NCERT Solutions Class 12 Biology Chapter 13
NCERT Solutions Class 12 Biology Chapter 13
The science of the study of all living things including plants, animals and microbes is referred to as Biology. Biologists generally study the structure, function, growth, origin, evolution and distribution of living species. Some scientific disciplines are intertwined with others. Biology is vital because it enables us to comprehend how living things function and interact on several levels. There was formerly an English term for biology called lifelore, but it is no longer used.
The 13th chapter of Class 12 Biology is Organisms and Populations. New ideas about organisms, populations, biomes and communities will be introduced in this chapter. When it comes to ecology, an organism’s definition and other terminology are different. Our planet has a diversified ecosystem that provides a haven for a wide range of flora and wildlife. Understanding what constitutes an ecosystem and how these words are linked will be essential.
New conceptual chapters are introduced in the Biology Class 12 syllabus. Chapter 13 is an important chapter in this syllabus. The teachers at Extramarks are aware of the questions and doubts young students may have about this chapter. Hence, they have introduced Class 12 Biology Chapter 13 NCERT Solutions. Chapter 13 Class 12 Biology NCERT Solutions helps students grasp the intricate details of the chapter.
Extramarks has a wide range of study material for students. In addition to Biology Class 12 Chapter 13 NCERT Solutions, students can use the Extramarks website to access several other study tools. For example, NCERT books, CBSE revision notes, CBSE sample papers, CBSE previous year question papers and other materials are available to students.
Key Topics Covered NCERT Solutions Class 12 Biology Chapter 13
Listed below are the key topics that are covered in NCERT Solutions Class 12 Biology Chapter 13- Organisms and Populations:
|Introduction to Ecology|
|Organism and Environment|
|Population and its Attributes|
|Population Growth and Growth Models|
Let us look at in-depth information on each subtopic in NCERT Solutions Class 12 Biology Chapter 13 given by Extramarks.
Introduction to Ecology
The branch of biology which studies the interactions between organisms and their surroundings is referred to as Ecology. The biological organisation is divided into four tiers:
NCERT Solutions Class 12 Biology Chapter 13 discusses the two levels of the organisation- Organisms and Populations in the section below.
Organism and Environment
NCERT Solutions Class 12 Biology Chapter 13 states that a continuous biological system in an environment can adapt and keep specific structures and behaviour as an organism. Fungi, bacteria, plants, animals and people are all part of it. A group of organisms forms a population. The population is organised into a community that manages the ecology. The Biotic and Abiotic components together constitute the ecosystem.
Due to genetic differences, all organisms have the potential to adapt to a variety of environmental situations. They have a better probability of surviving because of this. Polar bears have a variety of characteristics that help in their adaptation to the freezing temperature of Antarctica. They are protected from the cold and predators by their thick fur coats. The waxy covering keeps the body warm while repelling water. These characteristics aid in their adaptation to the cold.
Some major Abiotic factors
NCERT Solutions Class 12 Biology Chapter 13 gives some major abiotic factors below:
- Temperature is an essential abiotic element that influences both the environment and organisms. The temperature at the poles is always lower than that at the equator. The Arctic region’s temperature fluctuates from sub-zero to >15°C in tropical deserts throughout the summer. The temperature impacts the body’s metabolism and physiology, just as it does on enzyme kinetics.
- Water, which is essential for life, is another critical abiotic element. Deserts are areas where there is a lack of water, and only creatures with adaptations will be able to survive. The content and pH of the water are critical for aquatic organisms. Some species have a high salinity range, referred to as euryhaline.
- Another major abiotic element is light, which is necessary for photosynthesis and may be seen in autotrophs. The sun is the universal source of light. Light intensity requirements differ from organism to organism, with some requiring high light intensity and others requiring low light intensity.
- Soil is another key abiotic element that impacts organisms and their populations. The nature and characteristics of soil vary based on the kind of climate, the weathering process, soil formation, soil movement, and if the soil is sedimentary.
