Some scientists estimate around 2 million animal and plant species inhabit Earth. Others approximate around 8 million. Bug experts note there are probably some 5.5 million insects species alone. One recent analysis estimates tens of millions of animal species (this number is dominated by insects and tiny arthropods).
For centuries, biologists, ecologists, and taxonomists have documented critters around the globe. But, in a world teeming with biodiversity and remote ecosystems, they acknowledge there are plenty more species out there. The broad assumption is that humanity has named around 1.5 to 1.7 million plant and animal species, so far. Everything else is an educated guess.
"It's hard to even know quite how many species have been described -- never mind the difficult challenge of knowing how many haven’t been described," said Andy Purvis, a biodiversity researcher at the National History Museum of London.
"It is embarrassing," he added, noting that after some 260 years of scientific work, the number of known species is still uncertain -- and woefully incomplete.
"We’re muddling along with a fractional knowledge of species diversity."
"We’re muddling along with a fractional knowledge of species diversity," agreed Quentin Wheeler, an insect taxonomist and founding director of the International Institute for Species Exploration.
On a planet with a 24,901-mile circumference and little-explored pockets of extreme biodiversity, our grasp of species may forever be imperfect. "The debate about how many species there are is perpetual," said Seabird McKeon, an ecologist at the University of Central Florida.
But the healthy scientific debate continues. Purvis recently led an exhaustive UN assessment of the planet's biodiversity, compiled by 145 scientists, which concluded that 1 million plant and animal species on Earth are threatened with extinction -- a number much larger than the some 27,000 species known to be threatened. In a letter entitled "No inflation of threatened species" published last week in the journal Science, Purvis and other scientists defended their estimates, based upon the evaluation that some 8 million species likely inhabit Earth. Eight million species, he noted is "not the high end or low estimate," but a moderate approximation.
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Yet Mark Costello, a biodiversity expert at the University of Auckland, disagrees. He argued, via an earlier letter published in Science, that 8 million species is inflated, and instead estimates some 2 to 2.7 million animal and plant species inhabit the planet, of which about 27,000 are threatened ("Conservationists do not need to exaggerate the crisis facing the world’s biodiversity" -- as there's problem enough, Costello wrote online).
"There is not really consensus on how many species because many people do not study the evidence in detail and perpetuate meaningless 'up to' statement estimates for the sake of dramatic effect," Costello said over email. There is just not enough evidence to support larger estimates of potential species, he emphasized. In the face of so much environmental gloom, he argued such a high number (1 million threatened species) could cause people to feel emotionally fatigued about an overwhelming problem.
Though Purvis appreciates the genuine scientific dialogue and acknowledges the uncertainty in estimating undiscovered life, he said the known number of 27,000 threatened species doesn't capture the looming potential for extinction. "We know [27,000 species] is a big underestimate of the true extent of the problem," he explained.
The UN report -- undoubtedly grim -- sought to estimate the likely state of nature, which is different than the current state of knowledge, Purvis underscored. He summed this up in his Science letter with a quote from the statistician John Tukey: "Far better an approximate answer to the right question, which is often vague, than an exact answer to the wrong question, which can always be made precise."
Species estimations aside, biologists can all agree on this: We need to look harder.
"Instead of going to the moon and Mars -- which are ostensibly lifeless places -- what a worthwhile endeavor it would be to figure out how many species there are on Earth and to describe them all," said John Wiens, an ecologist at the University of Arizona.
"The bottom line is we have to do a lot more species exploration," said the insect taxonomist Wheeler.
As an undergraduate student, Wiens described nine new species of amphibians "living in muddy holes" in South America. Lots of people had visited the area before, but no one had really scrutinized what might lie hidden in the mud. "People are finding new things in places where people have been before," he said. "There's lots of stuff like that out there."
"There are things around us every day that people just haven’t taken a good look at yet," said the University of Central Florida's McKeon.
"Mites are insane."
Even many "known" species are little-known, McKeon emphasized. He recently ran the "National Biodiversity Championship" in the greater Louisville, Kentucky area. Over the course of four days about 200 people found 760 species, many of which were barely documented. Some had never been seen before in Kentucky.
"In the center of the U.S. we are surrounded by a depth and richness of biodiversity that humbles the best of our scientists," said McKeon. "Go to a rainforest or a coral reef, or any of the wild places left on the planet, and we are simply over our heads."
Some species estimates are hard to wrap your head around. Wiens' research estimates that perhaps some 100 million (or more) animal species inhabit the planet. That's because for every insect species (around 5.5 or 6 million), each bug is likely walking around with at least one mite species, he said.
"Mites are insane," said Wiens.
Mites are small arthropods that live in the nooks and crannies on insects. There are mites that live specifically on an army ant's antennas, noted Wiens. What's more, insects also carry little worms, called nematodes, in their stomachs. So when one accounts for the unique mite and nematode species that live on unique insect species, the number of potential species explodes.
"What we’re understanding now is that many species are structural species," said McKeon, referring to a species that provides a habitat for other species. "That ramps up our [species] estimations tremendously." (McKeon doesn't accept any one estimate, but advises folks to recognize that there is both a middle ground and wide spectrum of possibilities).
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Costello, though, is not too keen on higher, or highly extreme estimates, specifically from mite and nematode species. "Sounds like more of a wild extrapolation from very little data," he said.
Fortunately, the key to much of the planet's unknown biodiversity may be somewhat hard to reach, but it's no secret. "It's not like we don’t know where [unknown species] are going be," said the National History Museum's Purvis.
In remote archipelagos and mountain slopes there are bounties of life to reveal. Take the Andes Mountains, the longest continental mountain range on Earth. It's filled with valleys. "Every one of those valleys has a different species of frog," said McKeon. Imagine what else lives there, he mused.
The most critical question of all, however, isn't how many species there might be on Earth. "It's a distraction from the real topic: How many of those species are threatened or endangered because of human activity," asked McKeon.
"Saying a million species are threatened strikes me as extremely conservative."
The number accepted in the UN report by Purvis, 1 million, is a fair estimate, if not an underestimate, noted Wheeler. "Saying a million species are threatened strikes me as extremely conservative," he said.
"It's at least a million that are threatened," agreed Wiens. "It's not less than a million."
The primary culprits endangering animals and plants globally are destroyed wilderness, critters exploited for their horns and furs, accelerated climate change, and widespread pollution. About 70 percent of the ice-free land on Earth is now affected by humans, which inevitably means far less wild habitat. The modern extinction rate is the highest it's been in human history, and is "tens to hundreds of times" higher than the normal rate of extinction over the last 10 million years, the UN report concluded.
It's not just little-known species that have been hit with extinction in modern history. The report emphasized that larger, spined creatures have been impacted, at the rate of well over 100 species per century since the 1500s: "At least 680 vertebrate species have been driven to extinction by human actions since the 16th century," the report reads.
Elevated extinction levels are all the reason to look for new species -- before they're gone for good.
"There's every reason to ramp up species exploration," said Wheeler. "Very few fossils are left behind, so it's a now-or-never proposition."
Better get moving, said McKeon, who is presently identifying a species of florescent crustacean. "There's a huge amount of work to do."
Topics Animals Sustainability
