CBSE Class 12 Biology Notes Chapter 13 Organisms and Populations

Ecology is a discipline of biology that studies how organisms communicate with other organisms within their environment. 

• Ecosystem is used to represent the organisms and their environment in a particular area.
• Ecology has four biological organization levels, namely organisms, populations, communities and biomes. 
• Organisms refer to all living things in an environment that are able to retain specific behaviors and structures.
• The population refers to the group of the same species capable of interbreeding, residing in the same geographical region.
• Communities refer to groups of populations of different species present in any particular area that interact among themselves.
• The biome is the largest unit of ecology, which is composed of major vegetation types and associated fauna in a particular climatic zone.

CBSE Class 12 Biology Chapter 13 Notes on Organisms and Populations will help individuals prepare for the CBSE Class 12 Biology Examination, which comes under the unit named Ecology and Environment.

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Class 12 Biology Chapter 13 Notes – Organisms and Populations

Population

• Population is defined as the sum total of all living organisms of a particular species in a geographical area.
• Each species in the ecosystem competes with each other for their basic needs. 
• Intraspecific competition is defined as competition among members of similar species for limited resources.

Population Attributes

Various attributes of population are as follows:

• Age Pyramid: An age pyramid is a form of structure that plots the age distribution of individuals in a given population.
• Sex Ratio: The sex ratio is the ratio of the total number of males and females per thousand individuals. 
• Birth Rate: Birth rate is the ratio of the total number of individuals born in a particular time frame. It also indicates the ratio of the number of live-born births in the year and the average total population of that year.
• Death Rate: Death rate, also known as mortality rate, determines the number of individuals of a particular population loss due to death per unit of time.

Population Growth

Population growth refers to an increase in the number of organisms in a particular population. It depends on factors like weather, food availability, and predator pressure.

Factors affecting the Population Density

• Immigration: Immigration is defined as the number of organisms of the same species that enter the habitat from elsewhere over a certain period of time.
• Emigration: Emigration refers to the number of individuals who are relocated to another habitat in a certain time frame.
• Mortality: Mortality is the number of individuals who died in a population in a certain time period.
• Natality: Natality is the number of individuals giving birth in a population in a certain time period that, in turn, increases the population density.

Population Growth Models

The population growth model will determine the change in population mapped with respect to time. There are two population growth models, which are as follows:

Exponential Growth Model

The exponential growth model is used when there is a limited supply of food, and the population continues to grow at an exponential rate. It can be represented as:

N_t = N₀ ert 

• 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) 

Logistic Growth Model

The logistic growth model is used when the amount of resources becomes limited or finite, and the population continues to grow at a logistic rate. This Verhulst-Pearl Logistic Growth is represented by the following equation: 

dN/dt = rNK-N/K

• N : Population density at time t 
• r : Intrinsic rate of natural increase 

Population Interaction

Population interaction is a biological process where different populations of organisms interact with one another for energy and food sources. 

• Physical factors, also known as abiotic factors, include nutrients in the soil, water, temperature, carbon dioxide, atmospheric pressure, wind and osmotic balances.
• Biotic factors include all living things that are part of the ecosystem.

Types of Population Interactions

Various types of population interactions are as follows:

Competition: Competition refers to two or more species who are competing for a limited amount of resources. It will harm the participants as either of the species will have higher survival and reproduction in the absence of the other species.

Predation: Predation is a process where one member of a species, called a predator, will eat the other species, called prey. In this way, each organism depends on the other for food and survival.

Mutualism: In the process of mutualism, two or more species will form a long-term relationship to benefit from each other. This is the most common type of ecological interaction.

Commensalism: In commensalism, two or more species form a long-term relationship in which one organism benefits without having a positive or negative effect on the other (+/0 interaction). 

Parasitism: Parasitism is a type of symbiosis where two or more species will form a long-term relationship, and one organism will benefit from having a negative effect on the other's interaction. The one organism that feeds on another organism is called a parasite, whereas an organism that is fed upon by other organisms is called the host. 

Chapter 13, Notes on Organisms and Populations, will also provide links to important long-type and MCQ questions on the given topic to prepare students completely for the biology examination.

There are Some important List Of Top Biology Questions On Organisms and Populations Asked In CBSE CLASS XII