A bird showcase at the Museum of Natural History in Rouen, France.

The Contested World of Classifying Life on Earth


For centuries, taxonomists have cataloged every living thing they could find. Expeditions have traveled the globe, searching for unknown species; museums and universities maintain entire departments devoted to classifying specimens.

But there exists no single, unified list of all the species on Earth.

The lack of consistency in taxonomy has always bothered Stephen Garnett. Every 10 years, the conservation biologist has assessed the extinction risk of Australian birds. But he repeatedly ran into inconsistencies between lists: A single species might have multiple scientific names, or, conversely, a single name could refer to different organisms. The problem, he found, extended far beyond birds. Taxonomists in different fields didn’t define species the same way, and classification systems were largely inefficient and poorly governed.

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Eventually, Garnett spoke with an ornithologist friend, Les Christidis, who shared his concerns. “And then we wrote this thing in Nature that got people stirred up,” recalled Garnett.

Their 2017 commentary in the prestigious science journal was inflammatory from the opening salvo: “For a discipline aiming to impose order on the natural world, taxonomy (the classification of complex organisms) is remarkably anarchic.”

Garnett and Christidis proposed tidying things by creating a universal set of rules for classifying all life on Earth and assigning governance to a single organization: the International Union of Biological Sciences, a nonprofit comprising international science associations.

Conservation biologist Stephen Garnett attends the Convention on the Conservation of Migratory Species of Wild Animals in February 2024. In his work studying birds in Australia, Garnett has repeatedly run into inconsistencies in the scientific names of animals.

Visual: G. Barry Baker

The notion of imposed authority enraged taxonomists, a fastidious bunch who even Garnett concedes are the opposite of anarchists. In the most prominent rebuttal, 184 people from the global taxonomy community warned in the journal PLOS Biology that the proposed bureaucracy was not only unnecessary and counterproductive, but also a threat to scientific freedom. Such governance would result in “science losing its soul,” wrote a smaller group of Brazilian and French scientists in another journal, raising the specter of Joseph Stalin and his political rejection of established science in the early 20th century.

For their part, Garnett and Christidis politely acknowledged the criticism and conceded that the problem of differing species definitions may be insolvable. But at the very least, there “is a need for legitimised global checklists of species that conservation authorities can follow,” they wrote.

The dust could have settled there, into a heap of perpetual disagreement. But Garnett wasn’t satisfied. “When it was over, I thought, ‘Well I actually want to get some change here,’” he said. So, Garnett and Christidis struck up a dialogue with some of the PLOS Biology paper’s authors, including ichthyologist Richard Pyle.

The scientists shared a sense of urgency about the need for a common language to describe biodiversity. The approximately 2 million complex organisms that humans have identified so far represent only a fraction of life on Earth. And the rate of species loss is accelerating at an alarming pace. Up to 1 million species are now threatened with extinction, according to a 2019 United Nations report.

“And then we wrote this thing in Nature that got people stirred up.”

While the taxonomists made clear that different disciplines would never submit to a central authority telling them how to define species, the group could agree on the need to compile one universally accepted list. That way, when people discuss the fate of an endangered salamander, for example, everyone can be sure they are referring to the same creature. “We need to have a common shared understanding of that,” said Pyle, “in order to communicate between what the taxonomists are discovering about the diversity of nature and the conservation biologists are doing to prioritize what limited resources we have to protect it.”

In February 2020, those discussions culminated in a three-day workshop at Charles Darwin University in Australia, where Garnett is a professor. There, an international group of scientists hammered out set of principles to guide the creation of the global species list. The group hopes to get universal buy-in — something previous efforts to create a global inventory have lacked.

Today, their vision appears to be taking shape. Although the devil is in the details, a recent survey found strong support for the idea of a catalog comprising the most accurate, up-to-date species lists from each discipline. If all goes according to plan, the initial version should be available by 2030.

“It was one of those little nice opportunities for science to work the way it’s supposed to work,” said Pyle.

