Quantifying Biodiversity: a Phylogenetic Perspective

Quantification of biodiversity presents a two-fold problem. First, there is the question of what exactly we should be counting. If that question is not difficult enough, it leads straight to the second practical question of how we can possibly accomplish such a count. At the species level, for example, even if we can decide what a species is (for discussion see Wheeler & Meier 2000), we must in practice use surrogate information to quantify relative numbers of species in different places. These same issues recur when biodiversity is defined below the species level. Debates about taxonomic distinctiveness in biodiversity conservation (Vane-Wright et al. 1991; Faith 1992a; Williams et al. 1994), although initially concerned with quantification of biodiversity at the level of species, led to a recognition that what we would like to count when setting species priorities is biodiversity below the species level-broadly, "feature diversity" (Faith 1992a, 1992b; 1994a, 1994b; Williams et al. 1994). Furthermore, the rationale in such prioritization, conserving feature diversity to maximize "option value" (Faith 1992a, 1992b)-dictates that those "features" must include not only observed characters of organisms but also unobserved characters. Clearly, we cannot count all those features and so must turn to something like taxonomy and phylogenetic pattern to capture character diversification (Faith 1992a, 1992b; but also see Faith 1996). Representation of "evolutionary history" (Faith 1994b), encompassing processes of cladogenesis and anagenesis, is assumed to provide representation of the feature diversity of organisms. Specifically, the phylogenetic diversity (PD) measure estimates the relative feature diversity of any nominated set of species by the sum of the lengths of all those phylogenetic branches spanned by the set (Faith 1992a, 1992b, 1994b). Although that method has subsequently been used in various biodiversity toolboxes (BioRap [Faith & Walker 1996] and Worldmap [Williams