Responses to Abiotic factors
Abiotic factors affect different creatures in different ways. In the following section, NCERT Solutions Class 12 Biology Chapter 13 presents numerous abiotic factor responses:
- Regulators: They are creatures that can maintain homeostasis and control, resulting in consistent body temperature, osmotic concentration and other factors.
- Conformers: They are creatures that cannot maintain a constant body temperature. The process of thermoregulation, which is an energy process, occurs when the body releases or absorbs heat, resulting in a rise or reduction in body temperature.
- Partial regulators: They are those species that can regulate themselves, but only to a limited extent, depending on the environment. When the organisms reach this limit, they are conformed.
- Migration: The transfer of animals from one location to another according to their needs.
- Spores: Certain microorganisms, such as bacteria and fungus can halt their development in unfavourable environmental circumstances.
Population and its Attributes
According to NCERT Solutions Class 12 Biology Chapter 13, a population is a collection of individuals or creatures of any species living in a well–defined geographic region at a certain moment capable of interbreeding. For instance, consider the deer population in a forest
Some Attributes of Population
- Birth rate: The total number of people born in a population in a certain period of time is the birth rate of that population.
- Death rate: Death rates are the total number of people of the population who die in a certain period of time.
- Sex ratio: The total number of females and males per 1000 people is the sex ratio.
- Age pyramid: A plot representing the age distribution is called an age pyramid.
Population Growth and Growth Models
The rise in the number of people in a population is called population growth. This is dependent on several circumstances including the weather, food supply, predator pressure and so on. The density of the population fluctuates because of the following factors:
- Natality: Natality refers to the birth of individuals in a given population. The “natality rate” is calculated as the number of individuals produced per female in a given unit time.
- Morality: Mortality is the number of people who die in a certain period.
- Emigration: The number of people who relocated to a different environment in a certain period.
- Immigration: The number of people who have arrived in the habitat from other places.
NCERT Solutions Class 12 Biology Chapter 13 states that Population Growth involves two models: Exponential Growth and Logistic Growth.
Exponential Growth Model
The creation of exponential growth occurs when the nutrients available are in excess amount. If N is the population size, b is the birth rate (per capita births, not total births), and d is the death rate, then the rise or decrease in N over time period t will be (dN/dt).
dN/dt = (b-d)*(N)
Let (b-d) = r, then
dN/dt = rN
The ‘intrinsic rate of natural growth is referred to as r in this equation. The exponential growth pattern, also known as the geometric growth pattern causes the population to form a J-shaped curve. The following is the final equation that is obtained:
Nt = N0ert
Nt = population density after time t
N0 = population density at time zero
r = intrinsic rate of natural increase
e = the base of natural algorithms
When the number of nutrients and other available resources are restricted, population expansion is called Logistic Growth or Verhulst-Pearl Logistic Growth. As a result of this state, there will be increased competition among species. As a result, a lag phase will occur in the face of finite resources, followed by exponential growth, slowing, and eventually asymptote.
The logistic growth curve will be either S-shaped or sigmoid in form. Verhulst-Pearl Logistic Growth is depicted in the equation below:
dN/dt = rNk – N/K
r = intrinsic rate of natural increase
K = carrying capacity.
In the above section, NCERT Solutions Class 12 Biology Chapter 13 has discussed two models of Population Growth. All study material related to these is available on the Extramarks website. Register today to experience learning like never before.
The interplay of diverse populations is referred to as Population Interaction. There are a variety of ways for people to interact with one another. NCERT Solutions Class 12 Biology Chapter 13 explains some of them:
- Predation: Predator-prey interaction in which the predator kills the prey. This is critical for the survival of the prey and predator species. Herbivores play the role of predators in the case of plants. Various adaptations have been discovered in certain plants that assist them in protecting themselves from prey. For example, thorns as a defence or defensive agents such as caffeine, nicotine, quinine and other commercially available substances might be utilised.
- Competition: The interaction between organisms in which they compete for resources such as food, water, shelter, etc. The organisms may be of the same species or other species. When resources are few, this interaction happens.
