Revision: Organisms and Populations Biology HSC Science (General) 12th Standard Board Exam Maharashtra State Board

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

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

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

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

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 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.

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

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

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.

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.

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.)

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.

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

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

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

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.

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

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.”

• Niche is the functional role and position of a species in an ecosystem, including its food, shelter, and interactions with other organisms.
• Each species has a unique niche, and no two species can occupy the same niche, which helps reduce competition.
• Types of niche include spatial (habitat), trophic (food level), and multidimensional (interaction of many environmental factors).
• Fundamental niche is the potential role without competition, while realised niche is the actual role in the presence of competition.
• Niche is influenced by abiotic factors such as temperature, water, light, and soil.

• 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.

• 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.