Revision: Ecology and Environment >> Organisms and Populations Biology (Theory) ISC (Science) ISC Class 12 CISCE

Ecology is the branch of biology that studies the interactions among organisms and between organisms and their environment.

A biome is a large ecological region characterized by a particular climate and a group of associated plant and animal communities.

Percolation is the downward movement of infiltrated water through soil and rock layers.

Light is a form of Energy i. e. The external physical cause that affects our eye to produce the sensation of vision.

The ratio between the amount of water vapor in the atmosphere and the amount of water vapor it can hold is Relative humidity.

The amount of water vapour present in the air at a particular time and place is called Humidity.

The amount of water vapour in the atmosphere is called Absolute Humidity.

Niche of an organism can be defined as the total position and function of an individual in its environment.

Habitat is the natural place or environment where an organism lives and grows under suitable conditions.

A niche is the functional role of an organism in its environment, including how it uses resources and interacts with other organisms.

Organisms of the same kind inhabiting a geographical area constitute the population.

Individuals live in groups in a well-defined geographical area, share or compete for similar resources, and potentially interbreed and thus form a population.

The population is defined as a group of individuals of a species occupying a definite geographic area at a given time.

Camouflage, also called cryptic colouration, is a defense mechanism or tactic that organisms use to disguise their appearance, usually to blend in with their surroundings. Organisms use camouflage to mask their location, identity, and movement.

Species that tolerate narrow range of salinity are called stenohaline species.

Niche describes the position of a species in an environment and also the functional role played by an organism.

Parasitism is generally defined as a relationship between the two living species in which one organism is benefitted at the expense of the other. The organism that is benefitted is called the parasite, while the one that is harmed is called the host.

An interaction in which one species benefits while the other is neither benefited nor harmed is called commensalism.

A community is an assemblage of populations of different species living in the same area and interacting with one another.

The type of interaction in which both species are benefitted is called mutualism.

Mutualism refers to an interaction between two different species in which both benefit. (The term symbiosis is frequently used interchangeably with mutualism.)

Interspecific competition occurs when closely related species compete for limited resources such as food and habitat.

Interactions occurring among individuals of the same species within a community.

A social arrangement in which individuals of the same species are ranked according to their dominance status.

A form of competitive interaction in which an individual or group occupies and defends a fixed area for a certain period.

A type of social behaviour in which an individual sacrifices its own interest or safety for the benefit of other members of the group.

Interaction between populations of two different species living in the same habitat.

An interspecific relationship in which one organism benefits, while the other is neither benefited nor harmed.

It is a non-obligatory interaction between two different species in which both are benefited, but neither depends completely on the other for survival.

Mutualism is an interaction between two species in which both are benefited.

Parasitism is an interaction between two species in which one (known as parasite) obtains its food in ready‑made form from the other living organism (said to be host).

The rivalry between two or more organisms for the same limited resource, where the use of the resource by one reduces its availability to the other, is called competition.

Competition that occurs between individuals of the same species for limited resources is called intraspecific competition.

Competition that occurs between individuals of different species for the same limited resources is called interspecific competition.

A type of competition in which one organism directly prevents another from accessing a resource is called interference competition.

A type of competition in which organisms share a resource but one uses it more efficiently than the other is called exploitative competition.

A group of organisms of the same species occupying a particular area at a particular time is called a population.

The scientific study of the size, structure and growth of a population, generally human population, is called demography.

The number of individuals of a species present per unit area (or volume) at a given time is called population density.

The inherent capacity of a population to increase in number under ideal environmental conditions is called biotic potential.

The increase in the number of individuals of a species in a given area over a period of time is called population growth.

The rate at which new individuals are added to a population through reproduction is called natality.

The rate at which individuals die in a population during a given period of time is called mortality.

The rate of change in population size per individual per unit time is called population growth rate.

The maximum growth rate of a population under ideal conditions with maximum reproduction and minimum mortality is called intrinsic rate of growth.

The reproductive capacity of a population measured as the number of births during a given period is called fecundity.

The addition of individuals to a population from neighbouring populations is called immigration.

The movement of individuals out of a population to other areas is called emigration.

The maximum number of individuals that an environment can support or sustain with available resources is called carrying capacity.

The proportion of individuals belonging to different age groups in a population is called age distribution.

The arrangement or composition of a population based on different age groups is called age structure.

The graphical representation showing the proportion of individuals in different age groups of a population is called an age pyramid.

The proportion or ratio of males to females in a population is called sex ratio.

The flow and transformation of energy within a population is called bioenergetics

1. Organism: An individual living entity that interacts with its surroundings.
2. Population: A group of individuals of the same species living in a defined area.
3. Community: Different populations living and interacting together in an area.
4. Ecosystem: A functional unit where organisms interact with the physical environment.
5. Landscape: A land area containing a group of different ecosystems.
6. Biome: A large ecological region defined by climate and dominant vegetation.
7. Biosphere: The entire zone of life on Earth where all ecosystems exist.

• Organismic ecology studies how organisms are physiologically adapted to their environment for survival and reproduction.
• Variations in temperature, rainfall, and seasons lead to the formation of different biomes and habitats.
• Abiotic factors like temperature, water, light, and soil influence habitats, and organisms evolve adaptations to survive in them.

• Climatic factors such as precipitation, temperature, light, humidity, and wind determine climate and vegetation types.
• Rainfall is the most important factor influencing plant distribution and types of vegetation.
• Availability of water affects plant growth, metabolism, photosynthesis, and animal distribution.
• In aquatic habitats, salinity influences organism survival; organisms may be euryhaline or stenohaline based on tolerance.

1. Altitude: With increase in altitude, temperature decreases and wind and humidity variations increase.
2. Slope: Slope affects rainfall, sunlight exposure, soil moisture, erosion, and vegetation distribution.
3. Exposure: Areas exposed to sun and wind support different vegetation compared to sheltered areas.
4. Direction of mountain chains: Mountain ranges influence wind direction, rainfall patterns, and local climate.

• Organisms live in groups forming populations, and different populations together constitute a biotic community.
• Communities show species diversity, interdependence, dominance, and coexistence among different species.
• Communities interacting with their physical environment form an ecosystem with energy flow and nutrient cycling.
• All ecosystems together constitute the biosphere, the highest and most self-sustaining level of biological organization.