# Organisms and Populations Notes, Class 12 Biology | SwaVid Organisms and Populations, NCERT Class 12 Biology: notes with key ideas, key terms, common mistakes and 3 quick checks with worked answers. Canonical: https://www.swavid.com/cbse/class-12/biology/chapter/organisms-and-populations Source: https://www.swavid.com/cbse/class-12/biology/chapter/organisms-and-populations # Organisms and Populations ## Key terms ## Key ideas ## Common mistakes ## Quick checks ## Keep going ## Questions parents ask ## Class 12 Biology on SwaVid ### Population attributes ### Population growth ### Growth models ### Predation and herbivory ### Competition ### Parasitism, commensalism, and mutualism ### Next chapter ### Related chapter ### Related chapter ### All chapters SwaVid › CBSE › Class 12 › Biology › Organisms and Populations Notes written by SwaVid from the NCERT textbook. Class 12 Biology, Chapter 11. Key ideas, formulas and common mistakes. CBSE prints 10 marks for Unit X, not for this chapter alone. Short answer. This chapter explores populations, their growth attributes, and how different species interact within an ecological community. It explains how population size changes through vital rates and details biological interactions ranging from mutualism to competition. Source: NCERT Biology (Reprint 2026-27) , Chapter 11, Organisms and Populations A population is a group of individuals of the same species living in a well-defined geographical area and sharing or competing for similar resources. Populations possess attributes that individual organisms do not have, such as birth rates, death rates, sex ratio, and age distribution. The size of a population changes continuously depending on food availability, predation pressure, and weather. The four basic processes that alter population density are natality, mortality, immigration, and emigration. When resources are unlimited, populations exhibit exponential or geometric growth, yielding a J-shaped curve. When resources are limited, populations show logistic growth following the Verhulst-Pearl logistic equation, which results in a sigmoid curve reaching carrying capacity. Predation acts as a conduit for energy transfer across trophic levels and keeps prey populations under control. Herbivores act as predators on plants, leading to evolutionary defenses such as thorns, cardiac glycosides, and toxic alkaloids. Competition occurs when closely related or even unrelated species compete for limiting resources. Gause's competitive exclusion principle states that two closely competing species cannot coexist indefinitely, though species may evolve resource partitioning to coexist. Parasitism benefits the parasite at the expense of the host. Commensalism benefits one species while leaving the other unaffected, and mutualism benefits both interacting species, often leading to co-evolution. Try each question first. Then tap it to see the answer. Birth rate = Number of new individuals / Initial population Birth rate = 8 / 20 = 0.4 0.4 offspring per lotus per year Death rate = Number of deaths / Initial population Death rate = 4 / 40 = 0.1 0.1 individuals per fruit fly per week dN / dt = r × N × [(K - N) / K] dN / dt = 0.1 × 500 × [(1000 - 500) / 1000] dN / dt = 50 × [500 / 1000] = 50 × 0.5 = 25 25 individuals per year Ecosystem . Biodiversity and Conservation . Evolution . The 13 chapters of Class 12 Biology . CBSE does not give marks to one chapter. It prints 10 marks for Unit X, Ecology and Environment, which includes Organisms and Populations. No. They are written by SwaVid from the NCERT textbook. The CBSE curriculum is the reference for what is assessed. Ecosystem. SwaVid is an AI tutor for Indian students in Classes 6 to 12. It finds the earlier ideas a child missed, then teaches today’s chapter. Start free to see how it works. - Ch 11 NCERT book - 10 marks, Unit X - 3 quick checks - Confusing birth rate with absolute number of births . Birth rate is expressed as the change in numbers per capita, calculated by dividing the number of new individuals by the initial population size. - Treating carrying capacity K as a variable that changes with population size . Carrying capacity K is a constant limit set by the available resources in a specific habitat for a given species. - Assuming that competition occurs only between closely related species . Totally unrelated species can also compete for the same resource, such as visiting flamingos and resident fishes competing for zooplankton in South American lakes. - Mixing up commensalism and amensalism . Remember that commensalism is a (+, 0) interaction where one benefits and the other is unaffected, while amensalism is a (-, 0) interaction where one is harmed and the other is unaffected. ## Key Links - [CBSE](https://www.swavid.com/cbse) - [Class 12](https://www.swavid.com/cbse/class-12) - [Biology](https://www.swavid.com/cbse/class-12/biology/chapters) - [Start free →](https://www.swavid.com/signup) - [All 13 chapters](https://www.swavid.com/cbse/class-12/biology/chapters) - [NCERT Biology (Reprint 2026-27)](https://ncert.nic.in/textbook.php) - [Open notes](https://www.swavid.com/cbse/class-12/biology/chapter/ecosystem) - [Open notes](https://www.swavid.com/cbse/class-12/biology/chapter/biodiversity-and-conservation) - [Open notes](https://www.swavid.com/cbse/class-12/biology/chapter/evolution) - [Class 12 Biology](https://www.swavid.com/cbse/class-12/biology/chapters) - [Start free →](https://www.swavid.com/signup)