Populations

Individuals of a species are rarely found living entirely alone — most live in groups sharing a defined geographical area, competing for the same resources, and capable of interbreeding, which together is what defines a population. (Even asexually-reproducing organisms are treated as populations for ecological purposes, despite there being no literal interbreeding.) All the cormorants in a wetland, the rats in an abandoned building, the teak trees in a forest tract, the bacteria in a culture plate, the lotus plants in a pond — each is a population.

A population has properties an individual organism simply doesn't. An individual is born and dies; a population has birth rates and death rates — expressed per capita, as a change in numbers relative to the population's own size. For example, if a pond held 20 lotus plants last year and 8 new ones were added through reproduction, the birth rate works out to 8/20 = 0.4 new plants per lotus per year. …