Maharashtra Board Class 12 Biology Chapter 13 Organisms and Populations Solutions

Organisms And Populations Class 13 Exercise Question Answers Solutions Maharashtra Board

Class 12 Biology Chapter 13 Exercise Solutions Maharashtra Board

Biology Class 12 Chapter 13 Exercise Solutions

1. Multiple Choice Questions

Question 1.Which factor of an ecosystem includes plants, animals and microorganisms?
(a) Biotic factor
(b) Abiotic factor
(c) Direct factor
(d) Indirect factor
Answer: (a) Biotic factor
In simple words: Biotic factors are the living components of an ecosystem, such as plants, animals, fungi, and bacteria, which interact with each other and their environment.

🎯 Exam Tip: Understanding the difference between biotic (living) and abiotic (non-living) factors is fundamental for ecology questions.

Question 2.An assemblage of individuals of different species living in the same habitat and having functional interactions is
(a) Biotic community
(b) Ecological niche
(c) Population
(d) Ecosystem
Answer: (a) Biotic community
In simple words: A biotic community consists of all the different populations of species that live and interact in a particular area.

🎯 Exam Tip: Differentiate between population (same species), community (different species interacting), and ecosystem (community plus abiotic factors).

Question 3.Association between sea anemone and Hermit crab in gastropod shell is that of
(a) Mutualism
(b) Commensalism
(c) Parasitism
(d) Amensalism
Answer: (b) Commensalism
In simple words: Commensalism is a relationship where one species benefits, and the other species is neither harmed nor helped. In this case, the sea anemone gets a mobile home, and the crab is unaffected.

🎯 Exam Tip: Memorize classic examples for different types of species interactions (mutualism, commensalism, parasitism) as they are common exam topics.

Question 4.Select the statement which explains best parasitism.
(a) One species is benefited.
(b) Both the species are benefited.
(c) One species is benefited, other is not affected.
(d) One species is benefited, other is harmed.
Answer: (d) One species is benefited, other is harmed.
In simple words: Parasitism is an interaction where one organism (the parasite) lives on or in another organism (the host), benefiting by deriving nutrients at the host's expense.

🎯 Exam Tip: Clearly distinguish parasitism from commensalism and mutualism by focusing on whether one or both partners are harmed, helped, or unaffected.

Question 5.Growth of bacteria in a newly inoculated agar plate shows
(a) exponential growth
(b) logistic growth
(c) Verhulst-Pearl logistic growth
(d) zero growth
Answer: (c) Verhulst-Pearl logistic growth
In simple words: The Verhulst-Pearl logistic growth model describes population growth that starts slowly, accelerates, then slows down as it approaches a carrying capacity due to limited resources.

🎯 Exam Tip: Logistic growth is often referred to as Verhulst-Pearl logistic growth, especially in detailed biology contexts, and it's characterized by an S-shaped curve.

2. Very Short Answer Questions

Question 1.Define the following terms
a. Commensalism
Answer: The interaction between two species in which one species gets benefits and the other is neither harmed nor benefited is called commensalism.
In simple words: Commensalism is a relationship where one species gains advantages, while the other species involved remains completely unaffected.

🎯 Exam Tip: Provide a clear, concise definition and an example for full marks on such definition questions.

b. Parasitism
Answer: The interaction between two species in which one parasitic species derives benefit from the other host species by harming it is called parasitism.
In simple words: Parasitism is a biological interaction where one organism, the parasite, benefits by living on or in a host organism, causing harm to the host.

🎯 Exam Tip: Remember that in parasitism, one benefits (parasite) and the other is harmed (host), a key distinction from other interactions.

c. Camouflage
Answer: Camouflage is the disguising colouration or behaviour to merge with the surrounding so that prey or predator can remain hidden.
In simple words: Camouflage is an adaptation where an organism's appearance helps it blend into its environment, allowing it to hide from predators or sneak up on prey.

🎯 Exam Tip: When defining camouflage, include both "colouration" and "behaviour" as methods of blending in for a complete answer.

