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One-quarter of world’s mammals at risk, says new analysis of extinction rates

For this survey the journal Nature pulled together the most reliable data it could find on the current state of species endangerment worldwide.

Sunday’s death of a white rhino at the San Diego Zoo’s safari park brought the known worldwide population of the world’s rarest large mammal to five — two more in zoos, three in a Kenyan preserve.
REUTERS/Petr Josek Snr

Mass extinction has been a sobering theme in 2014’s flow of environmental news, starting with publication of Elizabeth Kolbert’s book “The Sixth Extinction”  in February, proceeding through a groundbreaking report on threatened bird species by the Audubon Society in September, and concluding now with a new survey by the journal Nature.

Nature is all about sound science and respecting scientific uncertainty, but also about making complex issues intelligible to nonexperts.

For this survey, it pulled together the most reliable data it could find on the current state of species endangerment worldwide, such as the latest listings by the International Union for Conservation of Nature, then mapped the grim tallies in a series of graphics, some of them interactive.

For example, it represents IUCN’s list of endangered animals as a vast field of colored dots, each representing a single threatened species of mammal (1,199 of them), bird (1,373), amphibian (1,957) or insect (993).

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Moving your mouse pointer over these fields turns up a species name beneath each dot, from the Abbott’s duiker (an African antelope) to the zugi (a Cuban frog).

On a percentage basis, amphibians are by far the most endangered of those four animal groups – about four out of 10 known species are in trouble, many of them because of chytrid fungi.

But more than one-quarter of all known mammals are endangered, as are about one-seventh of the world’s known birds.

The species we don’t know about

“Known” is an important modifier here, because another theme of the Nature assessment is how little is known, still, about the earth’s full range of biodiversity.  The 993 endangered insects, it notes, are only about one-half of 1 percent of the 1 million evaluated species — but another 4 million haven’t even been looked at.

The latest IUCN report, Nature notes, “evaluated more than 76,000 species, a big increase over earlier editions.

But that is just 4 percent of the more than 1.7 million species that have been described by scientists, making it impossible to offer any reliable threat level for groups that have not been adequately assessed, such as fish, reptiles and insects.

Even where efforts have been strongest, there are still known gaps and some of them are thought to be pretty big.

We’re doing best with plants; the official taxonomy includes about 307,700 species out of an estimated worldwide total thought to be no larger than some 450,000.

Not quite so well for animals; the tally of described species is 1,371,500, with estimates of the actual total ranging from about 2 million to perhaps 11 million.

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And with fungi, well, we’ve only just begun – 48,500 described, out of an estimated 600,000 to 10 million.

Discovery amid destruction

Turning to the question of whether today’s extinction rate is likely – as Kolbert argues  to qualify as a sixth “mass extinction,” defined as the loss of at least 75 percent of current species, Nature is making no bets for the moment:

Of all the species that have populated Earth at some time over the past 3.5 billion years, more than 95% have vanished — many of them in spectacular die-offs called mass extinctions. On that much, researchers can generally agree. Yet when it comes to taking stock of how much life exists today — and how quickly it will vanish in the future — uncertainty prevails.

Studies that try to tally the number of species of animals, plants and fungi alive right now produce estimates that swing from less than 2 million to more than 50 million. The problem is that researchers have so far sampled only a sliver of Earth’s biodiversity, and most of the unknown groups inhabit small regions of the world, often in habitats that are rapidly being destroyed.

As for future patterns, predictions are easier for some extinction scenarios – like birds threatened by loss of unique and necessary habitat – than for others. And there is considerable disagreement about the current rate of extinctions, too.

The effects of climate change, which are hard to forecast in terms of pace and pattern, will probably accelerate extinctions in as-yet unknown ways.

One simple way to project into the future would be to assume that the rate of extinction will be constant; it is currently estimated to range from 0.01% to 0.7% of all existing species a year. “There is a huge uncertainty in projecting future extinction rates,” says Henrique Pereira, an ecologist at the German Centre for Integrative Biodiversity Research in Leipzig.

If those percentages seem quite small, falling as they do to the right of the decimal point, consider that as applied only to animal species, lower estimates suggest that creatures are vanishing at a pace of about 500 per year, or 10 every week.

At the upper end, it’s 690 animal species per week, nearly 36,000 per year.

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Human activity is the driver

As for the driving factors behind these losses, there is much greater scientific consensus and the simple answer is human activity in one form or another.

Habitat pressures remain the No. 1 cause, driving some 44 percent of extinctions. Interestingly, Nature distinguishes “habitat degradation and change,” thought to be driving 31 percent of extinctions, from “habitat loss,” driving 13 percent.

The other factors, largest to smallest: exploitation, such as overfishing (37%), climate change (7%), pollution (4%), disease (2%).

And if Nature finds the patterns of widespread extinctions a challenge to scientific measurement and characterization, that does not lessen their gravity. In an accompanying commentary, the editors put it this way:

There are 22,413 species deemed at risk of extinction by the International Union for Conservation of Nature (IUCN). If some ambitious person tried to read out their names — without any breaks for food or water — it would take at least half a day. But that would be just the start.

The IUCN has assessed the status of only 76,199 of the 1.7 million species of animals, plants, fungi and protists on Earth that have been described by scientists. And some suggest that at least five times more species still wait to be discovered. Many of these are also threatened, and it would take months to read out all of their names. (Except that they do not, of course, have names.)

Before human populations swelled to the point at which we could denude whole forests and wipe out entire animal populations, extinction rates were at least ten times lower. And the future does not look any brighter. Climate change and the spread of invasive species (often facilitated by humans) will drive extinction rates only higher…

The pace of extinction is leading towards a crisis. If all currently threatened species were to go extinct in a few centuries and that rate continued, the die-offs would soon reach the level of a mass extinction — the kind of biological catastrophe that ended the reign of the dinosaurs and that has happened only five times in Earth’s history. The sixth mass extinction could come in a couple of centuries or a few millennia, but it lies somewhere in the future if nations keep to their present course.

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A marquee example on the verge:

Sunday’s death of a white rhino at the San Diego Zoo’s safari park brought the known worldwide population of the world’s rarest large mammal to five — two more in zoos, three in a Kenyan preserve.

Natural reproduction is now thought to be impossible, according to a report in the Guardian, but in-vitro fertilization is being considered. Mostly, one supposes, because there’s nothing else to be done.