How Many Kinds of Birds Are There and Why Does It Matter?

Abstract

Estimates of global species diversity have varied widely, primarily based on variation in the numbers derived from different inventory methods of arthropods and other small invertebrates. Within vertebrates, current diversity metrics for fishes, amphibians, and reptiles are known to be poor estimators, whereas those for birds and mammals are often assumed to be relatively well established. We show that avian evolutionary diversity is significantly underestimated due to a taxonomic tradition not found in most other taxonomic groups. Using a sample of 200 species taken from a list of 9159 biological species determined primarily by morphological criteria, we applied a diagnostic, evolutionary species concept to a morphological and distributional data set that resulted in an estimate of 18,043 species of birds worldwide, with a 95% confidence interval of 15,845 to 20,470. In a second, independent analysis, we examined intraspecific genetic data from 437 traditional avian species, finding an average of 2.4 evolutionary units per species, which can be considered proxies for phylogenetic species. Comparing recent lists of species to that used in this study (based primarily on morphology) revealed that taxonomic changes in the past 25 years have led to an increase of only 9%, well below what our results predict. Therefore, our molecular and morphological results suggest that the current taxonomy of birds understimates avian species diversity by at least a factor of two. We suggest that a revised taxonomy that better captures avian species diversity will enhance the quantification and analysis of global patterns of diversity and distribution, as well as provide a more appropriate framework for understanding the evolutionary history of birds.

Fig 1. Distribution of numbers of phylogenetic species of birds per biological species.

(a) for the total sample of 200, and (b–d) for the individual distributions of three authors.

Fig 2. Relationship between the number of generally recognized subspecies (e.g., [8]) within a biological species and the number of phylogenetic species recognized by the authors.

The Pearson correlation coefficient is 0.76. The dashed line corresponds to the expectation if all subspecies were equivalent to phylogenetic species. Solid points represent one occurrence, open circles represent two occurrences, and numbers in circles are numbers of occurrences of three or more; complete data are available in S1 Table.

Fig 3. Distribution of distinct taxa per biological species based on mtDNA data.

The average is 2.4 evolutionarily significant groups per species.

Fig 4. Relationship between geography and species richness.

Plot shows average number of phylogroups per biological species as a function of latitude.

Fig 5. Distribution of species diversity.

Number of biological species, number of biological species sampled in this study for mtDNA, and the number of phylogenetic species as a function of latitude.

Fig 6. A reconstructed plot [25] for 191 sister species of New World birds suggesting a latitudinal trend in diversification rates.

A. Divergence dates were determined using corrected (GTR +) mtDNA data and a 2%/my molecular clock. B. A plot of uncorrected mtDNA genetic distance vs. mid-point latitude for 260 pairs of sister taxa, irrespective of taxonomic rank, suggesting diversification rates do not vary by latitude.