Organisms and Populations NEET Notes

Complete Organisms and Populations Notes for Class 12 Biology | NEET Revision and NCERT Solutions

Ecology deals with interactions among different organisms and their environment.
Organisms get adapted to their environment for their survival and reproduction.
The rotation of the earth about its axis brings about changes in the environment, leading to different seasons. This leads to the formation of various biomes such as desert, grassland, etc.
Life not only exists in favourable habitats, but also in harsh and extreme conditions.
The environment of an organism can be divided into:
1. Abiotic factors
2. Biotic factors

Abiotic Factors

Some of the major abiotic factors that interact with the organisms are:

Temperature − It is the most relevant abiotic factor since all organisms require an optimum temperature for their metabolism and other body functions. Depending upon their ability to tolerate temperature range, organisms are of two types- stenothermal (restricted to a narrow range of temperature) and eurythermal (can tolerate a wide range of temperature).

Water − Water also is a major influencing factor. Life on earth is impossible without water as it forms the major constituent of all living cells. In oceans where quantity of water is not a limitation, the quality of water becomes one. Depending upon the ability to tolerate salinity, organisms can be stenohaline (restricted to narrow range of salinity) and euryhaline (tolerant to wider range of salinity).

Soil − The nature and composition of soil differs from one place to another depending upon the climate, weathering process, and soil development method. The characteristic features such as soil composition, grain size, percolation, water holding capacity, etc. determine the native of the organisms it can support.

Light − The major source of light on earth is the Sun. Light is essential for plants to perform photosynthesis. Certain plants become adapted to perform photosynthesis under very low light since they are constantly overshadowed by tall trees. Many plants also depend on light for their flowering (photoperiodism). The availability of light on land is comparatively higher than that in water.

Responses to Abiotic Factors

All organisms in order to sustain maximum functionality maintain a constant internal environment (homeostasis). An organism may adopt one of the following strategies for homeostasis:

Regulate − Certain animals have the ability to maintain a constant temperature and a constant osmolarity to keep up their homeostasis. Mammals have a constant body temperature (37°C) irrespective of the outside temperature. In summers, to maintain the temperature, we sweat and in winters we shiver, which produces heat.

Conform − Animals and plants except mammals do not have a constant body temperature and their body temperature changes in accordance with the outside temperature. Such organisms are called conformers. Conformers have not evolved. They have become regulators since regulation is energetically more expensive.

Migrate − The organism can move temporarily from stressful habitats to more hospitable areas and return once the period changes. Birds can migrate from cold regions to relatively warmer regions during winter and vice-versa during summers.

Suspend − Some organisms cease to be metabolically active during stressful period. They suspend all activity and enter a period of dormancy. For example − Spores in bacteria and lower plants; and hibernation (winter sleep) and aestivation (summer sleep) in animals Similarly, zooplankton enter diapause, a stage of suspended development during unfavourable conditions.

Adaptations

Adaptations are certain characteristics that organisms develop in order to survive and reproduce better in their habitat.

These adaptations can be physiological, behavioural, or morphological.

Some of the adaptations are:

Desert plants have thick cuticle on their leaf surface and stomata arranged in deep pits to reduce water loss. Their special photosynthetic pathway CAM enables their stomata to remain closed during day time. Their leaves are reduced to spines and photosynthesis is carried out by flattened stems.
Animals of colder regions have shorter limbs and ears to minimise heat loss (Allen’s rule) and the body is covered by thick fur to reduce the heat loss. Their body has a thick layer of fat (blubber) below their skin that acts as an insulator to minimise heat loss.
People living in high altitudes have high RBC production and increased breathing rates.
Some desert animals are capable of burrowing in order to escape the heat. In addition, some desert animals such as kangaroo rat are able to meet their water requirement through internal fat oxidation. They also have ability to concentrate their urine.

Population

It is a group of similar individuals living in a geographical area, sharing similar resources, and capable of interbreeding.

Population has certain attributes, which individual organisms do not possess:

Birth rate per capita births

Death rate per capita deaths

Sex ratio − Ratio of number of males to females in a population

Age distribution

A population can be composed of individuals of different age groups.
Age distribution plot for a given population is given by the age pyramid.
The structure of the age pyramid determines the growth status of the population, whether it is growing, stable, or declining.

