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

An adaptation is any advantageous structural or functional characteristic developed over time that enables a living organism to survive, reproduce, and constantly adjust to its specific changing environment.

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 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.

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.

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.

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

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.

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.

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.

A type of competition in which one organism directly prevents another from accessing a resource is called interference competition.

Competition that occurs between individuals of different species for the same limited resources is called interspecific 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: \[\mathbf{N_{t}}=\mathbf{N_{0}}\mathbf{e^{rt}}\]

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

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.

• Temperature - Ranges from subzero (polar) to >50°C (tropical deserts). Affects enzyme kinetics & metabolism. Eurythermal = wide range; Stenothermal = narrow range.
• Temperature Examples - Mango trees are absent in Canada; snow leopards are absent in Kerala; tuna are absent beyond tropical latitudes — all due to temperature limits.
• Water - Salinity: <5% inland, 30-35% sea, >100% hypersaline lagoons. Euryhaline = wide salinity tolerance; Stenohaline = narrow tolerance.
• Freshwater vs Seawater - Freshwater animals can't survive in seawater and vice versa due to osmotic pressure problems.
• Light - Used for photosynthesis & flowering. Herbs/shrubs adapted to low light under tall trees. Below 500m ocean depth, it is perpetually dark.
• Soil - Composition, pH, minerals & topography determine vegetation on land and benthic animals in aquatic environments.

• 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).

• According to Darwin’s natural selection theory, only the most viable and adaptable species successfully survive the struggle for existence.
• The continuous adjustment of a species to its new surroundings serves as the primary factor driving the formation and conservation of species.
• Organisms that are unable to adjust to shifting environmental conditions will gradually diminish in the course of evolution and eventually face extermination.
• These constant structural and functional adjustments directly match living forms to their changing environment, ensuring their survival within specific ecological limits.

• 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.

• Age Pyramid - Shows age distribution of males & females. Types: Growing (broad base), Stable, Declining (narrow base).
• Population Density (N) - Expressed as numbers, % cover, or biomass when counting is impractical. E.g., biomass used for dense bacterial cultures.
• Density Range - From <10 Siberian cranes (wetland) to millions of Chlamydomonas (pond).
• Relative Density - Fish caught per trap in a lake is sufficient for ecological studies without an absolute count.
• Indirect Estimation - Tiger census uses pug marks & faecal pellets instead of direct counting.

• 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 evolve life history strategies to maximise reproductive (Darwinian) fitness in their environment.
• Different species adopt different strategies based on selection pressures.
• Some organisms reproduce once in a lifetime (e.g., salmon, bamboo), while others reproduce multiple times (e.g., birds, mammals).
• Species may produce many small offspring (e.g., oysters) or few large offspring (e.g., mammals), depending on survival needs.
• These variations are shaped by biotic and abiotic factors, and studying them is an important area in ecology.

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)

• Predation (+/-) involves a predator killing and eating its prey, which acts as nature's main channel for transferring energy up the food chain.
• Predators naturally regulate prey numbers, preventing overpopulation - a principle used effectively in biological pest control.
• By consuming dominant prey species, predators prevent resource monopolisation and maintain overall species diversity.
• Predator and prey populations cycle together naturally; predators are "prudent" and do not hunt their food source to extinction.
• To survive, prey species continuously evolve physical defences (like camouflage or thorns) and chemical defences (like toxins).

• Competition is an interaction (-/-) where organisms struggle for the same limited resources, resulting in a negative impact on the biological fitness of both participants.
• It is generally more intense when it occurs between members of the same species (intraspecific) rather than between members of different species (interspecific) because their resource requirements are identical.
• The struggle can occur directly, in which one organism actively prevents another from accessing a resource (interference competition), or indirectly, in which one organism rapidly consumes a shared resource (exploitative competition).
• Gause's Competitive Exclusion Principle states that two closely related species competing for the exact same resource cannot coexist indefinitely; the competitively weaker species will eventually be eliminated.
• To avoid elimination, competing species often adapt to coexist through resource partitioning (using different parts of a resource or hunting at different times) or character displacement (evolving distinct physical traits to reduce overlap).
• Competition usually restricts a species to a smaller realised niche rather than its full-potential fundamental niche; however, if the dominant competitor is removed, the restricted species will rapidly expand its range in a process called competitive release.

• Parasitism is an interaction where one organism (the parasite) derives ready-made nourishment directly from another living organism (the host).
• Ectoparasites (like ticks and lice) live on the host's outer surface and retain functional organs, whereas endoparasites (like tapeworms and Plasmodium) live inside the host with reduced sensory and digestive systems.
• In brood parasitism, a bird lays its eggs in the nest of a host species, often using egg mimicry and earlier hatching times to ensure the host raises its young (e.g., the Koel bird and the crow).
• Plant parasites use specialised structures to extract nutrients; they can be total parasites lacking chlorophyll entirely (like the leafless Cuscuta) or partial parasites that still photosynthesise (like Mistletoe).
• Parasites possess unique survival adaptations, including specialised attachment organs (hooks and suckers), the ability to absorb food directly through their skin, anaerobic respiration, and massive reproductive capacities involving intermediate hosts.

• Commensalism (+/0) is an interaction where one species (the commensal) gains a benefit - such as food, shelter, or transport - while the other (the host) remains completely unaffected.
• It occurs in various forms, such as phoresy for transport (e.g., barnacles on a whale) or inquilinism for shelter (e.g., clownfish living safely inside a stinging sea anemone).
• Epiphytic plants, like orchids, grow on host plants (such as a mango tree) strictly for physical support and better sunlight, without absorbing any of the host's nutrients.
• Cattle egrets exhibit this relationship by closely following grazing cattle to easily catch insects that are flushed out of the grass by the cattle's movement.
• Remora fish attach themselves to the underside of sharks to gain free oceanic transport and scavenge leftover food scraps, causing absolutely no impact on the shark.

• Mutualism is an interaction where both species benefit from each other, represented as (+, +).
• Lichens are a mutualistic association between fungi and algae/cyanobacteria, where fungi provide shelter and algae provide food through photosynthesis.
• Mycorrhizae are associations between fungi and plant roots, where fungi help the plant absorb nutrients from the soil, and the plant provides carbohydrates to the fungi.
• Fig trees and wasps share a one-to-one relationship where the wasp pollinates the fig, and the fig provides developing seeds as food for the wasp larvae.
• The orchid Ophrys* uses "sexual deceit" by resembling a female bee, causing male bees to pseudocopulate with it and transfer pollen from flower to flower.