0.5 Species diversity  (Page 3/14)

An evolutionary significant unit (ESU) is defined, in conservation biology, as a group of organismsthat has undergone significant genetic divergence from other groups of the same species. According to Ryder, 1986 identification of ESUs requires the use of natural history information, range and distributiondata, and results from analyses of morphometrics, cytogenetics, allozymes and nuclear and mitochondrial DNA. In practice, manyESUs are based on only a subset of these data sources. Nevertheless, it is necessary to compare data from differentsources ( e.g. , analyses of distribution, morphometrics, and DNA) when establishing the status of ESUs. Ifthe ESUs are based on populations that are sympatric or parapatric then it is particularly important to give evidence of significant genetic distancebetween those populations.

ESUs are important for conservation management because they can be used to identify discrete components of the evolutionarylegacy of a species that warrant conservation action. Nevertheless, in evolutionary terms and hence in manysystematic studies, species are recognized as the minimum identifiable unit of biodiversity above the level of a singleorganism ( Kottelat, 1997 ). Thus there is generally more systematic information available forspecies diversity than for subspecific categories and for ESUs. Consequently, estimates of species diversity are used morefrequently as the standard measure of overall biodiversity of a region.

Species diversity as a surrogate for global biodiversity

Global biodiversity is frequently expressed as the total number of species currently living on Earth, i.e. , its species richness. Between about 1.5 and 1.75 million species have been discovered andscientifically described thus far ( LeCointre and Guyader, 2001 ; Cracraft, 2002 ). Estimates for the number of scientifically valid species vary partly because ofdiffering opinions on the definition of a species.For example, the phylogenetic species concept recognizes more species thanthe biological species concept. Also, some scientific descriptions of species appear in old, obscure, or poorlycirculated publications. In these cases, scientists may accidentally overlook certain species when preparinginventories of biota, causing them to describe and name an already known species.

More significantly, some species are very difficult to identify. For example, taxonomically "cryptic species" lookvery similar to other species and may be misidentified (and hence overlooked as being a different species). Thus, severaldifferent, but similar-looking species, identified as a single species by one scientist, are identified as completelydifferent species by another scientist. For further discussion of cryptic species, with specific examples of cryptic frogsfrom Vietnam, see Inger (1999) and Bain et al. , (in press) .

Scientists expect that the scientifically described species represent only a small fraction of the total number of specieson Earth today. Many additional species have yet to be discovered, or are known to scientists but have not beenformally described. Scientists estimate that the total number of species on Earth could range from about 3.6 million up to 117.7million, with 13 to 20 million being the most frequently citedrange ( Hammond, 1995 ; Cracraft, 2002 ).

The estimation of total number of species is based on extrapolations from what we already know aboutcertain groups of species. For example, we can extrapolate using the ratio of scientifically described species to undescribedspecies of a particular group of organisms collected from a prescribed area. However, we know so little about some groups oforganisms, such as bacteria and some types of fungi, that we do not have suitable baseline data from which we can extrapolateour estimated total number of species on Earth. Additionally, some groups of organisms have not been comprehensively collectedfrom areas where their species richness is likely to be richest (for example, insects in tropical rainforests). These factors,and the fact that different people have used different techniques and data sets to extrapolate the total number ofspecies, explain the large range between the lower and upper figures of 3.6 million and 117.7 million, respectively.

While it is important to know the total number of species of Earth, it is also informative to have some measure of theproportional representation of different groups of related species ( e.g. bacteria, flowering plants, insects, birds, mammals). This is usually referred to as thetaxonomic or phylogenetic diversity. Species are grouped together according to shared characteristics (genetic,anatomical, biochemical, physiological, or behavioral) and this gives us a classification of the species based on theirphylogenetic, or apparent evolutionary relationships. We can then use this information to assess the proportion of relatedspecies among the total number of species on Earth. contains a selection of well-known taxa.

Most public attention is focused on the biology and ecology of large, charismatic species such as mammals, birds, and certainspecies of trees ( e.g. , mahogany, sequoia). However, the greater part of Earth's species diversity is foundin other, generally overlooked groups, such as mollusks, insects, and groups of flowering plants.