Revision: Ecology >> Organisms and Populations Biology Science (English Medium) Class 12 CBSE

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

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

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.

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.

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.

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

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

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

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.

• Population - Group of same species in a given area, sharing resources & reproducing. E.g., cormorants in a wetland, lotus in a pond.
• Birth & Death Rate - Measured per capita. Birth rate = 8/20 = 0.4 offspring/lotus/year; Death rate = 4/40 = 0.1 individuals/fruit fly/week.
• Sex Ratio - Proportion of males to females. E.g., 60% females & 40% males.
• Population Density (N) - Number of individuals per unit area. Can be as low as <10 (Siberian cranes) or millions (Chlamydomonas). Sometimes measured as biomass or % cover.
• Indirect Estimation - Used when a direct count is difficult. E.g., fish per trap = lake density; tiger census uses pug marks & fecal pellets.
• Population Ecology - Connects ecology to genetics & evolution. Natural selection operates at the population level.

• 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.

• 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.

• 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.

• A population consists of individuals of the same species living in a defined area, sharing resources and capable of interbreeding.
• Population ecology studies populations because natural selection acts at the population level, linking ecology with evolution.
• Populations have birth rate and death rate (per capita), unlike individuals which have only birth or death events.
• Sex ratio and age structure (represented by age pyramids) are key attributes that indicate whether a population is growing, stable, or declining.
• Population size or density (N) reflects the status of a population and may be measured as number, biomass, or percent cover.
• Population density is often estimated indirectly (e.g., fish caught per trap, tiger census using pug marks) when direct counting is difficult.