Question 2.Give one example for each
a. Mutualism
Answer: a. Lichen is composed of alga (cyanobacteria) and fungus. They cannot survive independently. Their association is mutualistic alga synthesises food by photosynthesis and fungus does the absorption of moisture.
In simple words: Lichens are a classic example of mutualism, where fungi provide structure and moisture absorption, while algae or cyanobacteria perform photosynthesis to produce food for both.

🎯 Exam Tip: For examples of mutualism, ensure you specify how *both* species benefit from the interaction.

b. Interspecific competition
Answer: b. Leopard and lion competing for a same prey. Sheep and cow competing for grazing in the same land.
In simple words: Interspecific competition occurs when different species compete for the same limited resources, like food, water, or territory.

🎯 Exam Tip: Interspecific competition involves *different species* competing; intraspecific competition involves *the same species* competing.

Question 3.Name the type of association:
a. Clown fish and sea anemone
b. Crow feeding the hatchling of Koel
c. Humming birds and host flowering plants
Answer:
a. Commensalism
b. Brood parasitism
c. Mutualism
In simple words: These examples illustrate different ecological interactions: clownfish and sea anemone show commensalism, crows feeding koel hatchlings is brood parasitism, and hummingbirds pollinating flowers is mutualism.

🎯 Exam Tip: Be ready to identify the type of ecological interaction given various examples, focusing on who benefits, who is harmed, and who is unaffected.

Question 4.What is the ecological process behind the biological control method of managing with pest insects?
Answer:
1. Pest insects act as prey to predator birds or frogs.
2. The biological control method consists of releasing the predators in the farms so that they can control the pest population in the natural way.
3. This also eliminates the use of chemical pesticides.
4. Frogs are natural predators of locust, therefore the population of this hazardous insect is controlled by frogs and the produce from agricultural farm can be saved.
In simple words: Biological control uses natural predators or parasites to manage pest populations, leveraging the ecological relationship of predation to reduce pest numbers without chemical intervention.

🎯 Exam Tip: Biological control highlights the practical application of predator-prey relationships in sustainable agriculture.

Protocooperation:
1. Protocooperation is a type of population interaction where two species interact with each other.
2. Both are benefited but they have no need to interact with each other.
3. They can survive and grow even in the absence of other species.
4. Therefore this interaction is purely for the gain that they receive in such type of interaction.
5. The interaction that occurs can be between different kingdoms.
In simple words: Protocooperation is a loose, non-obligatory mutualistic relationship where both species benefit from the interaction but can still survive independently.

🎯 Exam Tip: The key differentiator for protocooperation is that the mutual benefit is not essential for the survival of either species.

3. Short Answer Questions

Question 1.How is the dormancy of seeds different from hibernation in animals?
Answer: In dormancy seed is not showing any metabolic activities. It can come back to life if and only if it gets suitable moisture and sunlight. Hibernation is suspended state, in which metabolic reactions do take place but at a very reduced pace. Animal arouses on its own after the winter sleep is over. This arousal is spontaneous and depends upon the ambient temperature. Dormant seed does not show such change unless it is planted or thrown in to moist place.
In simple words: Seed dormancy involves a complete metabolic halt, requiring specific environmental cues to resume growth, while animal hibernation is a state of reduced metabolic activity that is usually spontaneously reversible based on internal cycles and ambient temperature changes.

🎯 Exam Tip: Focus on the metabolic activity level and the triggers for exit from each state to differentiate dormancy and hibernation effectively.

Question 2.If a marine fish is placed in a fresh water aquarium, will it be able to survive? Give reason.
Answer: Marine fish has its own osmoregulation which is different from the osmoregulation seen in fresh water fish. In marine water, the ambient salinity is more than the concentration of ions in the body. But in fresh water reverse is the case. Therefore, marine fish has different machinery to cope up with high saline environment. Therefore, it cannot survive in fresh water as its osmoregulation is not possible in less saline waters.
In simple words: A marine fish cannot survive in freshwater because its body is adapted for a high-salinity environment, meaning its osmoregulation system is designed to prevent water loss and excrete excess salt, which would be detrimental in a low-salinity freshwater environment.

🎯 Exam Tip: Osmoregulation is a critical concept here; understand how organisms maintain water and salt balance in different environments.

