Where does biodiversity collide?

The variety of life in a place condensed into a single number that combines how many species are present, called richness, with how evenly individuals are spread among them, called evenness. Simpson's 1949 Measurement of diversity proposed one such index, the probability that two individuals drawn at random belong to different species, and many rival indices followed it. Hurlbert's 1971 The nonconcept of species diversity warned that these indices can rank the same communities in different orders and share no common scale, so calling any one of them the biodiversity of a site hides a choice among measures that need not agree. Here the referent is a measured community property, not a length on an evolutionary tree, a treaty's conservation goal, or a priced asset.[1][2]

Unit

One community's diversity, as a dimensionless index of richness and evenness

Recovery means

Not applicable

Measured by

Computed from a species-by-abundance count using an index such as Simpson's D or the Shannon function, which need not rank sites the same way

The variety of life measured as evolutionary history, summed along the branches of the tree that connects the species in a place, so that a set holding distantly related lineages scores higher than an equally rich set of close cousins. Faith's 1992 Conservation evaluation and phylogenetic diversity defined this quantity, phylogenetic diversity or PD, as the total branch length spanning a group of taxa, arguing that conservation should protect evolutionary history rather than mere species counts. Because a place rich in species can be poor in PD, and the reverse, this measure can rank sites in the opposite order to a species index. The referent is a length on a phylogeny, not a community's evenness, a legal target, or a priced service.[3]

Unit

A set of taxa's diversity, as total branch length on their phylogeny

Recovery means

Not applicable

Measured by

Summed from a phylogenetic tree as the branch length spanning the taxa of interest

The variety of life reframed as an economic asset, a stock of natural capital whose functioning yields flows of valuable services such as pollination, clean water, and climate regulation. Costanza and colleagues' 1997 The value of the world's ecosystem services and natural capital estimated those flows in monetary terms, placing biodiversity on the same ledger as produced capital and human capital. In this meaning what matters is the value that variety delivers, so a species that supplies no measured service can look economically redundant even when it is evolutionarily distinct or legally protected. The referent is a priced stock and its service flows, not a community index, a branch length, or a treaty's goal.[4]

Unit

A region's biodiversity, as a stock of natural capital priced by its service flows

Recovery means

Not applicable

Measured by

Valued by estimating ecosystem service flows and converting them to monetary units, for example per hectare per year