- Parasitism: It is a relationship in which one species relies on the other, resulting in one species benefiting while the other suffers. Ectoparasites are parasitic parasites that feed on the host organism’s outer surface.
- Commensalism: It is a species interaction in which one species benefits while the other species receives neither benefit nor damage.
- Mutualism: It is the interaction in which both species benefit from one another. Lichens (a reciprocal and symbiotic connection between algae and fungus) and mycorrhiza are two examples (symbiotic relationship between fungi and the roots of the higher plants).
- Amensalism: It is an organism interaction in which one species suffers harm while the other suffers neither gain nor injury.
NCERT Solutions Class 12 Biology Chapter 13 Exercise and Solutions
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By getting access to NCERT Solutions Class 12 Biology Chapter 13, students can easily understand all the concepts of Organisms and Populations.
Key Features of NCERT Solutions Class 12 Biology Chapter 13
Students are frequently advised to read through NCERT questions when studying for board examinations. These NCERT questions are an excellent combination of all the chapter’s concepts. Extramarks NCERT Solutions have been produced so that students can comprehend every concept of any difficult chapter with ease. Here are some of the benefits of using Extramarks:
- These solutions are a time saver for all students. They are easy to grasp and go.
- Organism and Population NCERT Solutions help students clear all their doubts and understand the manner in which the answers need to be written in the final exam.
- These solutions can help students build confidence and prepare them for the actual examination eventually.
Q.1 How is diapause different from hibernation?
Ans- Diapause and hibernation are both ways to avoid the stress of the unfavourable environment. When animals want to avoid cold winter season, they undergo a state of suppressed metabolic activities or become dormant and this is called hibernation. They hide in their shelters, thereby avoiding the harsh winter climate. This phenomenon is commonly seen in bears, rodents, etc. Diapause is a stage of suspended development and reduced physiological activity under unfavourable environmental conditions. This is mostly seen in zooplankton species in lakes and ponds.
Q.2 If a marine fish is placed in a fresh water aquarium, will the fish be able to survive? Why or why not?
Ans- The salt concentration in the sea is in the range of 30-35 (measured as salinity in parts per thousand) as compared to the freshwater aquarium in which salt concentration remains less than 5. The body of a marine fish is adapted to high salt concentration. If they are placed in freshwater, they cannot survive due to osmotic pressure problems. The water will enter their body through the process of osmosis (from low to high salt concentration) and the fish will not be able to bear the sudden change in the osmotic pressure. The body will swell and eventually burst.
Q.3 Most living organisms cannot survive at temperature above 45°C. How are some microbes able to live in habitats with temperatures exceeding 100°C?
Most living organisms cannot survive at temperatures above 45°C. However, some microbes, known as thermophiles are able to live in habitats with temperatures exceeding 100°C due to the presence of enzymes which are stable to heat. These thermostable enzymes do not denature under high temperatures allowing the metabolic activities of bacteria to go on.
Q.4 List the attributes that populations but not individuals possess.
Ans- The attributes that populations but not individuals possess are mentioned below:
a. Birth Rate: It is the rate at which new individuals are added to the population due to birth and is also known as natality. The birth rate is the total number of births per 1,000 individuals of a population each year.
b. Death Rate: It is also known as mortality and is the number of deaths in a given population per unit of time. The mortality rate is typically expressed in units of deaths per 1000 individuals per year.
c. Sex Ratio: The ratio of males and females in a population. The sex ratio is an important attribute of a population and tells about the percent of females and males in the population.
d. Age Distribution: Age distribution is the proportionate number of persons in successive age categories in a given population at any given time. If the age distribution (percent individuals of a given age or age group) is plotted for a population, the resulting structure is called age pyramid. The shape of the pyramid reflects the growth status of the population and tells whether it is expanding, stable or declining.
e. Population Density: Population density is the total number of individuals of a species per unit area or volume at a given time e.g. number of animals per square kilometre or the number of trees in a given area etc. The population density in an area may increase or decrease due to many factors like birth rate (natality), the death rate (mortality), migration (immigration and emigration) etc. Population density reflects the success of a species in any given area.