Question 3.How is the dormancy of seeds different from hibernation in animals?
Answer: In dormancy seed is not showing any metabolic activities. It can come back to life if and only if it gets suitable moisture and sunlight. Hibernation is suspended state, in which metabolic reactions do take place but at a very reduced pace. Animal arouses on its own after the winter sleep is over. This arousal is spontaneous and depends upon the ambient temperature. Dormant seed does not show such change unless it is planted or thrown into moist place.
In simple words: Seed dormancy is a state of complete metabolic inactivity awaiting specific external triggers, whereas animal hibernation involves significantly reduced but still active metabolism, with awakening often governed by internal rhythms or gradual environmental changes.

🎯 Exam Tip: Highlight the distinction between "no metabolic activity" for seeds and "reduced metabolic activity" for hibernating animals for a clear contrast.

Question 4.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:
1. Orchid is an epiphyte. It gets the support from the mango tree. But it does not cause any harm to the mango tree.
2. Mango tree does not derive any benefit from this association. Therefore, this interaction is of type of commensalism.
In simple words: This interaction is commensalism, where the orchid benefits by using the mango tree for support and position, but the mango tree is neither helped nor harmed.

🎯 Exam Tip: Commensalism examples often involve epiphytes (like orchids) or organisms using another for shelter/transport without affecting the host.

Question 5.Distinguish between the following:
a. Hibernation and Aestivation
Answer:

🎯 Exam Tip: When comparing hibernation and aestivation, focus on the season (winter vs. summer), duration, and primary stressors (cold/food vs. heat/water) they combat.

b. Ectotherms and Endotherms
Answer:

🎯 Exam Tip: Key differences lie in heat source (external vs. internal), temperature regulation (fluctuating vs. constant), and metabolic rate.

c. Parasitism and Mutualism
Answer:

🎯 Exam Tip: Focus on the outcome for each species (benefit, harm, neutral) to clearly distinguish between these two fundamental ecological interactions.

Question 6.Write a short note on
a. Adaptations of desert animals
Answer:
1. Animals which are well-adapted to live in deserts are called xerocoles. These animals show adaptations for water conservation or heat tolerance.
2. These animals show low basal metabolic rate. They obtain moisture from succulent plants and rarely drink water. E.g Gazella and Oryx.
3. Desert animals like camel produce concentrated urine and dry dung.
4. Many other hot desert animals are nocturnal, seeking out shade during the day or dwelling underground in burrows.
5. Smaller animals from desert, emerge from their burrows at night.
6. Mammals living in cold deserts have developed greater insulation through warmer body fur and insulating layers of fat beneath the skin.
7. Few adaptations to desert life are unable to cool themselves by sweating so they shelter during the heat of the day. Many desert reptiles are ambush predators and often bury themselves in the sand, waiting for prey to come within range.
8. Other animals have bodies designed to save water. Scorpions and wolf spiders have a thick outer covering which reduces moisture loss. The kidneys of desert animals concentrate urine, so that they excrete less water.
In simple words: Desert animals, or xerocoles, exhibit diverse adaptations like low metabolic rates, nocturnal behavior, concentrated waste excretion, and specialized body coverings to conserve water and tolerate extreme temperatures in arid environments.

🎯 Exam Tip: When discussing adaptations, categorize them into mechanisms for water conservation and heat tolerance, and provide specific animal examples.

b. Adaptations of plants to water scarcity
Or
Adaptations in desert plants.
Answer:
1. Thick cuticle on their leaf surfaces
2. Stomata of desert plants is sunken that is it is in deep pits to minimize loss of water through transpiration.
3. Desert plants also have a special photosynthetic pathway (CAM - Crassulacean acid metabolism) that enables their stomata to remain closed during daytime.
4. Some desert plants like Opuntia, have their leaves reduced or they are modified to spines. Loss of leaf surface helps in prevention of transpiration.
5. Photosynthetic function is taken over by the flattened stems called as phylloclade.
In simple words: Desert plants adapt to water scarcity with thick cuticles, sunken stomata, CAM photosynthesis (closing stomata during day), reduced leaves modified into spines, and flattened stems (phylloclades) to minimize water loss and maximize water retention.

