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.

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

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

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

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

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

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

The rate at which individuals die in a population over a given period is called mortality.

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

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

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

A community is an association of a number of different interrelated populations belonging to different species in a common environment. A community is characterised by species diversity, coexistence and interdependence.

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.

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.

A population is a group of individuals of the same species that are capable of interbreeding and are found in a geographical area. E.g., all the frogs identified as Rana tigrina living in a given pond constitute a population. Similarly, all water hyacinth plants (Eichhornia) growing in that pond form another population. Organisms of the same kind may form several populations inhabiting different geographical areas.

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

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

Commensalism is the interaction between two living individuals of different species in which one is benefited while the other is neither harmed nor benefited except to a negligible extent.

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.

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

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

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

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

Verhulst-Pearl Logistic Growth:

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

• Climate variations, specifically temperature and rainfall, determine the distribution of Earth's major biomes.
• An organism's survival depends on both abiotic factors (such as temperature and water) and biotic interactions (such as competition and mutualism).
• Organisms develop specific structural, physiological, or behavioural adaptations to overcome environmental challenges.
• To manage environmental stress, organisms can either regulate internal conditions, conform to the environment, migrate, or suspend their activities.
• A habitat is the physical address where a species lives, whereas an ecological niche is its functional role within that environment.

• 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.”

• Population is the basic ecological unit for studying genetics, evolution, and ecological change.
• Population density tells the number of individuals per unit area or volume.
• Natality increases population size, while mortality decreases it.
• Sex ratio describes the proportion of males and females in a population.
• The age pyramid helps in identifying whether a population is growing, stable, or declining.

• Population is the basic ecological unit for studying genetics, evolution, and ecological change.
• Population density tells the number of individuals per unit area or volume.
• Natality increases population size, while mortality decreases it.
• Sex ratio describes the proportion of males and females in a population.
• The age pyramid helps in identifying whether a population is growing, stable, or declining.

• 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.

Population interactions refer to the ways organisms influence one another within a shared ecosystem, categorised by positive (+), negative (-), or neutral (0) effects.

1. Mutualism (+/+): Both species benefit. (Example: Lichen - algae and fungi helping each other)
2. Competition (-/-): Both species are harmed when fighting for limited resources. (Example: Two plant species competing for sunlight)
3. Predation (+/-): Predator kills and eats the prey. (Example: Tiger hunting a deer)
4. Parasitism (+/-): Parasite slowly harms a living host for nutrients. (Example: Tapeworm inside human intestines)
5. Commensalism (+/0): One benefits, the other is entirely unaffected. (Example: Cattle egrets eating insects stirred up by grazing cattle)
6. Amensalism (-/0): One is harmed, the other is entirely unaffected. (Example: Penicillium fungus killing nearby bacteria)
7. Neutralism (0/0): Neither species affects the other. (Example: Rabbits and deer sharing the same forest)