Q.5 If a population growing exponentially double in size in 3 years, what is the intrinsic rate of increase (r) of the population?
Q.6 Name important defence mechanisms in plants against herbivory.
Ans- In nature, herbivores are the predators of plants. Plants cannot run away from predators unlike animals, so over the years, they have evolved several defence mechanisms to protect themselves from herbivores. These involve both morphological and chemical defence mechanisms, a few of which are listed below:
a. Morphological Defence Mechanisms
- Thorns e.g. Cactus, Acacia
- Mechanical protection on the surface of the plants e.g., hairs, trichomes, spines, and thicker leaves
b. Chemical Defence Mechanisms
- Many plants produce and store chemicals which are harmful to the herbivores e.g. weed Calotropis growing in the fields produce highly poisonous cardiac glycosides and thus, cattle do not eat it.
- Chemicals like nicotine, caffeine, quinine, opium, strychnine, that are extracted from plants for commercial reasons are actually produced by these plants as part of their defence mechanism.
- Indirect defences against insect herbivory are mediated by the release of a mixture of volatile compounds that specifically attract natural enemies of the herbivores and/or by providing food (e.g., extrafloral nectar) and housing to enhance the effectiveness of the natural enemies, thereby killing the herbivore.
Q.7 An orchid plant is growing on the branch of a mango tree. How do you describe this interaction between orchid and the mango tree?
Ans- This type of interaction, where one species gets benefitted and other is neither benefitted nor harmed, is called commensalism. Mango tree provides elevation to the orchid plant so that it can receive a sufficient amount of sunlight and air. The orchid plant receives nutrients mainly from the moist air around them and sometimes from decaying organic waste of host plant or other insects. They do not parasitize on mango tree, rather take only physical support. Thus, the orchid plant gets benefitted but it neither harms nor benefits the mango tree.
Q.8 What is the ecological principle behind the biological control method of managing with pest insects?
Ans- The ecological principle behind the biological control method of managing with pest insects is based on a very simple fact that the predators keep the prey population under check by consuming it e.g. all insect species are suppressed by naturally occurring organisms and environmental factors.
Q.9 Distinguish between the following:
- Hibernation and Aestivation
- Ectotherms and Endotherms
Ans- (a) Hibernation and Aestivation
|1. Hibernation is a dormant state during extreme winters in which hibernating animals go into a deep sleep and use stored body fat as the only source of energy.||1. Aestivation is a dormant state during extreme summer climate to avoid hot and desiccating environment.|
|2. They decrease their body temperature and metabolic activities to reduce energy consumption. By doing this they protect themselves from harsh cold weather and scarcity of food.||2. Aestivating animals usually go under the ground to avoid dehydration and high temperature. They also decrease their metabolism to reduce energy consumption.|
|Examples: chipmunks, hamster, polar bears||Example: snakes and frogs|
(b) Ectotherms and Endotherms
|1. Ectotherms are the animals that depend upon the temperature of the surrounding environment to maintain their body temperature.||1. Endotherms are the animals who can maintain constant optimum body temperature on their own.|
|2. They absorb heat from the environment to keep themselves warm or they aestivate by going under the ground to avoid high temperature during summer because that will increase their body temperature too.||2. They maintain their body temperature mainly by the heat produced inside the body as a byproduct of metabolic reactions.|
|Examples: reptiles, amphibians and fishes||Examples: Mammals and birds with a few exceptions.|
Q.10 Write a short note on
(a) Adaptations of desert plant and animals
(b) Adaptation 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.
Ans- (a) Adaptations of desert plant and animals: Adaptations of desert plants are the morphological and behavioural responses to maximise their chances of survival in desert (arid) regions. Scarcity of water is the main limiting factor that requires these plants to adapt to such an environment. Plants evincing adaptation to arid conditions are termed xerophytes.
- Succulent plants store water in fleshy leaves, stems or roots from which it is not lost easily. All succulent plants have extensive, shallow root systems that allow them to absorb water efficiently. Example: Cacti, agaves, aloes, elephant trees and many euphorbias.
