An Overview on The Amount of Biodiversity

The "species-scape" of [Wheeler 1990] , shown here, is one way of representing the answer to this question. The size of an organism illustrated in the species-scape is in proportion to the number of known species in the taxonomic group it represents; data for thenumber of species in each taxonomic groups comes from [Wilson 1992] .

The current level of biodiversity is thought to stand at around 1.75 milliondescribed species [Hawksworth & Kalin-Arroyo 1995] .

However, investigating biodiversity at the organismal level goes beyond the simple question of species numbers.

The amount of global biodiversity:

is difficult toestimate;
is not evenly distributed between differenttaxonomic groups;
is not at a constant level overtime;
and is not eveninvestigated in a systematic way.

As current levels of biodiversity are increasinglyunder threat, an understanding of the processes that affect organismal diversity becomes ever more important.

		Bacteria (or Monera): 4,800 described species. Back to top
		Annelids: 12,000 described species. Back to top
		Molluscs: 50,000 described species. Back to top
		Echinoderms: 6,100 described species. Back to top
		Insects: 751,000 described species Back to top
		Non-Insect arthropods (crustaceans, spiders, millipedes and centipedes etc.): 123,400 known species. Back to top
		Fishes and lower chordates: 18,800 described species. Back to top
		Amphibians: 4,200 described species. Back to top
		Reptiles: 6,300 described species. Back to top
		Birds: 9,000 described species. Back to top
		Mammals: 4,000 described species. Back to top
		Fungi: 69,000 described species. Back to top
		"Algae" (photosynthetic protists and lower plants): 26,900 described species. Back to top
		"Higher plants" (terrestrial plants or embrophytes): 248,400 described species. Back to top
		"Protozoa" (non-photosynthetic protists): 30,8000 described species. Back to top
		"Coelenterates" (Cnidaria and Ctenophora): 9,000 described species. Back to top
		Sponges: 5,000 described species. Back to top
		Platyhelminthes (flatworms): 12,200 described species. Back to top
		Nematodes (roundworms): 12,000 described species. Back to top
	Bibliography For further information on organismal biodiversity, see: Biodiversity. An Introduction, chapter 2, by [Gaston & Spicer 1988] ; The Diversity of Life, chapter 8, by [Wilson 1992] ; Species Diversity in Space and Time, by [Rosenzweig 1995] ; Species inventory, by [Hammond 1992] ; Magnitude and distribution of biodiversity, by [Hawksworth & Kalin-Arroyo 1995] ; Getting the measure of biodiversity, by [Purvis & Hector 2000] . Back to top

	How Many Species? Organismal diversity is often considered in terms of "how many species are there"? Typical questions asked about this topic include: how many species have been described globally? how many species are there estimated to be on Earth? where is the "missing" biodiversity to be found? at what rate are new species discovered? and how are estimates of species numbers arrived at?	Five kingdoms of living organisms

	Described Species Numbers Current estimates suggest there are about 1.75 million described species [Hawksworth & Kalin-Arroyo 1995] . What proportion of these described species belong to each of thefive kingdoms?	Described Species Numbers

	Described Species Linnaeus was the first true taxonomist, as he systematically named and recorded new species. In his book Systema Naturae, published in 1758, he recorded some 9,000 species of plants and animals. Nowadays, it is clearly beyond any one taxonomist to keep track of the 1.75 milliondescribed species. So how has this figure of 1.75 million species been obtained? How reliable are the figures for different taxonomic groups?	Linnaeus and a modern taxonomist

	Estimated Species Numbers [Hawksworth & Kalin-Arroyo 1995] give the figure of 13.6 million as a conservativeestimate for the total number of species. Look at the bar chart and calculate the ratio of described to estimated species numbers for each taxon. Which taxa show the greatest discrepancy between described and estimated species numbers? Why is this ratio higher insome taxa but low inothers? How accurate are the estimated species numbers for each taxon? See: [Hammond 1992] , [Hammond 1995] and [Hawksworth & Kalin-Arroyo 1995] to help answer these questions.	Estimated Species Numbers

	Missing Biodiversity Where is the "missing" biodiversity - i.e. the difference between described and estimated species numbers - to be found? Investigate the various possible sources of undisclosed species richness and critically evaluate them. See: [Hammond 1992] , [Hammond 1995] and [Hawksworth & Kalin-Arroyo 1995] to help answer these questions.	Possible sources of missing biodiversity

	Benthic Diversity Traditionally deep sea (benthic) communities have been regarded as being species-poor. However, extrapolations based on recent samples of benthic diversity have produced estimates of 10 million species of marinemacrofauna and similar numbers ofmeiofaunal species [Grassle & Maciolek 1992] , [May 1992] , [Lambshead 1993] .	Sampling benthic diversity

	Fungal Diversity [Hawksworth 1991] looked at the ratio of fungi to vascular plant species in the British Isles. The ratio was then extrapolated world-wide to give an estimate of 1.5 million fungal species. This figure may be either an overestimate [May 1995] or an underestimate [Frohlich & Hyde 1999] .	Diversity of fungi and vascular plants

	Microbial Diversity There are an estimated two millionmicrobial species [Hawksworth & Kalin-Arroyo 1995] , and at least half of these are thought to be bacteria. Part of the problem in measuring this unknown microbial diversity is determining exactly what is a microbial "species". Also, one estimate suggests that over 90 % of microorganisms have never been cultivated [Amann 1995] . Direct analysis of genetic material from environmental samples may help solve this problem [Olsen 1990] , [Furhman & Campbell 1998] .	Examples of microbial diversity

	Arthropod Diversity [Erwin 1982] estimated that there might be 30 million arthropod species. Is there any evidence for such high numbers? According to [Hawksworth & Kalin-Arroyo 1995] 65 % of all species arearthropods. Where might these arthropods be found? Some experts believe that maximum biodiversity occurs in tropical forest canopies. However, there is increasing evidence to suggest ground arthropod fauna may be equally or more diverse (see: [Hammond 1992] , [Stork 1997] ).	Arthropod diversity

	Symbionts and Parasites [May 1995a] suggests that every metazoan animal or vascular plant has one specialised parasitic virus, bacterium, nematode and protozoa. Is this asource for the missing diversity? Will less complex, smaller organisms offer similar opportunities to host specific parasites?	Examples of symbionts and parasites
	Description of New Taxa 9,000 species were known to Linnaeus in 1758, whereas now there are currently 1.75 million known species. What does this say about the rate at which new species are described? Current knowledge of levels of organismal diversity is heavily biased towards certain well-studied taxonomic groups, with less well-known groups missing out. Compare the number of mammal and arthropod species known now, to the number known by Linnaeus: in 1970 there were 4,500 known mammal species, at least half of which had beendescribed by 1852, [May 1990] ; in 1970 there were 96,000 known non- insect arthropod species but only half of these had beendescribed by 1960, [May 1990] . [Hammond 1992] assumed that the catalogue of the 9,881 bird species then known was virtually complete, whereas the 3,058 known bacterial species represented only a small proportion of those still to be described. Are all these newly described speciesvalid? Just occasionally, the headlines are dominated by the discovery of a new species of bird ormammal. However, in 1983 a microscopic animal, found between sand grains on the sea bed, was placed in a newphylum and another new animalphylum was described in 1995.

	How to Estimate Species Numbers Methods for evaluating global species numbers include: predictions from current species description rates; comparisons of known to unknown floras and faunas; extrapolations from samples of biodiversity; estimates inferred from theoretical and empirical patterns. [May 1992] provides an easily readable introduction to these various methods.