Organisms and Populations: Class 12 Notes Biology

Organism and Population is a group that represents the entire biosphere. The branch of biology involving the study of the interactions that take place between an organism and its environment is known as Ecology. 

• Organism and Population is one of the four levels of any biological organization.
• Biomes and Communities are the other two levels of ecology. 
• Habitat is defined as the place where an organism lives.
• All organisms have the ability to live according to the situation.
• Adaptation and reproduction are essential for the survival of organisms.
• The ecology is dependent upon the biotic and abiotic factors.
• A group of two or more organisms forms a population.
• A population is responsible for forming a community that controls the entire ecosystem.

Read More: Importance of Ecosystem

Key Terms: Organism and Population, Organism, Population, Ecology, Biotic Factors, Abiotic Factors, Habitat, Photosynthesis, Environment, Biomes, Community

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Organisms

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Organism is defined as an attached living system that lives in an environment. These living things are able to retain specific behaviours and structures. Some examples of organisms are bacteria, fungi, plants, animals, and humans. 

• A collection of these organisms leads to the formation of a population. 
• The collection of population forms a community that aids in the operation of ecosystems. 
• Each organism has the ability to adapt themselves to various conditions of the environment. 
• This ability of organisms is due to their genetic variations. 
• It is due to this only that their chances of survival increase.

Read More: Food chains

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Organisms and Environment

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Organisms grow and develop in their respective environments based on several different factors. There are certain variations that affect the environment and the organisms living in it. 

• Geological factors like the rotation of the earth around the sun are one of them.
• The tilt of the axis is responsible for the temperature variations (intensity and duration). 
• These are the factors that create distinct seasons on the planet. 
• Variations of the above factors and the annual variation in precipitation affect the formation of the major biomes(deserts, rainforests and tundras).
• It is important to note that certain biotic and abiotic factors affect the habitat formation in biomes. 
• These factors interact with each other at different levels, causing variations. 
• The biotic components include pathogens, parasites, predators and competitors within the habitat.
• They interact with the physio-chemical components of the habitat. 
• These physio-chemical components are known as abiotic factors and play a crucial role in habitat formation.

Biome distribution with respect to annual temperature and precipitation

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Abiotic Factors

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Certain abiotic factors lead to variations in the physical and chemical conditions of habitats. The four major abiotic factors affecting habitat formation are:  

Temperature

In ecological terms, temperature is the most important and relevant factor. It is the primary reason behind what makes something habitable. Temperatures are higher at the equator and decrease as you move towards the poles. 

• Polar regions can drop to sub-zero levels, while desert areas can go above 500 C. 
• There are certain unique habitats like thermal springs and deep-sea hydrothermal vents where temperatures can exceed 10000C. 
• The thermal tolerance of species helps decide their geographical distribution. 
• Most organisms have limited tolerance to extreme temperatures and can only function in a certain range. 
• These organisms are called stenothermal. 
• Few selective organisms can thrive in extreme temporal ranges called eurythermal. 

Water

Water influences the life of organisms in all stages as well as the habitat. Some of the world's greatest civilisations have thrived because they were established along water bodies. 

• Desert habitats, which have limited availability of water, have organisms specially adapted to live there. 
• The flora on the planet also exhibits their productivity and distribution, which are heavily dependent on water. 
• Several aquatic biomes are flourishing in the various water bodies across the planet.

Light

Light here refers to the natural sunlight on the earth's surface. This light is essential for Plants as the process of photosynthesis is only possible in the presence of sunlight. 

• Several plants need sunlight to meet their photoperiodic requirements for flowering. 
• Animals are also dependent upon the sunlight for their nourishment based upon the local flora.
• Animals establish their foragery, reproductive and migratory cycles based on natural sunlight changes and variations. 
• Light also impacts factors like variations in temperature, allowing access to the colour spectrum, etc. 

Soil

Soil all across the world has different natures and properties. Factors like climate, weathering process, soil movement, and soil development affect the soil. Soil composition, grain size, and aggregation are characteristics that influence various soils' percolation and water-holding capacity. 

• Parameters such as pH, mineral composition, and topography largely influence the vegetative capacity of a given area. 
• This also affects the type of animals that develop in that region. 

Read More: Ecosystem - What are its components?

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Response to Abiotic Factors

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It is already established that the abiotic factors are variable. The organisms living in the habitat have different responses to the changes in the environment. They have evolved to develop a constant internal environment(optimal temperature and osmotic concentration of body fluids).

• This allows for the biochemical reactions and physiological functions to proceed smoothly.
• There are certain different ways in which organisms respond to abiotic factors.

Regulate

The majority of birds and mammals, some lower vertebrate and invertebrate species have developed the ability to regulate their body temperature. It maintain homeostasis(thermoregulation and osmoregulation) state.

• Due to this reason mammals are able to survive in a variety of environments.
• Human beings maintain a constant temperature of 370 C.
• Plants however, are unable to maintain such a constant temperature. 

Read More: Food Chains and Webs

Conform

99% of animals almost all the plants lack the ability to maintain a constant internal environment. This means that their body temperature changes with ambient temperature.

• Even aquatic animals change their internal osmotic concentration according to the changes in the air and water osmotic concentration.
• Plants and animals are mainly conformers.
• The reason is that they lack the energy to regulate their internal environments. 

