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Organisms and Populations

Organisms and Populations explains how individual organisms respond to their physical environment and how groups of the same species behave as populations. The chapter connects abiotic factors such as temperature, water, light and soil with survival, reproduction and distribution of organisms. A population is studied through measurable attributes such as birth rate, death rate, sex ratio, age distribution and density. These attributes help predict whether a population is growing, stable or declining, which is why age pyramids and growth curves are important exam areas. Population growth is explained through exponential and logistic models. Exponential growth shows rapid increase under unlimited resources, while logistic growth shows how limiting resources slow growth near carrying capacity. Population interactions such as mutualism, competition, predation, parasitism, commensalism and amensalism show that species do not live in isolation. These interactions affect natural selection, population size, community structure and ecological balance. Adaptations and responses such as regulation, conformation, migration and suspension explain how organisms cope with changing environments. Examples such as kangaroo rat, hibernation and aestivation are often used to test cause-effect understanding.