The most distinctive birds are the ones most at risk of extinction

It’s bad enough that Earth could be losing thousands of species each year. Now two independent studies of birds have concluded the ones most likely to disappear are those that serve unique—and possibly irreplaceable—functions in their ecosystems. Consider the toucan: Its iconic beak lets it eat and disperse seeds and fruit too large for other birds in South American rainforests. Yet these striking creatures, as well as vultures, ibises, and others with distinctive physical traits, are likely to be the first to go extinct, homogenizing the avian world, according to one study. A second paper predicts communities will grow more alike as species flock to cooler regions in the face of climate change.

“That’s alarming because we know that diversity of sizes and shapes and behaviors is a signature of a healthy community,” says Scott Edwards, an evolutionary biologist at Harvard University who was not involved with the work. “This is laying out the grim world we are going to be facing.”

He and others hope the papers will spur conservationists to think more broadly about what biodiversity means. “The impacts of human actions can actually be worse than what we might think just based on species tallies alone,” says Jedediah Brodie, an ecologist at the University of Montana. “We haven’t focused on what kinds of functions we might be losing,” adds Marta Jarzyna, a macroecologist at Ohio State University.

Every ecosystem depends on diverse organisms to fill a variety of roles. Among birds, for example, some eat and disperse seeds whereas others eat carrion, helping to recycle remains. Special traits aid these tasks: long, pointed beaks help vultures tear into flesh whereas long legs keep wading birds’ bodies dry. “When communities are homogenized, they lose a lot of those ecological functions,” Brodie says.

To assess whether birds with certain sets of traits are disappearing, Emma Hughes, a newly minted Ph.D. at the University of Sheffield, spent several years measuring the beak size and shape, lower limb and wing length, and body size of museum specimens of almost 8500 birds from around the world. Working with conservation biologist David Edwards and macroecologist Gavin Thomas, both also at Sheffield, she used statistical techniques to chart similarities and differences among the species based on these traits. Many birds, such as songbirds, clustered together in shape. Outliers included big albatrosses and tiny hummingbirds, and ibises with their long, curved beaks.

Next, the researchers began to pare down this chart by removing the birds most likely to go extinct, using the International Union for Conservation of Nature’s Red List of Threatened Species, which ranks species according to their probability of disappearing. The most threatened species also turned out to be the most distinctive in body shape and ecosystem function, the group reports today in Current Biology. As the researchers sequentially removed species from most to least threatened, toucans, hornbills, hummingbirds, and other distinctive species dropped out; similar birds—think finches and starlings—remained.

To identify regions where the trend might hit hardest, the Sheffield team analyzed birds in 14 major habitats or biomes, such as tropical grasslands. They found that species homogenization will affect 12 of the 14 and will be most extreme in flooded grasslands and tropical forests. The “most imperiled” regions included South Vietnam, Cambodia, and the Himalayan foothills, as well as islands such as Hawaii, which has already lost all its distinctive honeycreepers, Hughes notes. “In some cases, there are no other organisms that can replace the unique ecological roles that these species play,” Brodie says.

A separate study led by Alke Voskamp, a macroecologist at the Senckenberg Biodiversity and Climate Research Centre, identified another homogenizing force: climate-driven shifts in birds’ ranges. Her team mapped the current ranges of 9882 bird species compiled by BirdLife International. Then they applied projections from global climate models to predict where those species could find hospitable habitat through 2080. Finally, the researchers analyzed how the altered distributions would change the makeup of bird communities. “[Moving] reshuffles the communities a lot,” Voskamp says.  

As expected, tropical and subtropical regions will likely lose the most species as they either go extinct or shift their range. A few species will move in, but most are expected to be closely related to one another and have similar traits for surviving in those conditions, Voskamp and colleagues reported 19 July in the Proceedings of the Royal Society B.

Northern North America and Eurasia will gain species as birds migrate away from regions that become too warm. But there, too, many of the newcomers will be closely related to species already present, the data showed. For example, the collared flycatcher (Ficedula albicollis) will move further into Northeast Europe, joining its cousin the pied flycatcher (Ficedula hypoleuca).  

Both studies drive home that humanity’s biggest impact on biodiversity as time goes on “will be the rearrangement of these species’ communities,” says Jonathan Davies, a biodiversity scientist at the University of British Columbia.

Taken together, the papers add up to a forecast of a more uniform avian world, a sad prospect for birdwatchers as well as a blow to ecosystems. They “add a largescale perspective to our understanding” of biodiversity loss, Jarzyna says. Already, some evidence suggests homogenization is also happening in amphibians and possibly mammals. In the future, “It’s the more unique species that will be at a disadvantage,” she says.

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