Scientists from around the world gathered in 2020 to attend a workshop at Charles Darwin University, including Stephen Garnett, Les Christidis, Richard Pyle, and Frank Zachos. There, the group created a set of principles to guide the creation of a global species list. Visual: Courtesy of Stephen Garnett

Taxonomy is more than just naming things; it is the art and science of classification. Frank Zachos, who helped develop the principles at the workshop in Australia, describes taxonomy as perhaps the most fundamental biological science because it reflects how humans think about and structure the world. “We will always put things in drawers,” he said. (Zachos was speaking figuratively but also literally; as head of the mammal collection at the Natural History Museum Vienna, in Austria, Zachos noted that he actually files specimens in drawers.)

An integral part of classifying organisms is giving them a scientific name — generally a two-part name, in Latin, using a system that dates back to the 18th century, and that now extends to everything from Canis familiaris (a dog) to the Ophiocordyceps unilateralis (a parasitic fungus that hijacks ants’ minds).

An example of the scientific name and main taxonomic ranks for the red fox, Vulpes vulpes.

Visual: Annina Breen/Wikimedia Commons

At least in theory, anyone can name a new species: For example, to name a new animal species, you need to publish the name, along with a description of the species and some additional details, in a scientific journal or book chapter. You also need to designate the location of a specimen — in a museum, for example — that others can refer to. The rule, according to the International Commission on Zoological Nomenclature — which sets the standards for scientific nomenclature of animals — is one organism, one name. If a species is mistakenly named twice, the oldest published name is considered valid. Similarly, if two species wind up with the same name, the first one named gets to keep it.

But with approximately 18,000 new species identified every year, the ICZN, which comprises 26 volunteer commissioners and one full-time staff member, can only check that a name conforms to its rules. They don’t weigh in on how to define or identify species. “We keep entirely out of science,” said Thomas Pape, the ICZN’s current president.

While the ICZN occasionally does rule on naming disagreements — a decade ago it famously settled a two-century-old dispute regarding a giant tortoise — they leave it to scientists in individual fields to work out what constitutes a species. The issue is that while nature is usually continuous, our classifications, like our language, are necessarily discrete, said Zachos: “If you draw lines along a continuum, ultimately there is some level of arbitrariness if you are talking about gray areas.”

“It was one of those little nice opportunities for science to work the way it’s supposed to work.”

One of the frustrations that led Garnett to co-author the Nature commentary is that criteria differ by field. Mammal taxonomists, for example, list two populations as different “species if they have a common ancestor but differ physically or genetically,” he wrote. Bird taxonomists, on the other hand, favor the more conservative criterion that differing species can’t produce fertile offspring together. If ornithologists followed mammalian criteria, research suggests that the number of bird species would more than double.

As Garnett has discovered inventorying Australian birds, even within a field, scientists don’t always agree where to draw lines. There are at least four major international lists of birds, for example, each reflecting several points of disagreement on species identification.

And while birds, mammals, reptiles, flowers, and ferns are well cataloged, much of life on Earth lacks a taxonomic champion: someone willing to spend months and years sifting through online databases, scientific journals, and ancient texts to create databases of identified species.

In addition to his work for the ICZN, Pape, a professor at the Natural History Museum of Denmark, has also taken on the task of cataloging the world’s flies. Over the last two decades he and collaborator Neal Evenhuis, an entomologist at the Bishop Museum in Hawaii have devoted much of their free time to the fly database, scouring references dating back to a 1758 text written by Swedish biologist Carl Linnaeus and resolving countless cases where names for an insect differed. To date, they have cataloged 250,000 names for species and groups of species, representing nearly 10 percent of all animals on Earth.

With the advent of the internet and the ability to share lists online, people began to envision compiling such efforts into a master list of species. Catalogue of Life, an international collaboration, has been working on one for the last two decades. But it’s a Herculean task, said Donald Hobern, a software engineer who chairs the organization’s taxonomy group. An amateur naturalist, Hobern also volunteers his time cleaning up species lists for moths and butterflies. Those and some other sections of the Catalogue of Life databases, he said, are still “very patchy.”

