while in some other season we shiver. Explain.
Answer. Human beings maintain a constant body temperature of 37°C. OU In summers: The outside temperature is much higher than our body temperature. Therefore, we sweat profusely. This results in evaporative cooling and our body temperature is brought down to normal (37°C). OU In winters: The outside temperature is much lower than our body temperature. Therefore, we start to shiver; this action (of shivering) is a kind of exercise (work) that produces heat and raises the body temperature.

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 aOs puinn tia. (Any two)

Question: Why do clown fish and sea anemone pair up? What is this relationship called?
Answer. The clown fish gets protection from predators which stay away from stinging tentacles of anemone but anemone does not derive any benefit from the fish. This relationship is called commensalism.

Long Answer Questions

Question: (a) List the different attributes that a population has and not an individual organism.
(b) What is population density? Explain any three different ways the population density can be measured, with the help of an example each.
Answer. (a) Attributes of population Birth rate, death rate, sex ratio, age pyramids/age distribution. (Any two)
(b) Population density: Number of individuals per unit area at a given time/period.
(i) Biomass/%Cover, e.g., Hundred Parthenium plants and 1 huge banayan tree.
(ii) Relative Density, e.g., Number of fish caught per trap from a lake.
(iii) Numbers, e.g., Human population
(iv) Indirect estimation, e.g., without actually counting/seeing them, e.g., tiger census based on pug marks and faecal pellets. (Any three)

Question: Differentiate between mutualism, parasitism and commensalism. Provide one example for each of them. 
Answer. Refer to Basic Concepts Point 13 (iii), (iv) & (vi).
Predation
- It is an interspecific interaction, where an animal, called predator, kills and consumes the other weaker animal called prey.
- Predation is nature’s way of transferring energy to higher trophic levels, e.g., a tiger (predator) eating a deer (prey), a sparrow (predator) eating fruit or seed (prey), etc.
Commensalism
- Commensalism is referred to as the interaction between two species where one species is benefited and the other is neither harmed nor benefited.

Mutualism
- Mutualism is referred to as the interspecific interaction in which both the interacting species are benefited.
- Some examples of mutualism

Question: List the various abiotic environmental factors.
Answer. (i) Atmospheric factors: Light, temperature, wind and water.
(ii) Lithosphere: Rock, soil.
(iii) Hydrosphere: Pond, river, lake and ocean.
(iv) Edaphic factors: Soil texture, soil water, soil air, soil micro-organisms, soil pH, minerals.
(v) Topographic factors: Slope, altitude, valley.

Question: (a) Explain any two defence mechanisms plants have evolved against their predators.
(b) How does predation differ from parasitism?
Answer. (a) Plants have developed the following defence mechanisms:
(i) Thorns as means of defence.
(ii) Plants may produce chemicals such as nicotine, caffeine, quinine, strychnine, opium for defence.
(b) Table 13.8 Differences between predation and parasitism

Question: How do organisms like fungi, zooplanktons and bears overcome the temporary short-lived climatic stressful conditions? Explain.
Answer. Fungi form thick-walled spores which help them survive in unfavourable conditions. On availability of suitable environment, these germinate. Zooplanktons in lakes and ponds under unfavourable conditions, enter diapause, a stage of suspended development.
Bears in extreme low temperatures, escape winter time by hibernating.

Question: (a) List any three parameters used by ecologists under different situations to measure the population size in a habitat.
(b) Mention what do the following stand for in the equation given below:
(i) Nt+1, (ii) B and (iii) E.
Nt+1 = N0 + [(B + I) – (D + E)]
Give an explanation for the above equation.
Answer. Refer to Basic Concepts Point 10.

Question: Interspecific interactions of two species of any population may be beneficial, detrimental or neutral. Explain each of them with the help of suitable examples.
Answer. Population interactions:

(i) + = Beneficial; – Detrimental; 0 Neutral. Both species ben, ee.fig.t licihne nsmutualism
(ii) Both species lose in competition, e.g. (detrimental)
(iii) In predation and parasitism the predator and the parasite gets benefit but it is detrimental to the other species (host and prey respectively)
(iv) In commensalism one species is benefitted but the other is neither harmed nor benefitted.

Question: Highlight the differences and a similarity between the following population interactions: Competition, predation and commensalism.
Answer. Differences between competition, predation and commensalism

Similarity: All these interactions leads to evolution as the fittest organism survives.

Question: (a) Why are herbivores considered similar to predators in the ecological context? Explain.
(b) Differentiate between the following interspecific interactions in a population:
(i) Mutualism and Competition (ii) Commensalism and Amensalism 
Answer. (a) Herbivores are animals feeding on plants. Although they are classed differently they are considered predators. Like predators, for transfer of energy across trophic levels, herbivores also do the same. Besides this, they also keep the population of their prey under control. For example, when the prickly pear cactus was introduced in Australia in early 1920s, they spread rapidly, causing havoc. Their population was controlled by introducing cactusfeeding predator (a moth).
(b) (i) Differences between mutualism and competition