Organisms and Populations — Class 12 Biology

This chapter introduces ecology at the levels of the organism and the population. It explains how organisms interact with their abiotic environment, with major factors being temperature, water, light and soil, and how these define habitats and biomes. Organisms respond to environmental stress by being regulators (maintaining homeostasis), conformers, by migration or by suspending activities through adaptations such as hibernation, aestivation, diapause and the formation of resistant stages. Adaptations, structural, physiological and behavioural, that fit organisms to their environment are described with examples like the kangaroo rat, desert plants, allen's rule and altitude sickness. At the population level, attributes such as population density, birth and death rates, sex ratio, age pyramids and dispersion are explained. Population growth is modelled by exponential growth (J-shaped curve, with intrinsic rate of natural increase r) and logistic growth (S-shaped curve constrained by carrying capacity K). The chapter then discusses population interactions, classifying them as mutualism, competition, predation, parasitism, commensalism and amensalism, with examples such as the predator-prey relationship, brood parasitism in the cuckoo, plant-pollinator mutualism in figs and wasps, and Gause's competitive exclusion principle.