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Measuring Biodiversity

From the definition alone, which is very broad, it is clear that no single measure of biodiversity will be adequate. Biodiversity can not be captured in a single number. Essentially, measurements have two components:
• The number of entities e.g. the number of individuals, the number of species, the number of different habitats etc.
• The degree of difference (dissimilarity) between those entities.
Species richness (the number of species) describes the number of elements but will not capture information on the number of individuals of the species. Rarity and conservation status, dependant on how threatened a species is, can also provide a measure of biodiversity.

Measures of biodiversity are commonly used as the basis for making conservation decisions or for planning more generally. Different measures of biodiversity may support different solutions. Often indicator species are used as a way of measuring biodiversity. Using this method can be very useful but it introduces an aspect of how we value different components of biodiversity. For example, we are more likely to use the abundance of birds or butterflies on a farm as a measure of biodiversity than the richness of microbes in the soil.

Genetic diversity can be measured directly by looking at genes and chromosomes or indirectly by looking at physical features of the organisms and assuming they have a genetic basis. Using the genetic code is arguably the strongest method of measuring biodiversity as it is looking at the building blocks of life.

Generally, multicellular organisms tend to have more DNA than single-celled organisms but there are exceptions. Similarly, although there appears to be an overall trend of increasing amount of DNA with increasing complexity of organisms, this is not invariant. The species with the greatest amount of DNA has about 100,000 times as much as that with the least, but the species with the largest number of genes has only 20 times as many genes as that found in many bacteria. In other words, much genetic variation is attributable not to differences in the number of functional genes, but in the amounts of non-coding DNA. One of the most striking findings is that there are many ‘universal’ gene segments in a wide range of organisms suggesting the existence of an ancient minimal set of DNA sequences that all cells must have.

Whilst biodiversity can be measured in a host of ways, in practice it tends to be measured in terms of species richness; the number of species.
The advantages of this are:
• Practical application: Species richness has proven to be measurable in practice, at least to the point where different workers will provide similar estimates of species numbers.
• Existing information: A substantial amount of information already exists on patterns in species richness, and this has been made available in scientific literature.
• Surrogacy: Species richness acts as a surrogate measure for many other kinds of variation in biodiversity. In general, as long as the numbers involved are at least moderate, greater numbers of species tend to embody more genetic diversity (in the form of a greater diversity of genes through to populations), more species diversity, and greater ecological diversity.
• Wide application: The species unit is commonly seen as the unit of practical management, of legislation and of political discourse. For a wide range of people, variation in biodiversity is pictured as variation in species richness.

The disadvantages are:
• Definition of species: The lack of agreement as to what constitutes a species. In major part this results because species can, to a large extent be regarded as hypotheses, opinions or concepts, as much as real robust entities. Click here to see the different definitions of species. The vast majority of groups of organisms have been, and are still being, described based on collections of preserved specimens using differences in morphological characteristics. Fortunately, this method of defining a species continues to be relatively effective for most multicelluar species. It is not an adequate method of defining single cell organisms.
• Different kinds of diversity: An additional limitation of species richness as a measure of biodiversity has frequently been illustrated with reference to the issue of whether an assemblage of a small number of closely related species, say two species of mouse, is more or less biodiverse than an equivalent sized assemblage of more distantly related species, say a species of mouse and a species of shrimp.


Photo Credit: Sammy, Leatherback Turtle

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