Revision: Ecology >> Organisms and Populations Biology Science (English Medium) Class 12 CBSE

Habitats are species-specific and have resources up to a limit that can support a maximum number of individuals to grow and reproduce; this limit of habitat to subsist a species is called carrying capacity.

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.

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

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

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.

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

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.)

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.

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

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.

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).

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

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

• 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.

• 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.

• A population consists of individuals of the same species living in a defined area, sharing resources and capable of interbreeding.
• Population ecology studies populations because natural selection acts at the population level, linking ecology with evolution.
• Populations have birth rate and death rate (per capita), unlike individuals which have only birth or death events.
• Sex ratio and age structure (represented by age pyramids) are key attributes that indicate whether a population is growing, stable, or declining.
• Population size or density (N) reflects the status of a population and may be measured as number, biomass, or percent cover.
• Population density is often estimated indirectly (e.g., fish caught per trap, tiger census using pug marks) when direct counting is difficult.