According to the United Nations-sponsored Convention on Biological Diversity, biodiversity means “the variability among living organisms from all sources … and the ecological complexes of which they are part; this includes diversity within species, between species and of ecosystems” [source: Convention on Biological Diversity].

While counting species is one of the most commonly-used measures of biodiversity, scientists have moved toward a more holistic approach, looking at diversity in the larger context of the ecosystem.

No species lives in isolation, and the health of a particular plant or insect depends on the network of vegetation, animals and microscopic organisms in the surrounding area. Ecosystems work to efficiently process nutrients, energy and waste in a continuing cycle. Every element of that cycle must be in place in order for it to work, so measuring the amount of one particular species does not give a clear view of the health of the overall environment.

Species richness, which examines the number of each species in a given area, is one aspect of a region’s biodiversity. But other elements include measuring the diversity within a certain species, as well as the number of other populations of that species in other geographic regions. A species may be well represented on a certain California coastline, for example, but found nowhere else on Earth. Should that coastline be damaged by environmental changes, the species would disappear. 

To make things even more complicated, scientists don’t always agree about what constitutes a species. New ones are discovered and named all the time, while debates rage about whether similar-looking insects or birds found in different areas constitute the same species or not. The very malleability of what it means to be a species highlights the difference between the unruly natural world and the rational, logical scientific process. As the environmentalist Edward O. Wilson writes, “The imperfections of the concept, and thereby of our classification system, reflect the idiosyncratic essence of biological diversity. They give even more reason to cherish each species as a world unto itself, worthy of lifetimes of study.” [source: Wilson].    

Sources:
Eldredge, Niles. Life in the Balance: Humanity and the Biodiversity Crisis. Princeton University Press. 1998.
Novacek, Michael (ed.). The Biodiversity Crisis. The New Press. 2001.
Spicer, John. Biodiversity. Oneworld Publications. 2006.
Wilson, Edward O. The Diversity of Life. Harvard University Press. 1992.

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