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

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 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 intrinsic rate of growth.

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

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

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 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.

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.

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

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.

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 type of competition in which organisms share a resource but one uses it more efficiently than the other is called exploitative 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 the same species for limited resources is called intraspecific 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).

• 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.

• 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, categorized 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 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 helps in energy transfer across trophic levels and keeps prey populations under control.
• Predators maintain ecological balance and species diversity by reducing competition among prey.
• In the absence of predators, some species may overgrow or become invasive (e.g., prickly pear in Australia).
• Overexploitation by predators can cause the extinction of prey and then predators, so natural systems remain balanced.
• Prey species develop defence mechanisms like camouflage and chemical protection (e.g., Monarch butterfly).
• Plants also defend against herbivores using thorns (morphological) and toxic chemicals (chemical defence).
• Example: Calotropis produces poisonous substances that deter grazing animals.

• Interspecific competition is a major force in evolution, reducing the fitness of species when resources are limited.
• Competition can occur between closely related or unrelated species for the same resources.
• It may occur even when resources are abundant as interference competition, where one species affects another’s efficiency.
• Gause’s Competitive Exclusion Principle: Two species competing for the same resource cannot coexist indefinitely; one will be eliminated.
• Competitive release: A species expands its range when the competing species is removed.
• Competition is generally more intense in herbivores than in carnivores.
• Species may avoid competition through resource partitioning, using resources in different ways (e.g., warblers feeding differently on the same tree).

• Parasitism is a relationship where the parasite benefits and the host is harmed (food, shelter).
• Many parasites are host-specific and co-evolve with their hosts.
• Parasites have special adaptations like loss of sense organs, hooks/suckers, and high reproductive capacity.
• Parasitism reduces the survival, growth, and reproduction of the host and may make it more vulnerable to predators.
• Parasites often have complex life cycles involving one or more intermediate hosts (e.g., liver fluke, malaria parasite).
• Two main types: Ectoparasites (live on host surface, e.g., lice, ticks) and Endoparasites (live inside body, e.g., liver, RBC).
• Brood parasitism: Birds like cuckoos lay eggs in another bird’s nest, and the host incubates them.

• Commensalism is an interaction where one species benefits and the other is neither harmed nor helped - represented as (+, 0).
• An orchid growing on a mango branch gets sunlight and support, while the mango tree is completely unaffected.
• Barnacles on a whale's body get free transportation and access to food, while the whale gains nothing and loses nothing.
• Cattle egrets follow grazing cattle because the movement of cattle stirs up insects, making it easy for egrets to catch them - cattle remain unaffected.
• Clownfish hide among sea anemone's stinging tentacles to stay safe from predators, while the sea anemone gets no benefit in return.

• 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.