- The stems and leaves of most species have waxy cuticles that make them nearly waterproof when the stomates are closed.
- Water is further saved by reduced surface areas; most succulents have few leaves, no leaves (most cacti), or leaves that are deciduous in dry seasons.
- Also, many succulents possess a water-efficient variant of photosynthesis called CAM (Crassulacean Acid Metabolism). CAM plants open their stomata for gas exchange at night. During the day, the stomata are closed. This helps the plant prevent water loss.
Desert animals face similar challenges for survival and have developed a multitude of adaptations to ensure their survival in the harsh conditions. Some of the adaptive responses to desert-dwelling are as follows:
- They are active in the early morning and late evening when the temperatures are not very high.
- Many animals are completely nocturnal such as bats, snakes, rodents foxes and skunks.
- Various animals use shade and burrows or dens during the heat of the day.
- Aestivation, dormancy during periods of heat and dryness is also witnessed.
- They keep their mouth open to exhaust body heat.
- They have long appendages and enormous ears that act like the radiator of a car.
- They are light coloured which reflects heat and acts as camouflage in desert surroundings.
- They excrete faeces on the legs, where evaporation cools the rest of the body (birds).
- Many burrows into moist soil where water is absorbed through the skin.
- Some obtaining their moisture needs from the food they eat.
- Excreting metabolic wastes in the form of uric acid to conserve water.
- Deriving water directly from plants, particularly succulents, such as cactus
- Specialised kidneys that extract water from their urine.
- Specialised organs that recapture exhaled moisture in the nasal cavities.
- Manufacturing water metabolically from the digestion of dry food items.
b. Adaptation of plants to water scarcity: Environmental stresses of low rain, low relative humidity with desiccating winds, and high summer temperatures characterise climates of deserts and coupled with low nutrient availability, produce severe limitations to the plant growth. Despite such stresses, desert contains surprisingly large amounts of plant biomass, and possess remarkable diversity of plant growth forms. The following strategies are adopted by various plants to survive the water scarcity of deserts:
- Drought-escaping plants: Plants that germinate and grow only when there is sufficient moisture available to complete their life cycle. Only their seeds persist during times of drought.
- Drought-evading plants: Non-succulent perennials which restrict their growth activity to periods when moisture is available. Typically, they are drought-deciduous shrubs which go dormant or die back during dry periods.
- Drought-enduring plants: Evergreen shrubs. The extensive root system is coupled with various morphological and physiological adaptations of their aerial parts enable these hardy xerophytes to maintain growth even in times of extreme water stress.
- Drought-resisting plants: succulent perennials. The water stored in their swollen leaves and stems is usually used very sparingly.
c. Behavioural adaptations in animals: Adaptations help organisms survive in their ecological niche or habitat and it can be anatomical, behavioural or physiological. Behavioural adaptations can be inherited or learnt, including tool use, language and swarming behaviour and allow animals to respond to life needs. Behavioural adaptations are the things organisms do to survive usually in response to some type of external stimulus. They can be instinctive or learnt. Instinctive behaviour is natural and need not be taught to animals. Some examples are what the animals eat, how it moves around and how it protects itself. Migration, hibernation, dormancy, ways of finding food, raising young ones and camouflage are ways of instinctive behavioural adaptations shown by various animals across a range of ecosystem. Learned behaviour is learnt by interaction with the environment and cannot be passed on to the next generation except by teaching the young ones. In animals, behavioural adaptation depends on the level of intelligence shown by the animal, the wise animal will quickly learn to make behavioural changes, in order to survive and this change can happen quickly over time. Moving in large groups is an example of behavioural adaptation; it helps protect the members of the group from predators.
d. Importance of light to plants: Light plays an important role in the development of a plant. Plants produce food by the process of photosynthesis and light is essential for this process as it is the source of energy. Without light, plants cannot carry out photosynthesis and thus, no food will be produced. The phototropism (hormones that control plant growth begin to accumulate in the plant’s stem, forcing the plant towards the light) shown by plants is also dependent upon the availability of light. Thus photosynthesis and phototropism depend on the availability of light sources for plants. The flowering in many plants is dependent upon the availability of sunlight and season. The vertical distribution of aquatic plants in any water/marine ecosystem is also dependent upon the availability of sunlight.
