Why lists of worldwide bird species disagree: Data gaps and species similarities may lead to undercounting biodiversity

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Biologists set out to compare four main lists of bird species worldwide to find out how the lists differ -- and why. They found that although the lists agree on most birds, disagreements in Southeast Asia and the Southern Ocean could mean that some species are missed by conservation ecologists.
How many species of birds are there in the world? It depends on whose count you go by. The number could be as low as 10,000 or as high as 18,000. It's tough to standardize lists of species because the concept of a "species" itself is a little bit fuzzy.

That matters because conserving biodiversity requires knowing what diversity exists in the first place. So biologists, led by University of Utah doctoral candidate Monte Neate-Clegg of the School of Biological Sciences, set out to compare four main lists of bird species worldwide to find out how the lists differ -- and why. They found that although the lists agree on most birds, disagreements in some regions of the world could mean that some species are missed by conservation ecologists.

"Species are more than just a name," Neate-Clegg says. "They are functional units in complex ecosystems that need to be preserved. We need to recognize true diversity in order to conserve it."

The results are published in Global Ecology and Biogeography.

On the origin of species

The definition of a species isn't clear-cut. Some scientists define populations as different species if they're reproductively isolated from each other and unable to interbreed. Others use physical features to delineate species, while yet others use genetics. Using the genetic definition produces many more species, but regardless of the method, gray areas persist.

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"Species are fuzzy because speciation as a process is fuzzy," Neate-Clegg says. "It's a gradual process so it's very difficult to draw a line and say 'this is two species' vs. 'this is one species.'"

Also, he says, physical features and genetic signatures don't always diverge on the same timescale. "For example," he says, "two bird populations may diverge in song and appearance before genetic divergence; conversely, identical populations on different islands may be separated genetically by millions of years."

Comparing the lists

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