Species Diversity

Species diversity refers to variation in the number and phylogenetic diversity (or evolutionary relatedness) of species present in an area. This is probably the most frequently used measure of total biodiversity (see Surrogate Measures of Overall Biodiversity, below).

To count species, we must define a species. There are several competing theories or "species concepts" (Mayden, 1997). The most widely accepted are the morphological species concept, the biological species concept, and the phylogenetic species concept.

The morphological species concept is the oldest. Although it is largely outdated as a theoretical definition, it is still widely used. This concept, as described by various authors (see, for example, Du Rietz [1930]; Bisby and Coddington [1995]), states that species are the "smallest natural populations permanently separated from each other by a distinct discontinuity in the series of biotypes."

The biological species concept, as described by Mayr (1982) and Bisby and Coddington (1995), states that "a species is a group of interbreeding natural populations unable to successfully mate or reproduce with other such groups, and which occupies a specific niche in nature."

The phylogenetic species concept, as defined by Cracraft (1983) and Bisby and Coddington (1995), states that "a species is the smallest group of organisms that is diagnosably [that is, identifiably] distinct from other such clusters and within which there is a parental pattern of ancestry and descent."

These concepts are not congruent, and considerable debate exists about the advantages and disadvantages of all existing species concepts. Some systematists take a pluralist theoretical approach: a species is a group of phy-logenetically distinct organisms (following the phylogenetic species concept) and repro-ductively isolated (following the biological species concept).

In practice, systematists group specimens together according to shared features (genetic, morphological, and physiological characters). When two or more groups show different sets of shared characters, and these differences cannot be attributed to intraspecific variation, the groups are considered different species. This approach relies on the objectivity of the phylogenetic species concept (that is, the use of intrinsic characters to define or diagnose a species) and applies it to the prac ticality of the morphological species concept, in terms of sorting specimens into groups. Kottelat (1995, 1997) used a similar approach for distinguishing species of European freshwater fish for which there was incomplete or confusing taxonomic information; he referred to his technique as the pragmatic species concept. By this, he meant that he was applying the most coherent and consistent way of defining species according to the taxonomic information available.

Regardless of their differences, all species concepts are based on the understanding that set parameters define a species and make it a discrete and identifiable evolutionary entity. If populations of a species become completely isolated, they can diverge, ultimately resulting in phylogenetic change and what is called speciation. During this process, we expect to see distinct populations representing so-called incipient species—species in the process of formation. These may be described as subspecies or some other infra-subspecific rank. However, it is very difficult to decide when a population is sufficiently different from other populations to merit its ranking as a subspecies. Difficulty also exists in defining the difference between a subspecies and a species. Categories such as subspecies, varieties, or populations are subjective measures of the magnitude of taxonomic difference and are not consistently discrete and identifiable evolutionary entities. Thus, in evolutionary terms, species are recognized as the minimum identifiable unit of biodiversity (above the level of a single organism) (Kottelat, 1997). This is the reason that species diversity represents an important and informative measure of biodiversity.

One aspect of species diversity is the number of species found in a particular region, often referred to as species richness. Global biodiversity is frequently expressed as the total number of species currently living on the earth. About 1.75 million species have been scientifically described thus far (Lecointre and Guyader, 2001), and estimates vary for the total number of species on the planet. This is partly because of differing opinions on the definition of a species. For example, the phylo-genetic species concept recognizes more species than does the biological species concept. Some scientific descriptions of species appear in old, obscure, or poorly circulated publications. In those cases, scientists may accidentally overlook certain species when preparing inventories of flora or fauna, causing them to describe and name a known species.

More significantly, some species are very difficult to identify. For example, taxonomically cryptic species look very similar to other species and may be misidentified (and hence overlooked as being a different species). Thus several different but similar-looking species, identified as a single species by one scientist, are identified as different species by another scientist. That does not, however, mean that contemporary taxonomic research is unreliable. Quite the contrary. As taxonomists obtain new collections of organisms and develop more techniques for investigating genetic and organismal diversity, they revise and refine their interpretation of species diversity and provide more reliable estimates of the total number of species.

Scientists expect that the 1.75 million scientifically described species represent only a small fraction of the total number of species on earth today. Many additional species have yet to be discovered, or are known to scientists but have not been formally described. (For a species to be recognized as valid, it must be described, according to precise rules set down by an international committee, and named in a publication.) Viral, bacterial, botanical, and zoological nomenclatures, and the nomen-

Taxon clature of cultivated plants, all have separate rules and committees (Bisby and Coddington [1995]). Scientists estimate that the total number of species on earth could range from 3.6 million up to 111.7 million (Hammond, 1995). The total number of species for any taxonomic group can be estimated from the ratio of the number of new species described each year to the number of previously described species. Estimates can also be extrapolated from the number of species collected per unit area from field samples (Stork, 1997). The range between the upper and lower figures is large because of the difficulty in estimating total species numbers for some taxo-nomically lesser known groups, such as bacteria, or groups not comprehensively collected from areas where their species richness is likely to be greatest—for example, insects in tropical rain forests. Consequently, authors have produced varying estimates for these groups. A reasonable estimate for the total number of species on earth seems to be about 13.6 million (Hammond, 1995).

Although it is important to know the total number of species on earth, it is also informative to have some measure of the different types of species that compose this biodiversity (for example, bacteria, flowering plants, insects, birds, and mammals). We do this through what is called taxonomy, the genetic, anatomical, biochemical, physiological, or behavioral features used to distinguish species or groups of species and that demonstrate diversity between species. Once ordered into a logical

Table 2

Estimated Numbers of Described Species

Number of Described Species

Percentage of Total Described Species*


Table 2

Estimated Numbers of Described Species

Number of Described Species

Percentage of Total Described Species*







Bryophyta (mosses)



Lycophyta (clubmosses)



Filicophyta (ferns)



Coniferophyta (conifers)



Magnoliophyta (flowering plants)






Porifera (sponges)









Platyhelminthes (flatworms)



Mollusca (mollusks)



Annelida (annelid worms)



Nematoda (nematode worms)















Chondrichthyes (cartilaginous fishes)



Actinopterygii (ray-finned bony fish)



Lissamphibia (living amphibians)



Mammalia (mammals)



Chelonia (living turtles)



Squamata (lizards and snakes)



Aves (birds)






*The total number of described species is assumed to be 1,747,851.

Source: Lecointre, G., and H. Le Guyader. 2001. Classification phylogénétique du vivant. Paris: Belin.



system, or classification, taxonomic diversity indicates the relatedness of groups of species, based on their shared characteristics.

Using this taxonomic information, we assess the proportion of related species among the total number of species on earth. Table 2 contains a selection of well-known taxa.

This table provides a measure of the evolutionary or taxonomic diversity of the species present in any given region. These studies correct common misconceptions about global biodiversity. For example, most public attention is focused on the biology and ecology of large, charismatic species such as mammals,

A clownfish hides in the protection of a sea anemone's tentacles. Clownfish and anemones have a symbiotic relationship—each provides the other the benefit of protection from predators. (Jeffrey L. Rotman/Corbis)

birds, and certain species of trees (for example, mahogany and sequoia). Far less public concern is paid to groups such as molluscs, insects, and, to some extent, flowering plants. However, Table 2 indicates that mammals and birds represent only a small portion of the total number of species (0.3 percent and 0.6 percent, respectively). Molluscs, on the other hand, represent about 7 percent of the total number of known species, and flowering plants 13 percent. Insects represent 47 percent of the total number of species; there are approximately 300,000 species of beetles alone, representing 17 percent of all species on earth. The greater part of earth's species diversity is often overlooked.

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