Science and Technological Issues
Has The Earth’s Sixth Mass Extinctional Ready Arrived?
With the steep decline in populations of many animal
species, from frogs and fish to tigers, some scientists have warned that
Earth is on the brink of a mass extinction like those that occurred only
five times before during the past 540 million years.
Each of these ‘Big Five’ saw three-quarters or more of
all animal species go extinct.
In a study published today (March 3) in Nature,
University of California, Berkeley, paleobiologists assess where mammals and
other species stand today in terms of possible extinction, compared with the
past 540 million years, and they find cause for hope as well as alarm.
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Risk of Extinction
“If you look only at the critically end angered
mammals—those where the risk of extinction is at least 50 per cent within
three of their generations — and assume that their time will run out, and
they will be extinct in 1,000 years, that puts us clearly outside any range
of normal, and tells us that we are moving into the mass extinction realm,”
said principal author Anthony D. Barnosky, UC Berkeley professor of
integrative biology, a curator in the Museum of Paleontology and a research
paleontologist in the Museum of Vertebrate Zoology.
“If currently threatened species — those officially
classed as critically endangered, endangered and vulnerable — actually went
extinct, and that rate of extinction continued, the sixth mass extinction
could arrive within as little as 3 to 22 centuries,” he said. Not too Late
Nevertheless, Barnosky added, it’s not too late to save
these critically end angered mammals and other such species and stop short
of the tipping point. That would require dealing with a perfect storm of
threats, including habitat fragmentation, invasive species, disease and
“So far, only 1 to 2 per cent of all species have gone
extinct in the groups we can look at clearly, so by those numbers, it looks
like we are not far down the road to extinction. We still have a lot of
Earth’s biota to save,” Barnosky said. “It’s very important to devote
resources and legislation toward species conservation if we don’t want to be
the species whose activity caused a mass extinction.”
Coauthor Charles Marshall, UC Berkeley professor of
integrative biology and director of the campus’s Museum of Paleontology,
emphasized that the small number of recorded extinctions to date does not
mean we are not in a crisis.
“Just because the magnitude is low compared to the
biggest mass extinctions we’ve seen in a half a billion years doesn’t mean
to say that they aren’t significant,” he said.
“Even though the magnitude is fairly low, present rates
are higher than during most past mass extinctions.”
“The modern global mass extinction is a largely
unaddressed hazard of climate change and human activities,” said H. Richard
Lane, program director in the National Science Foundation’s Division of
Earth Sciences, which funded the research. “Its continued progression, as
this paper shows, could result in unforeseen – and irreversible – negative
consequences to the environment and to humanity.”
The study originated in a graduate seminar Barnosky
organized in 2009 to bring biologists and paleontologists together in an
attempt to compare the extinction rate seen in the fossil record with
today’s extinction record. These are “like comparing apples and oranges,”
Barnosky said. For one thing, the fossil record goes back 3.5 billion years,
while the historical record goes back only a few thousand years. In
addition, the fossil record has many holes, making it is impossible to count
every species that evolved and subsequently disappeared, which probably
amounts to 99 per cent of all species that have ever existed. A different
set of data problems complicates counting modern extinctions.
Dating of the fossil record also is not very precise,
“If we find a mass extinction, we have great difficulty
determining whether it was a bad weekend or it occurred over a decade or
10,000 years,” he said. “But without the fossil record, we really have no
scale to measure the significance of the impact we are having.”
To get around this limitation, Marshall said, “This
paper, instead of calculating a single death rate, estimates the range of
plausible rates for the mass extinctions from the fossil record and then
compares these rates to where we are now.”
Mammals were chosen as a starting point as they are well
studied and well represented in the fossil record going back some 65 million
years. Biologists estimate that within the past 500 years, at least 80
mammal species have gone extinct out of a starting total of 5,570 species.
The team’s estimate for the average extinction rate for
mammals is less than two extinctions every million years, far lower than the
current extinction rate for mammals.
“It looks like modern extinction rates resemble mass
extinction rates, even after setting a high bar for defining ‘mass
extinction,’” Barnosky said.
After looking at the list of threatened species
maintained by the International Union for Conservation of Nature (IUCN), the
team concluded that if all mammals now listed as ‘critically endangered,’ ‘
endangered’ and ‘threatened’ go extinct, whether that takes several hundred
years or 1,000 years, Earth will be in a true mass extinction.
“Obviously there are caveats,” Barnosky said. “What we
know is based on observations from just a very few twigs plucked from the
enormous number of branches that make up the tree of life.”
“Our findings highlight how essential it is to save
critically endangered, endangered and vulnerable species,” Barnosky added.
If most of them die, even if their disappearance is stretched out over the
next 1,000 years, the sixth mass extinction will have arrived.” —
University of California, Berkeley