Population size, more technically, is referred to as population density (N), which indicates the number of individuals inhabiting a particular niche.

If the population is huge, then relative density is measured instead of absolute density whose measurement is time-consuming.

Population Growth

The size of a population is an ever-changing aspect since it depends upon availability of food, predation, weather conditions, etc.

This gives us an idea whether a certain population is growing or declining.

Some of the reasons for the increase or decrease in population:

Natality (B) − Number of births during a given period in the given population

Mortality (D) − Number of deaths during a given period in the given population

Immigration (I) − Number of individuals of the same species who have come into the habitat from elsewhere during a given period

Emigration (E) − Number of individuals of the same species who have left the habitat and gone elsewhere during a given period

If N is the population at time t, then its density at t + 1 is

Nt + 1 = Nt + [(B + I) − (D + E)]

Growth Models

Exponential Growth − When the resources are unlimited, population tends to grow in an exponential pattern.

If the population size is N and the birth and death rates (not per capita) are b and d respectively, then increase or decrease in N at t (time period) is given by,

dN /dt = (b − d) × N

If (b − d) = r, then

dN/ dt = rN

r is the “intrinsic rate of natural increase”.

Or,

Nt = N0 ert

Where,

Nt − Population density at time t

N0 − Population density at time 0

r − Intrinsic rate of natural increase

e − Base of natural logarithms (2.71828)

Logistic growth − When the resources are limited leading to competition between individuals and survival of the fittest, the population tends to grow in a logistic manner.

In this kind of growth, there is an initial lag phase followed by acceleration or deceleration phases and finally asymptote, when it reaches its carrying capacity (K).

When N in relation to t is plotted, it results in a sigmoid curve called the Verhulst − Pearl Logistic growth given by,

N − Population density at time t

r − Intrinsic rate of natural increase

K − Carrying capacity

Life History Variations

Populations tend to increase their reproductive fitness in order to survive better. This is known as Darwinian fitness (high r value).

Some of the trends they follow in course of achieving this:

Some organisms breed only once in their lifetime. Example - Salmon, Bamboo
Some breed many times. Example - Birds, mammals
Some produce a large number of small-sized offsprings. Example - Oyster
Some produce small number of large-sized offsprings. Example - Birds, Mammals

Population Interactions

A natural habitat consists of many organisms living together and these organisms communicate and interact with each other. For example, plants depend on insects for pollination.
Interspecific interactions are interactions between two different species of organisms. They can be either beneficial or harmful to one or both partners.

Interspecific interactions

Predation − It is beneficial to the predator while the prey is harmed.

It acts as a means of transfer of energy to the next higher trophic level and of maintaining balance in the ecosystem.

For plants, herbivores are predators and some plants produce secondary metabolites, thorns, or poisonous chemicals to ward off predators.

Similarly, animals also camouflage themselves to protect themselves from predators. Some preys are poisonous or distasteful (Monarch butterfly is highly distasteful because of a special chemical it acquires during its caterpillar stage by feeding on poisonous weeds) so as to avoid predators.

Competition − It occurs only in closely related species wherein they share the same type of habitat and food resources.

However, for competition to take place resources need not be always scarce and competition does not necessarily take place between same species.

In competition, the fitness of one species is significantly lower in presence of another species and survival of fittest ultimately takes place.

Gause’s Competitive Exclusion Principle states that two closely related species competing for the same resource cannot co-exist indefinitely and the competitively inferior will be eliminated eventually.

Moreover, some species may develop mechanisms to facilitate their co-existence.

Parasitism − In this interaction, one of the partners is benefited because it resides outside or inside the body of the host and gets free accommodation and food while the host is affected due to loss of nutrients.

Parasites in nature have developed a wide variety of adaptations such as hooks and suckers for attachment, loss of digestive system, high reproducing capacity, etc.

Parasites can live either outside (ectoparasites) or inside (endoparasites) the body of the host organisms.