Migrate

In certain cases, when the habitat becomes inhabitable, organisms will move away temporarily until favourable living conditions return. Many animals, and especially birds migrate to warmer climates for the winters. 

Suspend

Some organisms suspend their regular metabolic processes and go into a state of dormancy. This is generally done when the animal is unable to migrate and needs to escape a stressful environment.

Representation of organismic response

Read More: marine biology

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Adaptations

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Organisms respond to extreme variations in their environment. Among the various alternatives available, some respond by making physiological adjustments while others make behavioural adjustments (like migration). These responses of organisms are called adaptations. 

• Adaptation is the attribute of any organism that enables its survival and reproduction processes in the habitat.
• These attributes can be morphological, physiological or behavioural.
• Adaptations have evolved over the years and become genetically fixed in species. 
• The majority of organisms on the planet have managed to adapt to their respective habitats.
• Creatures respond to the variation in their environment to ensure optimum survival and continuity. 
• Organisms have been known to adapt physiologically as well.
• Those living in higher altitudes (where atmospheric pressure is low) compensate for low oxygen availability by increasing red blood cell production.
• There are also certain microbes that can flourish in extreme habitats like hot springs and deep sea hydrothermal vents. 

Read More: Taxonomic Hierarchy of Plants and Animals

Read More: Classification of Animal Kingdom

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Populations

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A group of organisms or individual species in a geographical area that is well-defined and capable of interbreeding is defined as a population. The individuals within the population are in competition with each other for their basic needs. 

• The process of competition for basic needs is called intraspecific competition.

Population Attributes

There are various attributes of population, which are as follows -

Age Pyramid 

Any given population will have individuals of different ages. An age pyramid is a structure that plots the age distribution of a population. 

Sex Ratio 

Sex Ratio refers to the total number of males and females per thousand individuals. It is a ratio of males to females in a population. 

Birth Rate 

Birth rate refers to the total number of individuals born in a given period of time. It is the ratio between the number of live-born births in the year and the average total population of that year.

Death Rate

Death Rate is a measure of the number of deaths (in general or due to a specific cause) in a particular population, scaled to the size of that population per unit of time. It is also known as the mortality rate.

Representation of Age Pyramids for Human Populations

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Population Growth

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An increase in the number of individuals in any population is known as population growth. The growth of a population depends on many factors such as availability of food, weather, etc. Immigration, emigration, mortality, and natality are mainly responsible for the changes in the population density.

Immigration

Immigration refers to the number of individuals of the same species that transfer in a certain period of time into a habitat from anywhere else.

Emigration

Emigration is the number of individuals that, in a certain time period, transfer to another habitat.

Mortality

Mortality refers to the number of deaths that occur in a certain time period.

Natality

Natality refers to the number of births that occur in a population in a certain time period that contribute to initial density.

Read More: Class 12 Biology Productivity in Ecosystem

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Population Growth Models

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The changes in population can be mapped over time. There are certain models that have been developed to do so. The two major models of population growth are:

Exponential Growth Model

When a habitat has unlimited resources, namely food and space, the growth then would be exponential. Impacted by Darwin’s theory of natural selection, the population grows in an exponential or geometric fashion. 

• If in a population of size N, the birth rates (not total number but per capita births) are represented as b and death rates (again, per capita death rates) as d.
• The increase/decrease in N during a unit time period t(\(\frac{dN}{dt}\)) can be calculated as 

dN/dt = (b – d) × N 

• Let (b–d) = r, then 

\(\frac{dN}{dt}\) = rN 

• Here, N = Population size, 
• r = Intrinsic rate of natural increase,
• b= birth rate and,
• d= death rate
• The above equation describes the exponential or geometric growth pattern of a population.
• The results is a J-shaped curve when we plot N in relation to time.

The integral form of the exponential growth equation can be derived as 

N_t = N₀ ert 

• Where
• N_t = Population density after time t 
• N₀ = Population density at time zero
• r = intrinsic rate of natural increase 
• e = the base of natural logarithms (2.71828) 

 Population growth curve

Logistic Growth Model

Realistically, it is not possible that any population of species would have unlimited resources at its disposal. Species have to compete with each other and individually for resources.

• Any habitat has the resources only to support a maximum possible number, after which there is no further growth.
• This limit becomes nature’s carrying capacity (K) for that species in that habitat. 
• In this, there are phases of lag, phases of acceleration, deceleration.
• It finally end at an asymptote when population density reaches carrying capacity. 
• This type of population growth is called Verhulst-Pearl Logistic Growth and is described by the following equation: 

 \(\frac{dN}{dt}\)= rNK-N/K

 Where 

• N = Population density at time t 
• r = Intrinsic rate of natural increase 
• K = Carrying capacity 
• Resources for most populations are finite and even becoming limited.
• Hence, the logistical growth model is considered as the more realistic model. 

Read More: Microbes in Human Welfare

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Things to Remember

• Organisms and Populations are levels of ecology.
• The relationship among organisms and its environment is called ecology.
• Tropical Rain Forest, Sea Coast, Desert and Decidious Forest are some Biomes in India.
• Population interaction is defined as interaction among different populations.
• Exponential and Logistics Growth Model are used to calculate population growth.

Read More: What is Biomass?

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Previous Years Questions