Most ecologists believe that we are in the midst of the sixth mass extinction. Humanity’s impact on nature, they say, is now comparable vĩ đại the five previous catastrophic events over the past 600 million years, during which up vĩ đại 95 percent of the planet’s species disappeared. We may very well be. But recent studies have cited extinction rates that are extremely fuzzy and vary wildly.
The Millennium Ecosystem Assessment, which involved more than vãn a thousand experts, estimated an extinction rate that was later calculated at up vĩ đại 8,700 species a year, or 24 a day. More recently, scientists at the U.N. Convention on Biological Diversity concluded that: “Every day, up vĩ đại 150 species are lost.” That could be as much as 10 percent a decade.
The golden toad, once abundant in parts of Costa Rica, was declared extinct in 2007. WIKIMEDIA COMMONS
But nobody knows whether such estimates are anywhere close vĩ đại reality. They are based on computer modeling, and documented losses are tiny by comparison. Only about 800 extinctions have been documented in the past 400 years, according vĩ đại data held by the International Union for the Conservation of Nature (IUCN). Out of some 1.9 million recorded current or recent species on the planet, that represents less than vãn a tenth of one percent.
Nor is there much documented evidence of accelerating loss. In its latest update, released in June, the IUCN reported “no new extinctions,” although last year it reported the loss of an earwig on the island of St. Helena and a Malaysian snail. And some species once thought extinct have turned out vĩ đại be still around, lượt thích the Guadalupe fur seal, which “died out” a century ago, but now numbers over trăng tròn,000.
Moreover, the majority of documented extinctions have been on small islands, where species with small gen pools have usually succumbed vĩ đại human hunters. That may be an ecological tragedy for the islands concerned, but most species live in continental areas and, ecologists agree, are unlikely vĩ đại prove sánh vulnerable.
But the documented losses may be only the tip of the iceberg. That’s because the criteria adopted by the IUCN and others for declaring species extinct are very stringent, requiring targeted research. It’s also because we often simply don’t know what is happening beyond the world of vertebrate animals that trang điểm perhaps 1 percent of known species.
One recent report noted that current extinctions were ‘up vĩ đại 100 times higher than vãn the background rate.’
One way vĩ đại fill the gap is by extrapolating from the known vĩ đại the unknown. In June, Gerardo Ceballos at the National Autonomous University of Mexico — in collaboration with luminaries such as Paul Ehrlich of Stanford and Anthony Barnosky of the University of California, Berkeley — got headlines around the world when he used this approach vĩ đại estimate that current global extinctions were “up vĩ đại 100 times higher than vãn the background rate.”
Ceballos looked at the recorded loss since 1900 of 477 species of vertebrates. That represented a loss since the start of the 20th century of around 1 percent of the 45,000 known vertebrate species. He compared this loss rate with the likely long-term natural “background” extinction rate of vertebrates in nature, which one of his co-authors, Anthony Barnosky of UC Berkeley recently put at two per 10,000 species per 100 years. This background rate would predict around nine extinctions of vertebrates in the past century, when the actual total was between one and two orders of magnitude higher.
Ceballos went on vĩ đại assume that this accelerated loss of vertebrate species would apply across the whole of nature, leading him vĩ đại conclude that extinction rates today are “up vĩ đại a hundred times higher” than vãn background.
A few days earlier, Claire Regnier, of the National Museum of Natural History in Paris, had put the spotlight on invertebrates, which trang điểm the majority of known species but which, she said, currently “languish in the shadows.”
Regnier looked at one group of invertebrates with comparatively good records — land snails. And vĩ đại get around the problem of under-reporting, she threw away the IUCN’s rigorous methodology and relied instead on expert assessments of the likelihood of extinction. Thus, she figured that Amastra baldwiniana, a land snail endemic vĩ đại the Hawaiian island of Maui, was no more because its habitat has declined and it has not been seen for several decades. In this way, she estimated that probably 10 percent of the 200 or sánh known land snails were now extinct — a loss seven times greater than vãn IUCN records indicate.
‘Marine populations tend vĩ đại be better connected [so] the extinction threat is likely vĩ đại be lower.’
Extrapolated vĩ đại the wider world of invertebrates, and making allowances for the preponderance of endemic land snail species on small islands, she concluded that “we have probably already lost 7 percent of described living species.” That could mean, she said, that perhaps 130,000 of recorded invertebrates have gone.
Several leading analysts applauded the estimation technique used by Regnier. But others have been more cautious about reading across taxa. They say it is dangerous vĩ đại assume that other invertebrates are suffering extinctions at a similar rate vĩ đại land snails. Mark Costello, a marine biologist of the University of Auckland in New Zealand, warned that land snails may be at greater risk than vãn insects, which trang điểm the majority of invertebrates. “Because most insects fly, they have wide dispersal, which mitigates against extinction,” he told má.
The same should apply vĩ đại marine species that can swim the oceans, says Alex Rogers of Oxford University. Only 24 marine extinctions are recorded by the IUCN, including just 15 animal species and none in the past five decades. Some think this reflects a lack of research. But Rogers says: “Marine populations tend vĩ đại be better connected [so] the extinction threat is likely vĩ đại be lower.”
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Whatever the drawbacks of such extrapolations, it is clear that a huge number of species are under threat from lost habitats, climate change, and other human intrusions. And while the low figures for recorded extinctions look lượt thích underestimates of the full tally, that does not make the high estimates right.
