In Chapter 21, we introduced the concept of population as a fundamental unit of evolution—it is populations, not individuals, that evolve. Here, we return to populations, this time as equally fundamental units of ecology. A population consists of all the individuals of a given species that live and reproduce in a particular place. Natural selection enables populations to adapt to the abiotic and biotic components of their environment. It is the traits evolved under natural selection that determine whether a population will grow or shrink under a given set of physical and biological conditions, perhaps disappearing entirely from a local environment.

How are populations characterized? What controls changes in population size? We first consider key features of populations. Then we look at what happens when there are more births than deaths, and what happens when resources such as food start to run out. We consider how individuals of different ages contribute to population growth, and how local populations are connected to one another. In the next chapter, we introduce interactions with other species and see how they shape the growth of populations. All these factors directly influence the flow of energy and materials through producers and consumers and, ultimately, the flow of genes through a population.