🎯 Exam Tip: Focus on adaptations that reduce transpiration (cuticle, sunken stomata, modified leaves) and those that allow for water-efficient photosynthesis (CAM pathway).

c. Behavioural adaptations in animals
Answer:
1. Behavioural responses to cope with variations in their environment are shown by few animals.
2. Desert lizards manage to keep their body temperature fairly constant by behavioural adaptations. They 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. Even snakes also show basking during winter months.
3. Since they are ectothermic, this kind of behaviour saves them from extreme temperatures.
4. Many smaller animals show burrowing behaviour to adapt to the temperature extremes.
5. Some species burrow into the sand to hide and escape from the heat.
6. Migrations shown by the birds and mammals are also behavioural responses for adapting to severe winter temperatures.
In simple words: Behavioural adaptations are actions animals take to survive environmental challenges, such as desert lizards basking or seeking shade to regulate temperature, smaller animals burrowing to escape extremes, and migrations to avoid harsh seasonal conditions.

🎯 Exam Tip: Behavioral adaptations involve actions, unlike physiological or structural adaptations. Provide examples like basking, burrowing, or migration.

Question 7.Define Population and Community.
Answer:
Population: Group of organisms belonging to same species that can potentially interbreed with each other and live together in a well-defined geographical area by sharing or competing for similar resources, is called population.
Community: Several populations of different species in a particular area makes a community.
In simple words: A population is a group of individuals of the same species living in a specific area and capable of interbreeding, while a community is an assemblage of different populations of various species interacting within a given habitat.

🎯 Exam Tip: The key distinction is "same species" for population and "different species" for community.

4. Long Answer Questions

Question 1.With the help of suitable diagram, describe the logistic population growth curve.
Answer:
ℹ️ चित्र व्याख्या (Diagram Explanation): यह आरेख एक लॉजिस्टिक जनसंख्या वृद्धि वक्र को दर्शाता है, जिसे एस-आकार के वक्र के रूप में जाना जाता है। इसमें चार मुख्य चरण होते हैं: लैग चरण (धीमी शुरुआत), लॉग या घातीय चरण (तेज वृद्धि), घटता वृद्धि चरण (वृद्धि धीमी होना), और अंततः स्थिर चरण (जनसंख्या स्थिर होती है क्योंकि यह वहन क्षमता तक पहुँच जाती है)। वाई-अक्ष पर 'वृद्धि (जीव का आकार या वजन)' और एक्स-अक्ष पर 'समय' दर्शाया गया है। 1. Naturally all populations of any species always have limited resources to permit exponential growth. Due to this there is always competition between individuals for limited resources. The most fit organisms succeed by survival and reproduction.
2. A given habitat has enough resources to support a maximum possible number, but beyond a particular limit the further growth is impossible.
3. This limit is called nature's carrying capacity (K) for that species in that habitat.
4. A population growing in a habitat with limited resources show following phases in a sequential manner, (a) A lag phase (b) Phase of acceleration (c) Phase of deceleration (d) An asymptote, when the population density reaches the carrying capacity.
5. A plot of N in relation to time (t) results in a sigmoid curve. This type of population growth is called Verhulst-Pearl Logistic Growth.
6. Since resources for growth for most animal populations are finite and become limiting sooner or later, the logistic growth model is considered as a more realistic one.
7. Logistic growth thus always shows sigmoid curve.
In simple words: Logistic population growth, represented by an S-shaped curve, describes how a population grows in an environment with limited resources. It includes an initial slow lag phase, a rapid exponential phase, a decelerating phase, and finally stabilizes at the environment's carrying capacity (K) due to resource scarcity and competition.

🎯 Exam Tip: When describing logistic growth, ensure you mention carrying capacity (K), the S-shaped (sigmoid) curve, and the four distinct phases of growth.

Question 2.Enlist and explain the important characteristics of a population.
Answer:
Important characteristics of a population are as follows:
1. Natality:
1. Natality is the birth rate of a population. Due to increased natality the population density rises.
2. Natality is a crude birth rate or specific birth rate.
3. Crude birth rate: Number of births per 1000 population/year gives crude birth rate. Crude birth rate is helpful in calculating population size.
4. Specific birth rate: Crude birth rate is relative to a specific criterion such as age. E.g. If in a pond, there were 200 carp fish and their population rises to 800. Then, taking the current population to 1000, the birth rate becomes 800/200 = 4 offspring per carp per year. This is specific birth rate.
5. Absolute Natality : The number of births under ideal conditions when there is no competition and the resources such as food and water are abundant, then it give absolute natality.
6. Realized Natality : The number of births under different environmental pressures give realized natality. Absolute natality will be always more than realized natality.
In simple words: Natality refers to the birth rate of a population, which increases its density. It can be measured as crude birth rate (overall) or specific birth rate (age-dependent), and further categorized into absolute natality (ideal conditions) and realized natality (actual conditions).