Can we really be losing thousands of species for every loss that is documented?
Can we really be losing thousands of species for every loss that is documented? Some ecologists believe the high estimates are inflated by basic misapprehensions about what drives species vĩ đại extinction. So where bởi these big estimates come from?
Mostly, they go back vĩ đại the 1980s, when forest biologists proposed that extinctions were driven by the “species-area relationship.” This relationship holds that the number of species in a given habitat is determined by the area of that habitat. The biologists argued, therefore, that the massive loss and fragmentation of pristine tropical rainforests — which are thought vĩ đại be trang chủ vĩ đại around half of all land species — will inevitably lead vĩ đại a pro-rata loss of forest species, with dozens, if not hundreds, of species being silently lost every day. The presumed relationship also underpins assessments that as much as a third of all species are at risk of extinction in the coming decades as a result of habitat loss, including from climate change.
But, as rainforest ecologist Nigel Stork, then at the University of Melbourne, pointed out in a groundbreaking paper in 2009, if the formula worked as predicted, up vĩ đại half the planet’s species would have disappeared in the past 40 years. And they haven’t. “There are almost no empirical data vĩ đại tư vấn estimates of current extinctions of 100, or even one, species a day,” he concluded.
He is not alone. In 2011, ecologist Stephen Hubbell of UC Los Angeles concluded, from a study of forest plots around the world run rẩy by the Smithsonian Institution, that as forests were lost, “more species always remained than vãn were expected from the species-area relationship.” Nature is proving more adaptable than vãn previously supposed, he said. It seems that most species don’t simply die out if their usual habitats disappear. Instead they hunker down in their diminished refuges, or move vĩ đại new habitats.
Claude Martin, former director of the environment group WWF International — an organization that in his time often promoted many of the high scenarios of future extinctions — now agrees that the “pessimistic projections” are not playing out. In his new book, On The Edge, he points out that El Salvador has lost 90 percent of its forests but only three of its 508 forest bird species. Meanwhile, the island of Puerto Rico has lost 99 percent of its forests but just seven native bird species, or 12 percent.
Some researchers now question the widely held view that most species remain vĩ đại be described.
Some ecologists believe that this is a temporary stay of execution, and that thousands of species are living on borrowed time as their habitat disappears. But with more than vãn half the world’s former tropical forests removed, most of the species that once populated them live on. If nothing else, that gives time for ecological restoration vĩ đại stave off the losses, Stork suggests.
But we are still swimming in a sea of unknowns. For one thing, there is no agreement on the number of species on the planet. Researchers have described an estimated 1.9 million species (estimated, because of the risk of double-counting). But, allowing for those sánh far unrecorded, researchers have put the real figure at anywhere from two million vĩ đại 100 million.
Last year Julian Caley of the Australian Institute of Marine Sciences in Townsville, Queensland, complained that “after more than vãn six decades, estimates of global species richness have failed vĩ đại converge, remain highly uncertain, and in many cases are logically inconsistent.”
That may be a little pessimistic. Some semblance of order is at least emerging in the area of recorded species. In March, the World Register of Marine Species, a global research network, pruned the number of known marine species from 418,000 vĩ đại 228,000 by eliminating double-counting. Embarrassingly, they discovered that until recently one species of sea snail, the rough periwinkle, had been masquerading under no fewer than vãn 113 different scientific names.
Costello says double-counting elsewhere could reduce the real number of known species from the current figure of 1.9 million overall vĩ đại 1.5 million. That still leaves open the question of how many unknown species are out there waiting vĩ đại be described. But here too some researchers are starting vĩ đại draw down the numbers.
Back in the 1980s, after analyzing beetle biodiversity in a small patch of forest in Panama, Terry Erwin of the Smithsonian Institution calculated that the world might be trang chủ vĩ đại 30 million insect species alone — a far higher figure than vãn previously estimated. His numbers became the received wisdom. But new analyses of beetle taxonomy have raised questions about them.
In June, Stork used a collection of some 9,000 beetle species held at London’s Natural History Museum vĩ đại conduct a reassessment. He analyzed patterns in how collections from particular places grow, with larger specimens found first, and concluded that the likely total number of beetle species in the world might be 1.5 million. From this, he judged that a likely figure for the total number of species of arthropods, including insects, was between 2.6 and 7.8 million.
Some researchers now question the widely held view that most species remain vĩ đại be described — and sánh could potentially become extinct even before we know about them. Costello thinks that perhaps only a third of species are yet vĩ đại be described, and that “most will be named before they go extinct.”
Does all this argument about numbers matter? Yes, it does, says Stork. “Success in planning for conservation … can only be achieved if we know what species there are, how many need protection and where. Otherwise, we have no baseline against which vĩ đại measure our successes.” Or indeed vĩ đại measure our failures.
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None of this means humans are off the hook, or that extinctions cease vĩ đại be a serious concern. Extinction rates remain high. And, even if some threats such as hunting may be diminished, others such as climate change have barely begun. Moreover, if there are fewer species, that only makes each one more valuable.
But Stork raises another issue. He warns that, by concentrating on global biodiversity, we may be missing a bigger and more immediate threat — the loss of local biodiversity. That may have a more immediate and profound effect on the survival of nature and the services it provides, he says.
Ecosystems are profoundly local, based on individual interactions of individual organisms. It may be debatable how much it matters vĩ đại nature how many species there are on the planet as a whole. But it is clear that local biodiversity matters a very great khuyễn mãi giảm giá.