In this chapter we change the emphasis of our approach. We will not be concerned so much with the interaction between individuals and their environment, as with the numbers of individuals and the processes leading to changes in the number of individuals. In this regard, there is a fundamental ecological fact of life:
This simply says that the numbers of a particular species presently occupying a site of interest (Nnow) is equal to the numbers previously there (Nthen), 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) that influence these, and the ways in which these demographic processes are themselves influenced by environmental factors.
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