🎯 Exam Tip: Differentiate between crude, specific, absolute, and realized natality, providing simple examples for each to illustrate understanding.

2. Mortality:
1. Mortality is the death rate of a population. It gives a measure of the number of deaths in a particular population, in proportion to the size of that population, per unit of time.
2. Mortality rate is typically expressed in deaths per 1,000 individuals per year. A mortality rate of 9.5 (out of 1,000) in a population of 1,000 would mean 9.5 deaths per year in that entire population or 0.95% out of the total.
3. Absolute Mortality : The number of deaths under ideal conditions when there is no competition, and all the resources such as food and water are abundant, then it gives absolute mortality.
4. Realized Mortality : The number of deaths under environmental pressures come into play gives realized mortality.
5. It must be remembered that absolute mortality will always be less than realized mortality.
In simple words: Mortality measures the death rate within a population, influencing its density. It distinguishes between absolute mortality (under ideal conditions) and realized mortality (considering environmental pressures), with the latter always being higher than the former.

🎯 Exam Tip: Similar to natality, understand the nuances of absolute versus realized mortality, noting that environmental challenges increase realized mortality.

3. Density:
The density of a population in a given habitat during a given period fluctuates due to changes in four basic processes, viz.
1. Natality i.e. birth rate (The number of births during a given period in the population that are added to the initial density).
2. Mortality i.e. death rate (The number of deaths in the population during a given period).
3. Immigration i.e. number of individuals of the same species that have come into the habitat from elsewhere during the time period under consideration.
4. Emigration i.e. the number of individuals of the population who left the habitat and gone elsewhere during the time period under consideration.
5. Natality and immigration increase in population density whereas mortality and emigration decrease it.
In simple words: Population density, or the number of individuals per unit area, constantly changes due to natality (births) and immigration (arrival), which increase density, and mortality (deaths) and emigration (departure), which decrease it.

🎯 Exam Tip: Remember the four factors (births, deaths, immigration, emigration) that directly influence population density and whether they increase or decrease it.

4. Sex ratio : Ratio of the number of individuals of one sex (male) to that of the other sex (female) is called sex ratio. In nature male, female ratio is always 1 : 1. This 1 : 1 ratio is called evolutionary stable strategy of ESS for each population.
In simple words: Sex ratio is the proportion of males to females in a population, which in natural settings typically approaches a 1:1 balance, known as an evolutionary stable strategy.

🎯 Exam Tip: The ideal 1:1 sex ratio is often stable in natural populations due to evolutionary pressures.

5. Age distribution and age pyramid : This parameter is important for human population. Each population is composed of individuals of different ages. The age distribution is plotted for the population, the resulting structure is called an age pyramid. For making the age pyramid, the entire population is divided into three age groups as Pre-Reproductive (age 0-14 years), Reproductive (age 15-44 years) and Post-reproductive (age 45 -85+ years).
In simple words: Age distribution describes the proportion of individuals in different age groups within a population, which when graphically represented, forms an age pyramid, categorizing individuals into pre-reproductive, reproductive, and post-reproductive stages.

🎯 Exam Tip: Understand how to interpret age pyramids to predict future population trends (growing, stable, declining) based on the relative sizes of age groups.

6. Growth : Growth of a population causes rise in its density. The size and density are dynamic parameters as they keep on changing with time, and various factors including food, predation pressure and adverse weather. From the density, one comes to know if the population is flourishing or declining.
In simple words: Population growth refers to the change in population size over time, which is influenced by factors like food availability, predation, and weather, and directly indicates whether a population is thriving or diminishing.

🎯 Exam Tip: Growth is a dynamic characteristic, constantly changing based on environmental and biological factors impacting natality and mortality.