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

View Article: PubMed Central - PubMed

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.

No MeSH data available.


Relationship between geography and species richness.Plot shows average number of phylogroups per biological species as a function of latitude.
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pone.0166307.g004: Relationship between geography and species richness.Plot shows average number of phylogroups per biological species as a function of latitude.

Mentions: The mtDNA data yielded an estimate of 2.4 taxa per biological species (S2 Table), and the distribution was also left skewed (Fig 3). Although species used in molecular analyses were not randomly sampled, using the same number as for the morphology study (9159) yields an estimate of ca. 22,000 species. Bootstrap re-sampling resulted in an empirical 95% confidence interval of 20,452 to 24,216 for the number of phylogenetic species of extant birds. A plot of the number of phylogroups per biological species versus latitude (Fig 4) showed that for all latitudes, biological species (on average) include more than a single phylogenetic unit. In the New World, we found that the average number of phylogroups per species ranges from 1.4 at 50° N to 3.5 near the equator, with an increasing gradient from high latitudes towards the tropics. Thus, there is not a single average that is appropriate for all geographic regions. Unlike our sampling of species for morphological analyses, the available sample of molecular studies was not random with respect to the geographic distribution of biological species diversity, reflecting some bias in the choices made by molecular systematists in selecting species for study (Fig 5). Our plot of percent sequence divergence vs. midpoint latitude for 260 pairs of sister taxa (Fig 6; S3 Table) revealed no relationship between latitude and rate of diversification (slope = 0.0001).


How Many Kinds of Birds Are There and Why Does It Matter?
Relationship between geography and species richness.Plot shows average number of phylogroups per biological species as a function of latitude.
© Copyright Policy
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC5120813&req=5

pone.0166307.g004: Relationship between geography and species richness.Plot shows average number of phylogroups per biological species as a function of latitude.
Mentions: The mtDNA data yielded an estimate of 2.4 taxa per biological species (S2 Table), and the distribution was also left skewed (Fig 3). Although species used in molecular analyses were not randomly sampled, using the same number as for the morphology study (9159) yields an estimate of ca. 22,000 species. Bootstrap re-sampling resulted in an empirical 95% confidence interval of 20,452 to 24,216 for the number of phylogenetic species of extant birds. A plot of the number of phylogroups per biological species versus latitude (Fig 4) showed that for all latitudes, biological species (on average) include more than a single phylogenetic unit. In the New World, we found that the average number of phylogroups per species ranges from 1.4 at 50° N to 3.5 near the equator, with an increasing gradient from high latitudes towards the tropics. Thus, there is not a single average that is appropriate for all geographic regions. Unlike our sampling of species for morphological analyses, the available sample of molecular studies was not random with respect to the geographic distribution of biological species diversity, reflecting some bias in the choices made by molecular systematists in selecting species for study (Fig 5). Our plot of percent sequence divergence vs. midpoint latitude for 260 pairs of sister taxa (Fig 6; S3 Table) revealed no relationship between latitude and rate of diversification (slope = 0.0001).

View Article: PubMed Central - PubMed

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.

No MeSH data available.