e. Effect of temperature or water scarcity and the adaptations of animals: The abiotic factors like temperature and water play a very important role in the distribution and adaptation of animals. As we know, the average temperature on land varies seasonally, across the globe. It decreases from the equator towards the poles and from plains to the mountain tops. It ranges from subzero levels in polar regions and high altitudes to >50°C in tropical deserts in summer. Other unique ecosystems are thermal springs and deep-sea hydrothermal vents where the average temperature exceeds 100°C. Many animals can tolerate a wide range of temperature however most animals are restricted to a narrow temperature range. Snow leopards and polar bears are adapted to polar regions – they have a thick layer of fat below their skin and have thick fur. Plants and animals found in desert areas have adapted themselves well so that they can face high temperatures and shortage of water.
Water is another very important abiotic factor that affects the living beings and depending upon its availability and quality, life has adapted itself. Where there is water scarcity, living beings have adapted themselves by making several modifications in their morphology and physiology. The succulent leaves, thorns, long roots in plants are few modifications that have occurred over the years in desert plants. Organisms living in lakes, oceans and rivers also face water problems – it is quality of water like chemical composition, pH, which makes the impact. The salt concentration in water also affects the life forms – some can tolerate a wide range of salinities (euryhaline) but others are restricted to a narrow range (stenohaline).
Q.11 List the various abiotic environmental factors.
Ans- The major abiotic environmental (non-living components) factors are temperature, water, light and soil.
Q.12 Give an example for:
(a) An endothermic animal
(b) An ectothermic animal
(c) An organism of the benthic zone
Ans- (a) An endothermic animal: Cow
(b) An ectothermic animal: Crocodile
(c) An organism of the benthic zone: Sea star
Q.13 Define population and community.
Ans- A population is defined as a group of all the organisms of the same species, who live in the same defined geographical area, have the capability of interbreeding and share common resources.
A community is defined as a group of various populations that share a common environment.
Q.14 Define the following terms and give one example for each:
(e) Interspecific competition
Ans- (a) Commensalism: The interaction between two species in a given area where one species is benefitted and the other is neither benefitted nor harmed is called commensalism. For example, the cattle and egret relationship is a very classical example of commensalism. The egrets always forage close to where the cattle are grazing because the cattle, as they move, stir up and flush out the vegetation insects that otherwise might be difficult for the egrets to find and catch.
(b) Parasitism: The relationship or interaction between two species in a given area which is beneficial for one species (parasite) but detrimental to the other species (host) is called parasitism. For example, a tick feeding on a dog, a mosquito feeding on a human or a leech feeding on a human.
(c) Camouflage: The act, means, or result of obscuring because of colouration or patterns that help an animal to appear to blend with its surroundings is called camouflage. Camouflage is common among invertebrates. It is also common in a variety of other animals. It is used by prey to help hide from predators and it is used by predators to help them conceal themselves when they are out to prey. For example, leaf mimic Katydids, Sea dragon, Australian Leaf Insect
(d) Mutualism: The relationship between two species that confer benefits on both the interacting species is called mutualism. For example, the association between the sea anemone and hermit crab is another example of mutualism. The sea anemones give protection to the crab using its stinging cells, and it remoulds its shell to fit the crab while the hermit crab allows the sea anemones to consume the remains of its food, thus it also provides the sea anemones with food supply.
(e) Interspecific competition: Interspecific competition is a form of competition in the ecosystem where individuals of different species living in the same area compete for the food or living space. For example, competition among carnivores for prey is an example of interspecific competition. It does not have an equal impact on each competitor: the larger competitor usually wins. Lions are much larger than hyenas, so in this case, hyenas are subordinate.
Another example is the competition between plants which is observed in forests where large trees dominate the canopy and thus allow little light to reach smaller competitors living below.
