Biodiversity has become something of a buzzword in recent years, used to denote the variation in species within an area. This entry will attempt to explain its exact meaning, as opposed to the fashionable usage, firstly by examining a few areas of high biodiversity, then by exploring the science behind measuring biodiversity.
Most Biodiverse Areas
The Amazon Rainforest1 is widely accepted as the most biodiverse environment on Earth, and not without justification. Some areas have as many as 400 species of tree in a square kilometre - for comparison, Europe or North America struggle to get ten species of tree in a square kilometre. By including bats (which are not always included in surveys of mammals), the number of species of native mammals surpasses the African plains. The bulk of its species, as with virtually all areas worldwide, are insects. A million species are believed to live there and in this Researcher's experience, most of them bite! Insect species are outnumbered only by bacteria; it is difficult to find even rough estimates of the numbers of species there may be. Some statistics claim that as many as one third of all the world's species live in the Amazon - see the US WWF report for instance.
However, as we shall see, it can be difficult - indeed impossible - to come up with an exact measurement of a region's diversity. The Darien in Panama is the most biodiverse area in some taxonomic groups. Costa Rica, Brazil, Indonesia and Australia each claim to be the most biodiverse country in the world, though a country can include more than one terrain type so this means very little in scientific terms - some of the 'best' sections of the Amazon are in its extreme west, in Peru and Bolivia, but it would make no sense for ecologists to count these areas as separate from the Brazilian Amazon.
Habitats such as cerrados (Brazilian grasslands) and coral reefs can rival rainforest for diversity - Australia's claim to high biodiversity, for example, centres on its having both offshore reefs and onshore rainforests.
An honorable mention should also go to the Congo. Although narrowly bested by the Amazon as the largest expanse of forest in the world, it is, if anything, even less explored. Madagascar, Southeast Asia and Central America all also score highly in terms of numbers of species.
It is noticeable that all these high biodiversity areas are high rainfall, and several are centred on large rivers. It appears that water, rather than soil fertility, is the primary factor limiting species multiplication. Further, access to this water causes an increase in numbers of species, not simply in one species dominating all others. This compares, for example, to the finches in the Galapagos Islands, which Darwin noticed were a single species diversifying to fill newly-available niches in these geologically young islands. South-east Asia's rainforests are believed to be the oldest ecosystem on Earth, at 70-100 million years old according to the fossil record. Stability is also believed to be a primary cause of species diversity.
The wooden spoon for biodiversity goes jointly to the Sahara Desert (especially in Mali/Niger), the Antarctic and the extreme north of Siberia as the least biodiverse places on Earth.
Scientific Definition and Measurement
One of the fundamental problems of defining an area's biodiversity is defining the area. This is a familiar problem in many areas of science. Though at first it may seem that the difference between jungle and grasslands is obvious, closer inspection will reveal that 'jungle' is actually composed of several types of terrain, with some plants common to all and other plants appearing in only one or two. Rather than the jungle ending in a sharp line at, say, grasslands, it will steadily thin out into forest and woodlands. This is usually resolved by dividing an area into grid squares and listing the species within each square, without assigning an area a 'terrain type'. Choosing the size and centre of the squares therefore can affect the overall results.
Then there is your definition of biodiversity - genetic diversity, species diversity and ecosystem diversity are all recognised variants.
Genetic diversity is the ultimate goal, a true measure of the ability of a species, or collection of species, to survive environmental change. However, this is very difficult to measure, as, without resorting to massively expensive genome-mapping projects, we can only judge by time-consuming studies of heredity. While it seems obvious that dogs have greater diversity than humans, for example, it can be impossible to tell whether two varieties of lizard are able to produce offspring together2 by inspection alone.
Species diversity is the most commonly used as it is easiest to measure - a simple count of species within an area. This may seem the obvious logical method, but it too has its drawbacks; it gives no indication of the relative abundance of each species, for example. A rainforest may have 300 species of tree, whereas a North American forest may only have five to 12, so the rainforest is more diverse on a species count. However, this does not take into account the fact that the rainforest might (hypothetically) be dominated by just one or two species of tree, with the remaining species represented by just one or two individual trees per square kilometre, making this a far more fragile system. There are mathematical methods for taking this into account to come up with an index number showing biodiversity in each grid square, but each method will give a differing result3. This also takes no account of how similar the species are to each-other (so five trees are as biodiverse in this system as a tree, an insect, a mammal, a fish and a bird - see 'genetic diversity' above for problems with this), and there can be huge practical difficulties with attempting to count every insect species in a given area, for example. A grid of a few tens of squares on a side, with each square containing a few hundred species, will rapidly lead to hundreds of thousands or millions of presence/absence records. No attempt is made to distinguish between native and foreign species; once a species is established in an area, it makes a contribution to current and future biodiversity, irrespective of how it was introduced.
Finally, ecosystem diversity is rarely used due to the difficulties with defining an 'ecosystem', as touched upon in the section on defining an area, above. This is the name given to any measure of the variation of morphologies and behaviours within an ecosystem. Although this gives another good indication of ecological viability of a system, it is all but impossible to measure objectively.
The most 'acceptable' method of measuring biodiversity remains species diversity. However, this is very time-intensive to measure accurately in practical terms. A shortcut is therefore often used, counting only families of species and using this to estimate how many individual species are represented.
Why Biodiversity Matters
Genetic diversity in particular indicates the health of an ecosystem and its survival potential. Any change in climate can cause serious damage to an ecology, and due to the complex network of interrelations, this damage tends to spread; removing one species leads to the deaths of any other species that feed on it, and remove controls on the growth of any species it fed on. Fragile ecosystems are at risk of collapse from even quite minor environmental changes, whether man-made or natural.
Measuring biodiversity can prioritise areas for conservation. With only limited funding available to any organisation, this function is absolutely vital.
Finally, there is a practical argument to go with the 'ethical' arguments above, repeated so often that it has becomes a cliché - elimination of the poorly-understood biological systems in these areas risks eliminating useful properties. Around a quarter of all medicines were developed from plants found in very high biodiversity areas. To explain why this has become such a cliché, it is worth pointing out that we do not yet have a full understanding of the biochemistry of any living organism. Every animal alive contains dozens of enzymes of unknown but useful function. Multiply this by the number of species and the implications for medicine, chemistry or even physics are obvious.
The Natural History Museum in the UK and the Smithsonian Institute in the USA both have excellent websites - the SI's is aimed primarily at schoolchildren and teachers, whereas the NHM's is more research-oriented.