Global taxonomic diversity of living reptiles

Abstract

Reptiles are one of the most ecologically and evolutionarily remarkable groups of living organisms, having successfully colonized most of the planet, including the oceans and some of the harshest and more environmentally unstable ecosystems on earth. Here, based on a complete dataset of all the world's diversity of living reptiles, we analyse lineage taxonomic richness both within and among clades, at different levels of the phylogenetic hierarchy. We also analyse the historical tendencies in the descriptions of new reptile species from Linnaeus to March 2012. Although (non-avian) reptiles are the second most species-rich group of amniotes after birds, most of their diversity (96.3%) is concentrated in squamates (59% lizards, 35% snakes, and 2% amphisbaenians). In strong contrast, turtles (3.4%), crocodilians (0.3%), and tuataras (0.01%) are far less diverse. In terms of species discoveries, most turtles and crocodilians were described early, while descriptions of lizards, snakes and amphisbaenians are multimodal with respect to time. Lizard descriptions, in particular, have reached unprecedented levels during the last decade. Finally, despite such remarkably asymmetric distributions of reptile taxonomic diversity among groups, we found that the distributions of lineage richness are consistently right-skewed, with most clades (monophyletic families and genera) containing few lineages (monophyletic genera and species, respectively), while only a few have radiated greatly (notably the families Colubridae and Scincidae, and the lizard genera Anolis and Liolaemus). Therefore, such consistency in the frequency distribution of richness among clades and among phylogenetic levels suggests that the nature of reptile biodiversity is fundamentally fractal (i.e., it is scale invariant). We then compared current reptile diversity with the global reptile diversity and taxonomy known in 1980. Despite substantial differences in the taxonomies (relative to 2012), the patterns of lineage richness remain qualitatively identical, hence reinforcing our conclusions about the fractal nature of reptile biodiversity.

Figure 1. Phylogenetic distribution of genera and species diversity among currently known families of living reptiles.

The six major reptile groups are differentiated in colours, as detailed in the top-right box. The lizard families Dibamidae and Hopolcercidae, and the amphisbaenian families Cadeidae and Rhineuridae are not shown because of conflicting phylogenetic information. Birds and other vertebrates have been excluded from the tree.

Figure 2. Historical rates of reptile species descriptions (dots) and rate of accumulation of new species (continuous line) since Linnaeus (1758), to the most recent species appeared to March 2012.

The two top plots show rates for reptiles as a whole and for the squamate clade, respectively, while the remaining ones focus on major reptile groups. The tuatara is not shown given the single-species richness of the order Rhynchocephalia. The time scale shown in the bottom plots is identical to the timescales of the plots above them.

Figure 3. Frequency distributions of reptile biodiversity.

The left plot depicts the overall frequency distribution of species per genera for all reptiles together, and the distributions for major clades separately (tuataras and amphisbaenians not shown) in the inset plots. For lizards and reptiles in general, the genera Anolis (A) and Liolaemus (L), and for snakes Atractus (At) and Typhlops (T) are indicated with black arrows. The right plots depict the same distributions, but for genera within families. Crocodilians and tuataras are not shown given the low number of families and genera.

Figure 4. Species richness in reptilian taxa.

Overall, the number of species per genus in a family is not directly correlated with the number of genera per family (A). However, the number of genera is proportional to the number of species per family in all major reptile groups (B). Each data point represents a family.

Figure 5. Distribution of the world’s reptile species as the accumulation of relative diversity from the richest (Anolis) to the poorest genera based on a 2012 and a 1980 dataset (main plot).

The 2012 relationship reveals that 50% of global reptile diversity is accounted for by the 93 richest genera only, all of them squamates, and 92 being lizards and snakes. The inset plot displays the accumulation of species for both datasets as absolute species numbers per genera. Anolis is treated as a single large genus (see text for details).