Q.15 With the help of a suitable diagram describe the logistic population growth curve.
Ans- A population growing in a habitat with limited resources initially shows a lag phase, followed by a phase of exponential growth. The exponential growth of all populations is eventually curtailed by food availability, competition for other resources, predation, disease, or some other ecological factor. Once growth is limited by resources such as food, the exponential growth of the population begins to slow as competition for these resources increases. The fittest individual survives and reproduces. The growth of the population eventually slows to zero as the population reaches the carrying capacity (K) for the environment which is the stage where the resources available in that habitat are just sufficient to sustain that population level in that geographical area. The result is an S-shaped curve of population growth known as the logistic curve. A plot of N (population density at time t) in relation to time (t) results in a sigmoid curve graph. This type of population growth is called Verhulst-Pearl Logistic Growth.
Q.16 Select the statement which best explains parasitism.
(a) One organism is benefitted.
(b) Both organisms are benefitted.
(c) One organism is benefitted, other is not affected.
(d) One organism is benefitted, other is affected.
Ans- (d) Parasitism is best described as an interaction where one organism is benefitted while the other is affected. It is the interaction between two species in a given area which is beneficial for one species (parasite) but is detrimental to the other species (host).
Q.17 List any three important characteristics of a population and explain.
Ans- Three important characteristics of a population are population size, density, and growth rate.
1. Population size: In nature, single individuals of any species are not found in isolation. They live in well-defined geographical areas and share and compete for similar resources. At a particular time, they function as a unit that can be termed as population. The size of the population is governed by birth rates and death rates. The rates are expressed as change in numbers with respect to members of the population. The population also is characterized by sex ratio which means the number of males versus the number of females in a given population. The age distribution (percent individuals of a given age or age group) is another characteristic that is shown by a population. This governs the growth status of the population.
2. Population Density: Population density is the total number of individuals of a species per unit area or volume at any given time. For example the number of animals per square kilometre or the number of trees per hectare. The population density in an area may increase or decrease due to many factors like birth rate (natality), the death rate (mortality), migration (immigration and emigration) etc. Population density reflects the success of a species in any given area. Population density increases if the number of births and the number of immigrants is more than the number of deaths plus the number of emigrants, otherwise, it decreases. Under normal condition, birth and death are two factors which influence population density the most.
3. Growth Rate: The rate at which a population increases in a given time is called the growth rate. The availability of food and space is the most important criteria that affect the growth rate. When resources in a habitat are unlimited, each species can grow to its maximum capacity. This kind of population growth shows exponential growth. Nature cannot sustain exponential growth due to limitation in resources. This leads to competition among the members of the populations. The fittest survives and reproduces. A population growing in a habitat with limited resources initially show a lag phase, followed by a phase of exponential growth. The exponential growth of all populations is eventually curtailed by food availability, competition for other resources, predation, disease, or some other ecological factor. Once growth is limited by resources such as food, the exponential growth of the population begins to slow as competition for those resources increases. The growth of the population eventually slows nearly to zero as the population reaches the carrying capacity (K) for the environment which is the stage where the resources available in that habitat are just sufficient to sustain that population level in that geographical area. The result is an S-shaped curve of population growth known as the logistic curve. A plot of N (population density at time t) in relation to time (t) results in a sigmoid curve graph.
FAQs (Frequently Asked Questions)
1. What topics are discussed in Biology Chapter 13 for Class 12?
The ecology, population density and calculation are covered in Chapter 13: Organisms And Populations. An introduction to organisms and their environments, primary abiotic variables, responses to abiotic factors, adaptations, population-related features, growth, growth models and population interactions are prominent themes addressed by the chapter in the recommended textbook. These subjects are covered in the same order as the NCERT Biology book.
2. Are there a lot of equations to understand in Biology Class 12 Chapter 13?
A common misunderstanding is that Chapter 13 of Biology in Class 12 has many formulas. Mostly, these calculations are straightforward and may be used to determine different population elements. Refer to NCERT Solutions Class 12 Biology Chapter 13 to easily comprehend the formulas.