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Unifying Measures of Biodiversity: Understanding When Richness And Diversity and Distributions, (Diversity Distrib.) (2013) 1–10 BIODIVERSITY Unifying measures of biodiversity: RESEARCH understanding when richness and phylogenetic diversity should be congruent † Caroline M. Tucker1,2 and Marc W. Cadotte1,2*† 1Department of Biological Sciences, ABSTRACT University of Toronto, Scarborough, 1265 Aim Biogeographical theory and conservation valuation schemes necessarily Military Trail, Toronto, ON M1C 1A4, 2 involve assessing how biodiversity is distributed through space and ‘biodiversity’ Canada, Department of Ecology & Evolutionary Biology, University of Toronto, encapsulates many different aspects of biological organization and information. 25 Willcocks Street, Toronto, ON M5S 3B2, While biogeography may try to explain biodiversity patterns, successful conserva- Canada tion strategies should attempt to maximize different aspects of diversity. Ulti- mately, diversity patterns are the product of evolutionary history, and research and conservation efforts seek to understand the unequal distribution of evolu- tionary history. For conservation efforts, results have been inconsistent as to A Journal of Conservation Biogeography whether species richness (SR) provides sufficient surrogacy for evolutionary his- tory. Here, we provide a conceptual framework allowing for the direct compari- son of taxonomic richness and phylogenetic diversity (PD), both in terms of their mechanistic relationship and the relationship between their spatial distributions. Location Global. Methods We present a framework that relates regional SR, PD, biogeographi- cally weighted evolutionary distinctiveness and biogeographically weighted SR. Further, we use simulations to illustrate how the size of the species pool, topo- logical patterns within the phylogeny and autocorrelation in spatial distribu- tions affect the correlation among metrics. Results In regions that include both recently diversified groups and ancient species poor lineages, large species pools and low spatial autocorrelation, the correlation between biodiversity measures is lower than regions with low rich- ness, balanced phylogenetic trees and high spatial autocorrelation. Main conclusions We can now understand and predict when regional richness and PD should be strongly correlated. This congruency is the product of evolu- tionary and ecological processes that determine species pool membership and *Correspondence: Marc W. Cadotte, community assembly. Further, in regions where SR is not expected to be Department of Biological Sciences, University congruent with phylogenetic distinctiveness, re-examining how existing reserve of Toronto-Scarborough, 1265 Military Trail, networks protect the multiple aspects of biodiversity is critically important. Toronto, ON, Canada. E-mail: [email protected] and Distributions Keywords † Biogeography, conservation prioritization, evolutionary history, habitat protec- Both authors contributed equally to this work. tion, phylogenetic diversity, spatial distributions. extremely large areas. Understanding this inequality in the INTRODUCTION distribution of species has been the focus of the creative Global patterns of biological diversity reveal stark contrasts. energy of numerous scientists (e.g. MacArthur & Wilson, Diversity Some regions contain thousands of species in relatively small 1967; Gaston & Blackburn, 2000) and has served as the basis areas, while elsewhere there may only be a few species over of global conservation prioritization (Myers et al., 2000; DOI: 10.1111/ddi.12087 ª 2013 John Wiley & Sons Ltd http://wileyonlinelibrary.com/journal/ddi 1 C. M. Tucker and M. W. Cadotte Fleishman et al., 2006). The recognition that the term diver- the degree of phylogenetic conservatism among traits and sity is not synonymous with species richness (SR), but the degree that trait divergence follows Brownian motion instead encompasses organismal variety at all levels, from evolution. Specific traits and lineages often fail to meet these genetic variation to the differences in the richness of higher assumptions, and some researchers have found functional taxa, and includes the diversity in ecosystem structure and diversity and PD vary independently (Safi et al., 2011). function (Wilson & Peter, 1988), has led researchers to mea- Regardless, researchers often use phylogenetic information to sure the spatial distribution of different aspects of diversity represent unknown aspects of species ecologies or simply as (Faith, 1992; Forest et al., 2007; Devictor et al., 2010; Huang a representation of similarities in the information contained et al., 2011; Tucker et al., 2012). Such comparisons aim to within their genomes. To this end, a number of studies have understand the biogeographical relationship