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Much of ecology is concerned with numbers and changes in numbers. Which species are common and which species rare? Why? Which species remain constant in abundance and which vary? Why? How can we reduce the numbers of a pest? Or prevent reductions in the numbers of a rare but valued species? At the heart of all such questions, there is a fundamental ecological fact of life:

(4.1)

That is, the numbers of a particular species presently occupying a site of interest (N_now) is equal to the numbers previously there (N_then), plus the number of births between then and now (B), minus the number of deaths (D), plus the number of immigrants (I), minus the number of emigrants (E).

This defines the main aim of ecology: to describe, explain and understand the distribution and abundance of organisms. Ecologists are interested in the number of individuals, the distributions of individuals, the demographic processes (birth, death and migration – also often referred to as vital rates) that influence these, and the ways in which these demographic processes are themselves influenced by environmental factors.