Brood parasitism is seen in birds in which the parasitic bird lays its egg in the nest of the unassuming host bird, which takes care of them until they hatch. For example, Koel lays its eggs in the nest of the crow.

Commensalism − In this interaction, one of the partners is benefited while the other is neither benefited nor harmed.

For example, an orchid growing as an epiphyte on the mango tree

The orchid gets support while the mango tree is unaffected.

Mutualism or symbiosis − In this interaction, both the partners are benefited.

For example, lichens, interaction of algae and fungi, where both are benefited

The fungi give support to the algae while the algae prepare the food for the fungi.

NCERT Solutions

Ecology is a subject that studies the interactions between organisms and their physical (abiotic) environment, as we have learned in previous classes. Ecology studies biological organisation at four levels: organisms, populations, communities, and biomes. This chapter delves into Ecology at the organismic and population levels. It also covers subtopics like Organism and Environment and Populations. Population interactions - mutualism, competition, predation, parasitism; population attributes - growth, birth and death rates, age distribution are among the topics covered in Organisms and Populations. (Topics omitted: Organism and Environment, Major Abiotic Factors, Abiotic Factor Responses, Adaptations)

1. How is diapause different from hibernation?

Solution:

Diapause is a phase of suspended development to deal with undesirable conditions. Several species of Zooplankton and insects display diapause to pass through extreme climatic conditions while in their development stage. On the other hand, winter sleep or Hibernation is a resting phase wherein animals escape winters by hiding themselves in their shelters. They do so by entering a state of inactivity achieved by decreasing their metabolism. This process of hibernation is observed in squirrels, bats and some rodents.

2. If a marine fish is placed in a fresh water aquarium, will the fish be able to survive? Why or why not?

Solution:

The chances of survival of marine fish will reduce if placed in a fresh water aquarium as their bodies are altered to higher salt concentrations as provided by marine environments. In a fresh water environment, fishes fail to regulate the water which enters the body through the process of osmosis. Due to the presence of a hypotonic environment outside the fish’s body, water enters their body which causes their body to swell leading to the death of the marine fish.

3. Most living organisms cannot survive at temperature above 45 degree C. How are some microbes able to live in habitats with temperatures exceeding 100 degree C?

Solution:

Thermophiles or Archaebacteria are ancient forms of bacteria that are present in deep sea hydrothermal vents and hot water springs. They are able to withstand the high temperature (exceeding 100 degree C) as their bodies have adapted to these extreme environmental conditions. Such entities comprise of specialized thermo-resistant enzymes that perform metabolic functions which do not get destructed at these extreme temperatures.

4. List the attributes that populations possess but not individuals.

Solution:

A group of entities belonging to the same species, residing in a specific geographical area at a particular time, together functioning as a unit can be termed as a population.

Listed below are the attributes that a population exhibits:

• Natality or Birth rate

It can be given by the ratio of live births in a particular area to the population of that area. Birth rate can be expressed as the number of individuals added to the population in terms of members of the population

• Mortality or Death rate

It is the ratio of deaths in a region to the population of a region. Death rate can be expressed as the loss of individuals in terms of members of the population

• Age distribution

It can be given by the percentage of individuals of various ages in a given population. A population consists of individuals at any given time, and are present in different age groups. Typically, an age pyramid can be used to depict the age distribution pattern.

• Sex ratio

It is the count of females or males per thousand individuals

• Population density

It is given by the number of individuals of a population per unit are at a particular time.

5. If a population growing exponentially double in size in 3 years, what is the intrinsic rate of increase (r) of the population?

Solution:

If adequate quantity of food resources are available to individuals in a population, it grows exponentially. The integral form of exponential growth equation can be used to estimate the exponential growth, which is as follows:

Nt = No ert ————– equation (1)

Where, Nt is the population density after ‘t’ time

No is the population density at time zero

e is the base of natural logarithm = 2.71828

r is the intrinsic rate of natural increase

Let the current population density be ‘x’

∴ The population density after two years will be 2x and t given is 3 years

Substituting these values in equation (1)

Therefore, the intrinsic rate of natural increase of the population is 0.2311

6. Name important defence mechanisms in plants against herbivory.

Solution:

A state of feeding on plants is known as herbivory. Many plants have evolved mechanisms both chemical and morphological, to safeguard themselves against the act of herbivory. Listed below are defence mechanisms of few plants:

Chemical defence mechanisms:

• Caffeine, nicotine, opium, and quinine are some chemical substances that are produced in plants in response as part of their defence mechanism

• All of the parts of Calotropis weeds consists of lethal cardiac glycosides that demonstrate to be fatal if consumed by herbivores.

