The last word in ignorance is the man who says of an animal or plant: 'what good is it?' If the land mechanism as a whole is good then every part is good whether we understand it or not. If the biota in the course of eons has built something we like but do not understand then who but a fool would discard seemingly useless parts. To keep every cog and wheel is the first precaution of intelligent tinkering.

The word "biodiversity," a merger of biological and diversity, is one of those terms that has been used in so many situations that its true meaning is difficult to pin down. There are many definitions, both explicit and implied. The term was probably first coined by W.G. Rosen in 1985. Rosen's original intent was to propose a word that encompassed all components of life, as a way to explicitly capture the idea that "everything is linked to everything else." Historically, geneticists communicated with geneticists, game managers communicated among themselves, and ecologists talked with their ilk. Coining the term "biodiversity" was an attempt to pull them all together, making explicit the need to consider all biological scales when undertaking conservation planning. It was, in essence, an early declaration that ecosystems are important.

The most straightforward definition is "the sum total of all living things – the immense richness and variation of the living world" (Orians and Groom 2006). While both simple and elegant, this definition is not very informative, and really makes little sense to non-biologists. A second, and probably the most commonly understood definition, holds that biodiversity is a measure of the relative diversity among organisms present in different areas, ecosystems, or regions. This definition, by focusing on species richness – that is, simply the number of species – ignores biological levels both above species (i.e. communities, ecosystems, landscapes) and below (i.e. genetic diversity).

Herein, biodiversity will be defined by a third definition that is often used by ecologists: Biodiversity refers to the totality of genes, species, ecosystems and natural landscapes of a region (Some would add "And the relationships among these components."). An advantage of this definition is that it describes most circumstances and presents a unified view of the levels at which biodiversity is commonly identified. Figure 2.1 (Noss 1990) exhibits some common attributes in terms of composition, structure, and function of each of these levels.

Proceed to Next Section: Components of Biodiversity of Concern to Land Managers