Organisms and Populations

Organisms experience environmental variation in temperature, water, light and soil-related conditions. Their responses depend on physiology, life history and habitat.

• Regulate: maintain a relatively stable internal state despite external change within physiological limits.
• Conform: internal conditions vary with external conditions.
• Migrate: move temporarily to a more favourable habitat.
• Suspend: reduce activity/metabolism through dormant states during unfavourable periods.

Do not imply that every species uses one strategy exclusively.

A population is a group of individuals of the same species occupying a defined area in a defined time context and functioning as an ecological unit.

• Density (N): population size or abundance measure in a defined area/volume/context.
• Birth rate: addition through births per population/time relationship.
• Death rate: loss through deaths per population/time relationship.
• Sex ratio: proportion of males and females where biologically applicable.
• Age distribution: proportions in pre-reproductive, reproductive and post-reproductive categories.

The official 2026 wording explicitly names growth, birth rate, death rate and age distribution. Density and sex ratio are retained as direct NCERT population-attribute context.

Population change

births + immigration - deaths - emigration. This relationship is conceptual and does not create real-world human population predictions.

Under effectively unlimited resources, population growth can be modelled as dN/dt = rN, where N is population density/size at time t, r is the intrinsic rate of natural increase, and dN/dt is the instantaneous rate of population change. The integrated form is N_t = N_0 e^(rt).

The key condition is near-unlimited resources. It is not a universal expectation in nature.

When resources become limiting, NCERT uses the logistic model dN/dt = rN((K - N)/K), where K is the carrying capacity of the habitat for the population under the stated conditions. As N approaches K, the growth term decreases, and the idealised curve is sigmoid.

Age structure can be displayed using age pyramids. The inference is based on relative pre-reproductive, reproductive and post-reproductive proportions, not on a single individual's age.

• Mutualism (+/+): both interacting species benefit. NCERT examples include lichen as a fungal plus photosynthetic partner association, mycorrhiza between fungi and plant roots, and the fig tree and pollinating wasp as a highly specific plant-pollinator mutualism.
• Competition (-/-): both populations experience reduced fitness relative to absence of competition. NCERT example: flamingoes and resident fishes competing for zooplankton in shallow South American lakes.
• Predation (+/-): the predator benefits and prey is harmed. Predation transfers energy across trophic levels and can regulate prey populations. A tiger-deer relationship is an intuitive example, while herbivory also fits the broad predator-consumer logic in NCERT.
• Parasitism (+/-): the parasite benefits while host fitness is reduced. NCERT examples include Cuscuta growing on host plants and brood parasitism in cuckoo/koel-host bird relationships.

• Commensalism (+/0): one species benefits and the other is neither clearly benefited nor harmed. NCERT examples: orchid epiphyte on mango tree; barnacles on whale.
• Amensalism (-/0): one species is harmed and the other is unaffected.

These remain useful for NCERT comparison, but this page labels them NCERT context, not explicit in the 2026 interaction list, rather than silently expanding official scope.

Population equations are simplified models. They require stated assumptions and cannot be used as direct prediction engines for real human populations without empirical parameters and broader demographic modelling.