Morphological defence mechanisms:

• Opuntia or cactus leaves are altered into thorns or sharp spines to prevent herbivores from feeding on it

• Margins of leaves in some plants are spiny, having sharp edges, preventing herbivores to feed on them

• Sharp thorns with leaves are found in Acacia to prevent herbivores from feeding on them

7. An orchid plant is growing on the branch of mango tree. How do you describe this interaction between the orchid and the mango tree?

Solution:

An epiphyte or air plants are entities growing on other plants. An orchid growing on the branch of a mango tree is an epiphyte. Such plants derive their nutrition and moisture from air, water and rain or from the debris around it and not from the plant on which it is growing. Hence, the relationship between a mango tree and an orchid is an example of commensalism wherein one species receives its benefits and the other stays unaffected. In the given interaction, the orchid is benefitted as it gets physical support from the mango tree but the mango tree is unaffected.

8. What is the ecological principle behind the biological control method of managing with pest insects?

Solution:

Predation is the ecological principle behind the biological control method of managing with pest insects. Predation is referred to as the biological interaction between a predator and a prey wherein the predator feeds on the prey, thereby regulating the population of pest insects. Example – The Gawbusia fish checks the mosquito larvae in water bodies.

9. Distinguish between the following:

(a) Hibernation and Aestivation

(b) Ectotherms and Endotherms

Solution:

The differences are as follows:

(a) Hibernation and Aestivation

Hibernation Aestivation

Also known as winter sleep, it is a state of reduced activity observed in some entities to escape the extreme cold climatic conditions Also known as summer sleep, it is a state of reduced activity noticed in some entities to escape the dehydration as a result of heat in summers

Examples – Squirrels and bears found in cold regions hibernate in winters Example – Snails and fishes aestivate in summers

(b) Ectotherms and Endotherms

Ectotherms Endotherms

The body temperature varies with their surroundings Body temperature remains constant

They are cold blooded-entities They are warm-blooded entities

Examples – reptiles, fishes, amphibians Example – mammals and birds

10. Write a short note on

(a) Adaptations of desert plants and animals

(b) Adaptations of plants to water scarcity

(d) Importance of light to plants

(e) Effect of temperature or water scarcity and the adaptations of animals.

Solution:

(a) Adaptations of desert plants and animals

Desert plants:

Some of the severe conditions typically found in a deserts are scorching heat and water scarcity. Plants that are found in deserts have suitably adapted to this as they have an extensive root system to tap the underground water. These plants possess sunken stomata on leaf surfaces and thick cuticles to decrease transpiration. A plant known as Opuntia has leaves that are completely alerted into spines where photosynthesis is performed by the green stems. These plants of deserts have specialized pathways known as CAM or the C4 pathway to manufacture food which facilitates the stomata to stay closed during the day to decrease the water loss caused through the transpiration process.

Desert animals:

Lizards, kangaroo rats, snakes are some animals found in deserts which have adapted appropriately to their habitat. For instance, the Kangaroo rat inhabiting the Arizona deserts never drinks water in its life and has the potential to concentrate its urine to conserve water. Snakes and Desert lizards have adapted in a way that they lounge in the sun early in the morning and burrow themselves in sand in the afternoons to escape the heat of the day, to prevent water loss.

(b) Adaptations of plants to water scarcity

Some cold blooded animals, ectotherms such as amphibians, fish, reptiles have their temperatures varying with their surroundings as seen in Desert lizard, where they lounge in the sun in the early hours of the day and burrow themselves in sand when the temperature begins to rise in the noon. Some warm-blooded animals or endotherms such as mammals or birds escape the hot and cold weather conditions by hibernating in winters and aestivating in summers. They take shelter in burrows, caves etc to safeguard against these temperature variations.

