Revision: Organisms and Populations Zoology HSC Science Class 12 Tamil Nadu Board of Secondary Education

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.

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 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 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.

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.

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.

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

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.

• Habitat is the place or environment where an organism lives and survives, including both living and non-living factors.
• Factors like sunlight, temperature, rainfall, and soil determine the type of organisms present in a habitat.
• Examples of habitats include ponds, rivers, oceans, and forests, which may be terrestrial, aquatic, aerial, or arboreal.
• Niche refers to the functional role of an organism in its environment, including its diet, behaviour, and interactions.
• In simple terms, habitat is the “address” of an organism, while niche is its “role or profession.”

• 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.