The concept of biodiversity (or "biological diversity" to give the full name) is slippery! Some biologists simply say, "I know it when I see it," others dismiss it as a 'non-concept'. The grand definition - "nothing less than the sum of all life on earth" - is not much help.

Intuitive meanings

"I know it when I see it" implies an intuitive feel for biodiversity. But different people may have different intuitions, especially if they come from different cultures or speak different languages: does "kepelbagaian" in Malay have exactly the same meaning as "diversity" in English?

Take a moment to look at the following pairs of diagrams: imagine that these are islands with different mammal populations. According to your intuition, which island is more diverse - left or right - or are they equal? And why?

First pair: Almost everyone agrees that the fauna of the right-hand island is more diverse, as it has more species (5 species vs 2).

Second pair: These have the same two species in the same proportions, so many people say there's no difference. But some folks point out that no two animals are exactly the same (unless they are clones!), so the larger population on the left-hand island has greater within-species or genetic diversity.

Third pair: These have the same five species, but the island on the right is dominated by civets, with only a few representatives of other species. High dominance is equated with low diversity, but that is not intuitively obvious!

Fourth pair: These have the same number of species in the same proportions, so some people see these as equally diverse. Others point out that the right-hand island has only ungulates, while the other has representatives of four different orders and is taxonomically more diverse. Still others note that the right-hand island has only herbivores, which the left-hand island has carnivores too, and is more functionally diverse.

Formal definitions and measures

According to the Convention on Biological Diversity "Biological diversity" means 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.

Thus, three levels of diversity are generally recognised:

• within species (genetic diversity),
• among species (species richness, biodiversity indices),
• ecosystem diversity - which last is difficult to measure as ecosystems are ill-defined.

Common meaning

In spite of these sophisticated differences in meaning, one interpretation seems paramount in popular accounts and even in much technical literature.

Claims that some location has high biodiversity are almost always followed by lists of species.

So we find that Mulu National Park : "contains 8 different types of forest - including peat swamp, health and mixed dipterocarp, moss forest and stunted upper montane vegetation - 4,000 species of fungi, 1,500 species of flowering plants. 1,700 species of mosses & liverworts and estimated 3,500 species of plants... 75 species of mammals, 262 species of birds, ... 74 species of frogs, 47 species of fish, 281 species of butterflies, 52 species of reptiles, 458 species of ants and 20,000 species of invertebrates." (Well, at least there's a nod in the direction of ecosystem diversity!)

The number of species - species richness  - seems a simple and intuitively sensible measure of species diversity.