The most straightforward definition is “variation of life at all levels of biological organization”. A second definition holds that biodiversity is a measure of the relative diversity among organisms present in different ecosystems. “Diversity” in this definition includes diversity within a species and among species, and comparative diversity among ecosystems.
A third definition that is often used by ecologists is the “totality of genes, species, and ecosystems of a region”. An advantage of this definition is that it seems to describe most circumstances and present a unified view of the traditional three levels at which biodiversity has been identified:
- genetic diversity – diversity of genes within a species. There is a genetic variability among the populations and the individuals of the same species.
- species diversity – diversity among species in an ecosystem. “Biodiversity hotspots” are excellent examples of species diversity.
- ecosystem diversity – diversity at a higher level of organization, the ecosystem. Diversity of habitat in a given unit area. To do with the variety of ecosystems on Earth.
The 1992 United Nations Earth Summit in Rio de Janeiro defined “biodiversity” as “the variability among living organisms from all sources, including, ‘inter alia’, terrestrial, marine, and other aquatic ecosystems, and the ecological complexes of which they are part: this includes diversity within species, between species and of ecosystems”. This is, in fact, the closest thing to a single legally accepted definition of biodiversity, since it is the definition adopted by the United Nations Convention on Biological Diversity.
If the gene is the fundamental unit of natural selection, according to E. O. Wilson, the real biodiversity is genetic diversity. For geneticists, biodiversity is the diversity of genes and organisms. They study processes such as mutations, gene exchanges, and genome dynamics that occur at the DNA level and generate evolution.
For ecologists, biodiversity is also the diversity of durable interactions among species. It not only applies to species, but also to their immediate environment (biotope) and their larger ecoregion. In each ecosystem, living organisms are part of a whole, interacting with not only other organisms, but also with the air, water, and soil that surround them.