(d) Importance of light to plants

The ultimate source of energy for plants is sunlight. Plants are referred to as autotrophic entities. They require light to perform photosynthesis. Light has a critical role in producing photoperiodic response that takes place in plants. These plants respond to the changes in the light intensity in different seasons to be able to meet their photoperiodic requirements for the process of flowering. Also, light has a significant role in aquatic habitats for vertical distribution of plants in the sea.

(e) Effect of temperature or water scarcity and the adaptations of animals

One of the most important ecological factors is temperature which varies from place to place. The variation in temperature affects the distribution of animals on the planet. Eurythermals are the animals that can tolerate a range of temperatures while stenothermal animals are the ones which can withstand only a narrow range of temperature. In order to suit their natural habitats, animals also undergo adaptations. For instance, animals in cooler regions have short limbs and ears, helping to prevent heat loss from the body. Animals that are found in colder regions have thick fat layers underneath their skin, thick fur coats to prevent heat loss.

Some entities that are affected by the fluctuations in temperature experience adaptations namely aestivation, hibernation, migration to escape environmental pressure to adapt to their natural habitat. These adaptations in the behavior of an entity is termed as behavioral adaptations. Example, the Desert lizards are ectotherms, this means to say they do not possess a temperature regulatory mechanism to escape the variations in temperature. These lounge in the sun in the early hours of the day and burrow themselves in sand when the temperature begins to rise in the noon.

Another factor that expels animals to experience several adaptations to familiarize to their natural habitats is water scarcity. Lizards, kangaroo rats, snakes are some animals found in deserts which have adapted appropriately to their habitat. For instance, the Kangaroo rat inhabiting the Arizona deserts never drinks water in its life and has the potential to concentrate its urine to conserve water. Snakes and Desert lizards have adapted in a way that they lounge in the sun early in the morning and burrow themselves in sand in the afternoons to escape the heat of the day, to prevent water loss.

11. List the various abiotic environmental factors.

Solution:

Abiotic environmental factors are formed by all the non-living components of an ecosystem which includes the following:

• Temperature –

It is one of the most significant factors which varies seasonally on land, progressively decreases from the equator towards the poles, from plains to the mountain tops. From sub-zero levels in polar areas to temperatures greater than 50 degree Celsius in tropical deserts in summers, their range is wide. Some unique habitats such as deep sea hydrothermal vents, thermal springs sees average temperature exceeding more than 100 degree Celsius. Entities who can withstand a range of temperatures are termed as eurythermals example – birds and mammals whereas stenothermals are entities who can bear a narrow range of temperature, example – polar bears.

• Water –

Life cannot sustain without water. For marine entities, pH, water temperature, chemical composition etc is significant. It is also affected by water salinity that is less than 5 parts per thousand in inland water, 30-35 parts per thousand in sea etc

Euryhaline are entities which can bear a wide salinity range whereas stenohaline are entities that can tolerate a narrow range of salinity. Most of the fresh water animals cannot sustain in sea water for long due to osmotic issues that arise due to high salinity and vice-versa

• Soil –

Properties and nature of soil varies from place to place, depending on the following factors

-climate

-soil development process

-Weathering process

Characteristics such as grain size, aggregation, soil composition determine the water holding capacity and percolation of soil. Along with these, some other features such as pH, topography, mineral composition etc determine the type of type of plant that can grow in a specific habitat.

• Light –

All autotrophs depend on light to carry out photosynthesis, releasing oxygen during the process. In forests, small herbs and shrubs are adapted to photosynthesis under very low light intensities as they are overshadowed by tall trees. Also, most plants depend on sunlight to meet their photoperiodic necessities for flowering. Several animals are dependent on diurnal and seasonal differences in light intensity as prompts for timing their reproductive, foraging and migratory activities. Light availability on land is closely associated with that of temperature, as Sun is the ultimate source. In deep oceans, it is dark perpetually. Spectral quality of solar radiation is necessary for life. For many entities, UV component of light is harmful. Different components of visible spectrum is available for marine plants active at different oceanic depths. Hence, different algae types such as brown, green and red algae occur at various depths in the middle, upper and lowest levels of water respectively.

