Strictly speaking, species diversity is the number of different species in a particular area (species richness) weighted by some measure of abundance such as number of individuals or biomass. However, it is common for conservation biologists to speak of species diversity even when they are actually referring to species richness.

Another measure of species diversity is the species evenness, which is the relative abundance with which each species is represented in an area. An ecosystem where all the species are represented by the same number of individuals has high species evenness. An ecosystem where some species are represented by many individuals, and other species are represented by very few individuals has a low species evenness. Table 6.1 shows the abundance of species (number of individuals per hectare) in three ecosystems and gives the measures of species richness (S), evenness (E), and the Shannon diversity index (H).

Shannon's diversity index H=–(∑(ρ_ilnρ_i))

See Gibbs et al., 1998: p157 [link] and Beals et al. (2000) [link] for discussion and examples. Magurran (1988) [link] also gives discussion of the methods of quantifying diversity.

In Table 6.1, ecosystem A shows the greatest diversity in terms of species richness. However, ecosystem B could be described as being richer insofar as most species present are more evenly represented by numbers of individuals; thus the species evenness (E) value is larger. This example also illustrates a condition that is often seen in tropical ecosystems, where disturbance of the ecosystem causes uncommon species to become even less common, and common species to become even more common. Disturbance of ecosystem B may produce ecosystem C, where the uncommon species 3 has become less common, and the relatively common species 1 has become more common. There may even be an increase in the number of species in some disturbed ecosystems but, as noted above, this may occur with a concomitant reduction in the abundance of individuals or local extinction of the rarer species.

Species richness and species evenness are probably the most frequently used measures of the total biodiversity of a region. Species diversity is also described in terms of the phylogenetic diversity, or evolutionary relatedness, of the species present in an area. For example, some areas may be rich in closely related taxa, having evolved from a common ancestor that was also found in that same area, whereas other areas may have an array of less closely related species descended from different ancestors (see further comments in the section on Species diversity as a surrogate for global biodiversity).

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

Although the morphological species concept (MSC) is largely outdated as a theoretical definition, it is still widely used. According to this concept:

In other words, morphological species concept states that "a species is a community, or a number of related communities, whose distinctive morphological characters are, in the opinion of a competent systematist, sufficiently definite to entitle it, or them, to a specific name" (Regan, 1926: 75 [link]).

The biological species concept (BSC), as described by Mayr and Ashlock (1991) [link], states that

According to the phylogenetic species concept (PSC), as defined by Cracraft (1983) [link], a species :

These concepts are not congruent, and considerable debate exists about the advantages and disadvantages of all existing species concepts (for further discussion, see the module on Macroevolution: essentials of systematics and taxonomy).

In practice, systematists usually group specimens together according to shared features (genetic, morphological, physiological). When two or more groups show different sets of shared characters, and the shared characters for each group allow all the members of that group to be distinguished relatively easily and consistently from the members of another group, then the groups are considered different species. This approach relies on the objectivity of the phylogenetic species concept (i.e., the use of intrinsic, shared, characters to define or diagnose a species) and applies it to the practicality of the morphological species concept, in terms of sorting specimens into groups (Kottelat, 1995 [link], 1997 [link]).

Despite their differences, all species concepts are based on the understanding that there are parameters that make a species a discrete and identifiable evolutionary entity. If populations of a species become isolated, either through differences in their distribution (i.e., geographic isolation) or through differences in their reproductive biology (i.e., reproductive isolation), they can diverge, ultimately resulting in speciation. During this process, we expect to see distinct populations representing incipient species - species in the process of formation. Some researchers may describe these as subspecies or some other sub-category, according to the species concept used by these researchers. However, it is very difficult to decide when a population is sufficiently different from other populations to merit its ranking as a subspecies. For these reasons, subspecific and infrasubspecific ranks may become extremely subjective decisions of the degree of distinctiveness between groups of organisms (Kottelat, 1997 [link]).

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

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

Scientists expect that the 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. Scientists estimate that the total number of species on Earth could range from about 3.6 million up to 117.7 million, with 13 to 20 million being the most frequently cited range (Hammond, 1995 [link]; Cracraft, 2002 [link]).

The estimation of total number of species is based on extrapolations from what we already know about certain groups of species. For example, we can extrapolate using the ratio of scientifically described species to undescribed species of a particular group of organisms collected from a prescribed area. However, we know so little about some groups of organisms, such as bacteria and some types of fungi, that we do not have suitable baseline data from which we can extrapolate our estimated total number of species on Earth. Additionally, some groups of organisms have not been comprehensively collected from 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 of species, 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 the proportional representation of different groups of related species (e.g. bacteria, flowering plants, insects, birds, mammals). This is usually referred to as the taxonomic 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 their phylogenetic, or apparent evolutionary relationships. We can then use this information to assess the proportion of related species among the total number of species on Earth. Table 6.1 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 certain species of trees (e.g., mahogany, sequoia). However, the greater part of Earth's species diversity is found in other, generally overlooked groups, such as mollusks, insects, and groups of flowering plants.