But the biggest stumbling block for Catalogue of Life — or any organization that tried to assemble a global list — is that until now, scientists haven’t been compelled to resolve disputes within disciplines to create one consensus list for each field, said Hobern. “There’s no single, completely uncontroversial frame of reference that says ‘this is the right view.’” And unless the global scientific community accepts the component lists, the Catalogue of Life is just another inventory, not the authoritative list of life on Earth.

For Garnett, the problem was simple: A decentralized system of species lists has left scientists unable to talk about nature in a way that everyone understands. And that communications gap, Garnett and others suspect, has led to real-world problems.

One issue is that international treaties to conserve and protect species don’t always work from the same lists. For example, white-naped cranes are listed as Grus vipio on one conservation agreement and Antigone vipio on another. Countries also don’t always agree on how to identify a species or its protective status. In his Nature commentary, Garnett pointed out that China’s outdated official wildlife lists have left some two dozen now-endangered species exposed to illegal trade. He and his colleagues hope that the availability of a global list would help the world’s governments in keeping checklists and regulations up to date.

The white-naped crane has been listed with different species names on two international conservation agreements. Such discrepancies can leave endangered species exposed to illegal trade.

Visual: Roshan Patel/Smithsonian’s National Zoo

In a paper published after the workshop, several of the attendees highlight how disagreement over names and identifications can pose grave risks to humans as well. Taxonomy is a dynamic process: As scientific knowledge evolves, taxonomists modify species definitions and names. But changes can take years to trickle down through various bureaucracies. Currently, plants and animals that could harbor pests or disease sometimes clear customs because the name on the quarantine list doesn’t match the name on the cage or seeds, said Garnett. It “can have devastating economic effects if diseases get through.”

Garnett and others also expect that the need for one consensus list from each discipline will spur people in those fields to fill gaps and work out discrepancies. The bird folks are already talking with one another to produce a single global list, he told me in a follow-up email.

And those vetted lists will no doubt weed out spurious entries from “taxonomic vandals,” people who name things without the support of scientific peers. In one recent notable case of alleged vandalism, members of the herpetology community accused amateur Australian herpetologist Raymond Hoser of using suspect science in naming (and, in some cases, renaming) scores of snakes, geckos, skinks, and crocodiles after himself, family members, pets, or whatever else strikes him, bestowing names like Dannyleeus rayhammondi, Ctenophorus sharonhoserae, Funkichelys funki, and Hosmeria shuddafakup.

It sounds comical, but the wrong taxonomy can have deadly consequences, said Garnett. He pointed to a paper citing case studies in Africa and Papua New Guinea where confusion over snake taxonomy led to people dying of snake bites after receiving the wrong antivenin.

But perhaps most important to the group that traveled to the workshop in Australia, a global species list represents a shared language for communicating about our world.

“There’s no single, completely uncontroversial frame of reference that says ‘this is the right view.’”

I caught up to ichthyologist Richard Pyle while he was on an expedition to record fishes around Wake Island, a tiny coral atoll in the middle of the Pacific Ocean, about 1,500 miles northeast of Guam. He and his crew were taking a baseline inventory of shallow reef fishes to be able to see how populations change over time.

Pyle likens biodiversity to the world’s greatest library, an information storehouse containing 4 billion years’ worth of wisdom shaped by evolution. In this age of mass extinction, the library is on fire. Losing a species is like the last copy of a book going up in flames, said Pyle. For each identified species that goes extinct, five or 10 more disappear that humans never knew existed, he said. “What secrets are lost when we burn the last copies of those books?”

Conservationists are doing a valiant job as firefighters, but they can’t work efficiently because they are only familiar with 10 to 20 percent of the library, said Pyle. “What taxonomy does is we try to build the card catalog of that library, as quickly as we can, to help guide the efforts of conservation biology.” A continuously updated global species list ensures that everyone has the key to what we know about life on Earth.