12. Give an example for:

(a) An endothermic animal

(b) An ectothermic animal

Solution:

(a) An endothermic animal – Crows, sparrows, cranes, cows, rabbits, rats etc

(b) An ectothermic animal – Fish such as amphibians, sharks, frogs, snakes, tortoise, lizards

13. Define population and community.

Solution:

Population – it can be defined as a group of individuals of the same species inhabiting a particular geographical area at a given time, functioning as a unit.

Community – it can be defined as a group of individuals of various species, living in a certain geographical region. These individuals can be dissimilar or similar, but cannot reproduce with members of other species.

14. Define the following terms and give one example for each:

(a) Commensalism

(b) Parasitism

(d) Mutualism

(e) Interspecific competition

Solution:

(a) Commensalism

The interaction between two species wherein one species is benefited and the other remains unaffected in known as commensalism. Examples – Barnacles attached to a whale’s body and an orchid growing on the branches of a mango tree

(b) Parasitism

The interaction between two species wherein one species is positively affected (typically the smaller one) and the other is negatively affected (typically the larger one) is known as Parasitism. Example – Liver fluke is a parasite living within the body of the host and deriving nutrition from it. Here, the parasite benefits from the host, as it derives nutrition from it while the host is affected negatively as the parasite reduces the host fitness causing its body to get weak.

It is a tactic taken up by the prey to escape from predators. Intrinsically, organisms are colored to easily blend with their surroundings and escape from their predators. Several insects and frog species camouflage to escape their predators.

(d) Mutualism

An interaction between two species wherein both species involved are benefited is Mutualism. Example – lichens exhibit a mutual symbiotic relationship between blue green algae and fungi. Here, both are equally benefited from each other.

(e) Interspecific competition

An interaction between individuals of different species wherein both are negatively affected is interspecific competition. Example – The competition between resident fishes and flamingoes in South American lakes for shared food resources, that is, zooplankton

15. With the help of suitable diagram describe the logistic population growth curve.

Solution:

The logistic population growth curve is usually observed in yeast cells cultivated in laboratory conditions and includes five phases, namely:

• The lag phase – the population of yeast cell is small initially as resources are limited in the habitat

• Exponential phase – in this stage, the population of the yeast cell suddenly rises as a result of rapid growth leading to an exponential population growth due to availability of enough food resources, consistent favorable environmental conditions without any interspecific competition. This results in the curve rising upwards steeply.

• Positive acceleration phase – at the start of this phase, the cell growth is limited. The yeast cell adjusts to the new environment and grows its population

• Negative acceleration phase – the environmental resistance increases and the growth rate of the population declines as a result of an increased competition between the yeast cells for shelter and food.

• Stationary phase – The population is stable in this phase. The count of cells generated in a population is equivalent to the number of cells that die out. Additionally, the species’ population is said to have touched nature’s carrying-capacity in its habitat. S-shaped growth curve is also referred to as the Verhulst- pearl logistic curve.

16. Select the statement which explains best parasitism.

(a) One organism is benefited.

(b) Both the organisms are benefited.

(d) One organism is benefited, other is affected.

Solution:

(d) One organism is benefited, other is affected

Parasitism is a type of interaction between two species wherein one species (parasite) gets benefited while the other species or the host is negatively affected/harmed. Example – Lice or ticks(parasites) found on human body(host) is an example wherein lice derives its nutrition by feeding on the humans’ blood causing the fitness of the individual(host) to reduce, harming the human body.

17. List any three important characteristics of a population and explain.

Solution:

A group of entities belonging to the same species, residing in a specific geographical area at a particular time, together functioning as a unit can be termed as a population. Example – All humans living in a particular area at a specific time comprise the population of humans.

Listed below are the attributes that a population exhibits:

• Natality or Birth rate

• Mortality or Death rate

It is the ratio of deaths in a region to the population of a region. Death rate can be expressed as the loss of individuals in terms of members of the population