between differ- examined the spatial distribution of PD and delineate sites ent facets of diversity. This type of research has been moti- with disproportionately high PD (Moritz, 2002; Rodrigues & vated, in part, by the fact that historically reserves have not Gaston, 2002; Forest et al., 2007; Devictor et al., 2010; focused on aspects of diversity beyond richness and ende- Tucker et al., 2012). mism. Therefore, it is reasonable to examine the efficacy of On its own, SR is not ecologically meaningful, and con- existing reserves in protecting other facets of biodiversity sidering other forms of diversity that capture species differ- (Devictor et al., 2010; Huang et al., 2011; Tucker et al., ences becomes important. With a particular focus on 2012). In addition, comparing different biogeographical conservation, a number of studies have questioned the effi- distributions of diversity allows researchers to potentially infer cacy of richness as a surrogate for other types of diversity different mechanisms generating and maintaining different and have called for more multifaceted approaches to conser- aspects of diversity. For example, studies examining latitudi- vation (Crozier, 1997; Bonn & Gaston, 2005; Fleishman nal gradients of SR often infer the influence of climate on et al., 2006; Devictor et al., 2010; Davies & Cadotte, 2011). speciation rates (Weir & Schluter, 2007), whereas biogeo- Studies that examine the congruence between species (or graphical studies that focus on genetic diversity often find generic) and PD have been inconsistent. For example, Devic- that vicariance or natural barriers are critically important tor et al. (2010) found a large spatial mismatch between the (Kuo & Avise, 2005). species, functional and PD of birds across France; these There is a long history of measuring and mapping pat- measures were congruent in some areas and incongruent in terns of SR across biogeographical regions throughout the others, possibly depending on the history of the regional world (Wallace, 1876; Whittaker, 1954, 1960; Preston, 1960; species pool in each area. They found that phylogenetic and Stevens, 1989). As the importance of alternative forms of functional diversity was underrepresented in the current diversity is increasingly recognized (Faith, 1992, 1994; Diaz reserve network, relative to SR. Two papers that compared & Cabido, 2001; Cadotte et al., 2011), documenting patterns the spatial distribution of generic or species diversity in the of other measures of diversity such as phylogenetic and Cape Floristic Region of South Africa (Forest et al., 2007; functional diversity become an important exercise. For diver- Tucker et al., 2012) similarly found evidence of spatial sity and conservation research, having a precise estimate of incongruence between SR and PD. Conversely, several stud- ecological or functional diversity is beneficial. However, eco- ies found that PD and taxonomic diversity to have similar logically meaningful functional diversity is often difficult to spatial distributions: for example, Rodrigues & Gaston quantify due to a lack of comprehensive trait information (2002) found that phylogenetic and generic richness of birds for species in a region, or an incomplete understanding of in north-west South Africa showed high spatial congruence, how traits correspond to ecological differences. and reserve site selection was complementary. Perez-Losada A related measure that is used as a surrogate for functional et al. (2002) found little difference in conservation priorities diversity is phylogenetic or evolutionary diversity, which for Chilean freshwater crabs, regardless of whether SR or PD quantifies the amount, distribution or evenness of evolution- was considered (although Faith & Baker, 2006 raise doubts ary information contained within species assemblages. There about these results). Similar conclusions were made regard- are a number of ways to measure phylogenetic diversity ing Ozark crayfishes (Crandall, 1998). This marked variation (PD) in communities (Webb et al., 2002; Cavender-Bares in the observed relationship between SR and PD appears to et al., 2009; Cadotte et al., 2010b), but methods that quantify makes it difficult to draw conclusions regarding the relation- either the amount of evolutionary history or the evolutionary ship between these measures. distinctiveness of a set of species are most appropriate to The relationship between PD and species diversity depends examine spatial patterns of diversity (Faith, 1992; Isaac, on the phylogenetic topology and the geographical distribu- 2007; Cadotte & Davies, 2010; Davies & Cadotte, 2011). The tion of species (Rodrigues et al., 2005). For example, in most often used measure is Faith’s (1992) PD, which is the regions with large, diverse species pools, particularly
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