The Main Candidates
As noted in the previous section, current broad characterisations of biodiversity permit a range of targets of measurement including species richness, species diversity, ecosystem function, species function, population relations, ecosystem diversity, biomass, genetic diversity, phylogenetic diversity, and many more. In this section, for the sake of manageability, we categorise that large array of strategies into four broad groups for consideration as general measures of biodiversity.
Species Diversity and Species Richness
Species diversity is an intuitively simple concept that has yielded numerous mathematical explications combining species richness, the number of species in an area, species evenness, and the relative abundance of species (see Maurer and MacGill 2011). Species richness is extremely common as a measure of biodiversity, partly due to its relative ease of discovery. It is a key variable from which many diversity metrics are constructed influencing the output of species diversity, functional, genetic, and phylogenetic measures. It is, in many contexts, a good indicator of biodiversity. Holmes Rolston goes as far to claim that species richness is biodiversity as “(s)pecies are a more evident, mid-range, natural kind” as opposed to other proposed units of biodiversity like genetic diversity or ecosystem diversity (p. 402, Rolston 2001).
Species richness is usually supplemented with other information as just counting the species present gives limited insight into the dynamics of an assemblage. Often species richness is combined with species evenness to create many of the common species diversity measures.[1] This is based on the idea that, given a species richness in an area, species diversity increases when the populations have more even abundances and vice versa. Information theory has provided the most common indices of species diversity, the Shannon evenness and the Simpson evenness indices. Other measures include: Hill's Indices, Hurlbert's “Interspecific encounter Index”, Rao's “Quadratic Entropy” Index, and Fager's Indices (See Justus 2011; Maurer and Macgill 2011).
While there is a range of ways that species diversity is calculated there is one feature common to these measures. Measures of species richness and diversity are blind to each individual species' identity. No species is treated as being more valuable to than any other. This assumption is directly rejected by measures that prioritize species by any of their individual features including morphology, genetics, or phylogeny.
- [1] For a sceptical take on the success of such measures see Justus (2011)
