Conservation: Evolutionary Values for All 10,000 Birds Many biologists and conservation practitioners believe that preserving evolutionary diversity should be a priority. An innovative new study measures the evolutionary distinctness of all the world’s birds and identifies the species and locations that capture the highest fraction of avian evolutionary history. Irby J. Lovette How do we assign a value to a species? Among the many potential answers to this challenge, one of the most straightforwardly applicable is to determine how much independent evolutionary history each species represents. Following this logic, a species with no close living relatives has very high value, because it alone retains the outcome of many millions of years of independent evolution. In contrast, a species that is a member of a very recently radiated ‘species flock’ would have lower value, because much of its evolutionary history is also shared by many closely related species. Conservation prioritization metrics that incorporate measures of phylogenetic distinctness have been around for a long time, but their application has been spotty [1–3]. This is in part because, like all metrics, they capture only part of what makes each species unique, and because they look backwards through past evolutionary history rather than forwards by assessing future evolutionary potential [4–6]. These metrics are nonetheless attractive because they index a fundamental component of evolutionary diversity, one that many conservationists find inherently worth preserving. In practical terms, the lack of reliable and complete evolutionary trees — phylogenies — that include every species within large groups of conservation concern have limited the broad-scale application of these phylogeny-based conservation weightings. Recently, however, supertrees for entire groups of organisms have been generated from combinations of genetic and taxonomic information, including a phylogeny for all of the nearly 10,000 species of the world’s living birds . A paper in a recent issue of Current Biology  by Walter Jetz and colleagues takes advantage of this complete bird phylogeny to determine which avian lineages are most
evolutionarily distinct, and then to identify the regions of the world that contain a disproportionately high proportion of avian evolutionary diversity. It goes on to provide more integrative weightings of each species’ evolutionary distinctness that incorporates information on the breadth of its geographic distribution, thereby identifying as particularly high-priority conservation targets those birds that have both high evolutionary uniqueness and very small geographic ranges. No previous study has ever attempted this approach on a comprehensive world-wide scale, and the resulting patterns are sometimes expected, sometimes surprising. By these measures, the single-most evolutionarily distinct bird species is the Oilbird (Steatornis caripensis), a bizarre, cave-nesting frugivore found broadly across Northern South America (Figure 1). Runners-up for this status of high evolutionary distinctness include other odd birds like the Cuckoo-roller (Leptosomus discolor) of Madagascar and the Hoatzin (Opisthocomus hoazin) of Amazonia; as the sole representatives of relatively ancient lineages, all three of these species have generally been placed alone in their respective taxonomic families . More surprisingly, the list of the top 50 most phylogenetically distinctive birds includes some that fall within more diverse families, such as the Red-throated Loon (Gavia stellata), Pied-billed Grebe (Podilymbus podiceps), and several species each of mousebirds, tinamous, and cuckoos. While perhaps a bit counterintuitive, the evolutionary distinctness of these latter species results from them falling within relatively low-diversity groups with relatively ancient origins. Even more surprising is the relatively low distinctness score of birds like the New Zealand wrens, two closely related species that together represent the notably ancient sister lineage to the group that comprises all other nearly 6000 species of passerine songbirds.
The outcomes of these weightings illustrate the inherently relative nature of phylogeny-based conservation metrics, as a species’ distinctness score depends fundamentally on its evolutionary relationships to other living species. For example, were one of the two New Zealand wren species to go extinct, the other species would instantly jump much higher in the rankings. Similarly, had the late Pleistocene extinctions not decimated the diverse assemblage of scavengers found until recently in the Americas, birds like the Andean Condor (Vultur gryphus) and King Vulture (Sarcoramphus papa) would likely be ranked far lower than they are now, in their roles as the last remnants of a once diverse group. In their Current Biology paper, Jetz and colleagues  go beyond simple phylogenetic distinctness to invent several new conservation metrics that also integrate geographic range size and current extinction risk assessments. The rationale here is that for a given level of evolutionary distinctness, species with small distributions, specialized habitat requirements, or low population numbers merit higher conservation attention than do species with broader or healthier populations. When geography is factored in, island species jump out as conservation targets of particular importance, which is not surprising given that endemic island birds naturally have some of the smallest ranges and population sizes. Measured in this way, large island nations like Indonesia, New Zealand, and Madagascar have overall bird conservation priorities that rival those of hyper-diverse countries like Peru, Colombia, and Brazil. These analyses provide a powerful new perspective on categorizing the world’s birds and habitats on the basis of evolutionary distinctness and conservation status. They deserve to be repeated and expanded as our information on avian diversity improves; for example, it was only discovered in the past month that the nondescript Spotted Wren-babbler (Spelaeornis formosus) of mainland Asia is not a babbler at all, but rather the sole member of an ancient songbird lineage, a finding that will substantially upgrade its evolutionary distinctness score . Similarly, there are many avian ‘species complexes’ around the
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prized by birdwatchers. The circle would then be closed when birders start to visit places like New Zealand and Indonesia specifically because they harbor birds with high evolutionary distinctness scores, and their ecotourism dollars feed back to underwrite conservation initiatives in those evolutionarily most special locations. References
Figure 1. One of a kind. Measures based on phylogenetic distinctness identify the Oilbird (Steatornis caripensis) of South America as the most evolutionarily distinct living bird species. On average, extant bird species result from about 6 million years of independent evolutionary history, whereas the Oilbird lineage has been evolving separately for at least 60 million years.
world in which closer study generally leads to finer splitting into multiple species; each of these changes in what we classify as a distinct bird species will change the phylogenetic distinctness metrics for those species. A further and exciting possibility is that these metrics of evolutionary distinctness have potential applications beyond weighting species and regions for conservation. For example, recreational birdwatchers worldwide
are avid list keepers, often retaining detailed records of the birds seen on a daily basis as well as a master ‘life list’. Like most of us, birders typically view the species on their lists as static entities. Yet using the same approach presented in the paper by Jetz and colleagues , every such bird list could be weighted by its total phylogenetic diversity and thereby capture the evolutionary processes that have generated the very diversity that is so
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