Revision: Class 12 >> Organisms and Populations NEET (UG) Organisms and Populations

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

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 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.

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.

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

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.

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

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.

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.

\[\frac{\mathrm{dN}}{\mathrm{dt}}=\mathrm{rN}\]

Integral form: \[\mathbf{N_{t}}=\mathbf{N_{0}}\mathbf{e^{rt}}\]

Verhulst-Pearl Logistic Growth:

\[\frac{\mathrm{d~N}}{\mathrm{d~t}}=\mathbf{rN}\left[\frac{\mathrm{K}-\mathrm{N}}{\mathrm{K}}\right]\]

\[\mathbf{N_{t+1}}=\mathbf{N_{t}}+[(\mathbf{B+I})-(\mathbf{D+E})]\]

• 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.

• Abiotic factors are non-living components like water, temperature, light, and soil that affect survival and reproduction.
• Regulate → Organisms maintain constant internal conditions (homeostasis), such as body temperature (e.g., birds and mammals).
• Conform → Organisms cannot maintain internal balance; their bodies change with the environment (e.g., plants).
• Migrate → Temporary movement to a more suitable place during unfavourable conditions (e.g., Siberian birds).
• Suspend (Dormancy) → Temporary stopping of metabolic activities to survive stress (e.g., seeds).
• Hibernation → Dormancy during winter to escape cold (e.g., bear).
• Aestivation → Dormancy during summer to avoid heat (e.g., snails, fish).

• Adaptation is any morphological, physiological, or behavioural feature that helps an organism survive and reproduce in its habitat.
• Desert plant adaptations → Thick cuticle, sunken stomata, and reduced leaves (spines in Opuntia) help to reduce water loss.
• CAM pathway → Special photosynthesis in desert plants where stomata remain closed during the day to minimise transpiration.
• Behavioural adaptation → Desert lizards regulate body temperature by basking in sun or moving to shade/burrowing.
• Cold region adaptations (Allen’s Rule) → Animals have shorter ears and limbs to reduce heat loss; aquatic mammals have a thick fat layer (blubber) for insulation.

• A population is a group of individuals living in a specific area that shares resources, competes, and can interbreed.
• Population attributes include size (number of individuals) and density (number of individuals per unit area), as well as natality, mortality, and age structure.
• Natality (birth rate) is the number of births in a population; it can be crude (per 1000 individuals) or specific (based on age or condition). Absolute natality is higher than realised natality.
• Mortality (death rate) is the number of deaths per population; it is usually measured per 1000 individuals per year. Absolute mortality is lower than realised mortality.
• Sex ratio is the proportion of males to females in a population, and a 1:1 ratio is generally considered stable.

• Population growth is studied mainly by two models: exponential growth and logistic growth.
• Population density (N) changes due to four factors: Natality (B), Mortality (D), Immigration (I), and Emigration (E).
• Exponential growth → Occurs when resources are unlimited; population increases rapidly and forms a J-shaped curve.
• Logistic growth → Occurs when resources are limited; growth slows down and forms an S-shaped (sigmoid) curve.
• Phases of logistic growth → Lag phase → Log (exponential) phase → Diminishing growth phase → Stationary phase.
• Carrying capacity (K) → Maximum population size that the environment can support; population stabilises at this level.

• 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.