CoachA population is the set of individuals of one species living in a given area at a given time and able to interbreed. Its size is the number of individuals; its density the number per unit area or volume; its dispersion the pattern of their spacing — clumped where resources or social life gather them (herds, schools, patches of a plant), uniform where they repel one another (territorial birds, desert shrubs competing for water), random where neither operates. Populations also have an age structure, a sex ratio, and rates of birth, death, immigration and emigration, whose balance is the growth.
Examples
Example 25.12 (Our own species)
The human population took all of history to reach one billion, in about 1800, and two centuries to reach eight. Its growth rate peaked near a year in the 1960s (doubling in 35 years) and has since fallen below , not through deaths but through births: where child mortality falls and women are educated, families shrink within a generation, and the age pyramid turns from a triangle to a column. The arithmetic of applies to us; what changed is .