Microscopic slide-mounted animals on display at the Grant Museum of Zoology, in London, which houses over 68,000 specimens. Biodiversity, says ichthyologist Richard Pyle, could be described as the world’s greatest library, and “what taxonomy does is we try to build the card catalog of that library.” Visual: Dan Kitwood/Getty Images

Mammal taxonomist Frank Zachos arrived at the Australian workshop extremely jetlagged, after traveling 30 hours from his home in Vienna. But the group’s collaborative attitude and laser focus was a bracing tonic. “People put everything on the table saying: ‘Okay, this, this, and this — where do we agree?’” said Zachos. They quickly found consensus around key points, he said.

One point was clear from the start: Despite Garnett and Christidis’ original vision of universal rules for defining and naming species, bureaucrats would have no authority over taxonomic science. “We are not telling the insect guy how to do insect taxonomy. That’s up to the entomologists,” said Zachos. “What we want is a certain quality management for a final insect or bird list.”

Mammal taxonomist Frank Zachos, who is also head of the mammal collection at the Natural History Museum Vienna, in Austria, was one of many who attended the 2020 workshop in Australia.

Visual: Frank Zachos

During their 2020 meeting, the group settled on 10 principles that spell out the criteria that individual lists must meet to be included in the master inventory of species. The individual lists must be based on science, rather than optimized for political or even conservation considerations, for example. And the groups creating them must record and report on their methods.

In addition, the group emphasized that lists should be built with local expertise. As it stands, says Zachos, too often people from the Global North are talking about biodiversity in the Global South without including the perspective of people who live there.

In 2021, members of the group published a series of papers that justified the need for a global list and detailed how it would work. The goal is to get people who create and use species lists on board, said Zachos. “All the authority that we will have comes from the quality of the work.”

So far, the idea seems popular. A 2022 survey of more than 1,000 people, mostly taxonomists and other scientists, found widespread enthusiasm, with more than three-quarters of respondents supporting the development of a governance system to create and maintain a single list of life on Earth.

“We did not expect to see as much agreement as we did,” said lead author Aaron Lien, an environmental scientist at the University of Arizona. Because of the early controversy, he said that he was pleasantly surprised that so many taxonomists backed the global list — though respondents were still divided over who, precisely, should oversee the project.

Catalogue of Life seems like an obvious home for the global list. “Not taking Catalogue of Life on board would be, in many regards, also reinventing the wheel and also probably disrespecting all the work that has gone on to into that,” Zachos said. “So, I think it makes a lot of sense to team up with them.” Currently, most of the financial support for the organization has come from short-term grants, particularly from the European Commission and the National Science Foundation in the U.S. But it would likely need additional funding, possibly from international conservation and scientific organizations, to take it on. 

“We are not telling the insect guy how to do insect taxonomy. That’s up to the entomologists.”

Ichthyologist Lynne Parenti, who’s listed as an author on the PLOS Biology comment criticizing the original proposal by Garnett and Christidis, but who was not involved in crafting the principles, thinks that there is great value in a global list and is also cautiously supportive of putting Catalogue of Life in charge of it. Wherever the list lives, it must be updated regularly, said Parenti. “Our understanding of the world is not static.”

As for how to handle disputes, Garnett envisions a separate organization comprised of taxonomists and users of taxonomy that could act as an arbitrator.

These days, Pape, the entomologist and ICZN president, is feeling optimistic. His database of fly species is nearly complete, so he and his colleague are mostly focused on housekeeping tasks such as fixing misspellings and inserting references as well as adding the 1,000 or so new species identified every year. After we first spoke, I followed up to ask Pape why he has devoted his life to cataloging insects. He is driven, he said, by the words of his taxonomic predecessor Carl Linnaeus: “If you do not know the names of things, the knowledge of them is lost, too.”


Teresa Carr is a Colorado-based investigative journalist and the author of Undark's Matters of Fact column.