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Michael's Blog: Zoology
Articles
Extinction:
Bye Bye, Birdie ...
By Sarah DeWeerdt,
World Watch. Posted August 7, 2006.
...
and thousands of other creatures. Prominent biologists say we're
on par with the five previous mass extinctions in the history of
life on earth.
In
April 2004, a computer and electronics professor named David Luneau
paddled a canoe through a swamp forest in eastern Arkansas and captured
a blurry video of a crow-sized bird perching on the trunk of a tupelo
tree and then flying off into the woods. The bird had large white
patches on the trailing edges of its wings and a vee of white stripes
on its back -- characteristic features of the ivory-billed woodpecker,
last seen in the United States 60 years before and widely believed
to be extinct.
Since
then, numerous search parties have been launched to comb that patch
of forest for more evidence of the bird's existence, and scientists
have been examining the video frame by frame and debating whether
it really depicts an ivory-billed woodpecker or just a more common,
similar-looking pileated woodpecker. Has this lost creature revealed
itself to human eyes again after six decades -- or is the bird a
figment of our wishful thinking? One thing is certain, says Duke
University conservation biologist Stuart Pimm: "If it survives,
it's a lonely bird."
Lonely,
except that in one sense it has lots of company: species that are
lost, or nearly so, are increasingly common because human activities
are driving them to extinction 1,000 times faster than the normal
rate, according to the just-released report, Global
Biodiversity Outlook 2. The report echoes the United Nations'
Millennium Ecosystem Assessment, published last year, and proclaims
that a "sixth mass extinction" is under way, the worst
loss of species since the dinosaurs died out 65 million years ago.
Such
dire claims have attracted some skeptics, however. Mostly journalists
and economists, they start with the argument that nobody even knows
how many other species share the planet, so how can anyone claim
to know what the extinction rate is? Taxonomists have named and
described around 1.5 million species, but estimates of the actual
total range from 5 million to over 15 million. A frequently cited
mid-range estimate is 7 million species, but that's by no means
an exact or universally accepted figure.
Because
of this uncertainty, says Pimm, the aim should be simply to calculate
a relative extinction rate rather than the absolute number of disappearing
species. Pimm and a group of colleagues first laid out these ideas
in a 1995 paper in Science that has become probably the most widely
accepted approach to quantifying species loss.
Only
a few of those 1.5 million described species are known well enough
to assess how they're doing; what's known about many species derives
from single specimens hiding in dusty museum cabinets somewhere.
So in order to say something meaningful about extinction rates,
it's necessary to pick a well-known group of organisms and treat
them as a sample of the larger total. Fortunately for Pimm, an ornithologist,
birds make a good sample group. Although there are still occasional
surprises, it's generally well known how many kinds of birds there
are, which ones have disappeared, and when. And, he says, the fact
that there are just about 10,000 species of birds in the world greatly
simplifies the arithmetic involved.
About
130 kinds of birds have vanished around the world over the past
century and a half. That's a pretty firm number: "We have a
body count and we have names," Pimm says. There's the great
auk, for example, driven to extinction in the 19th century by hunters
who sought its feathers, meat, and oil. There's the Lana'i hookbill,
lost in the early 1900s when its habitat was destroyed for pineapple
plantations, and the New Zealand bush wren, a ground nester that
proved easy prey for introduced rats and was last sighted in 1972.
The bird extinction rate is about one per 10,000 species per year,
or 100 extinctions per million species per year, since the middle
of the 19th century. Of course, extinction is a natural process;
no species lives forever. So the real question is how the current
extinction rate compares to the usual rate at which species come
and go (the back-ground rate).
To
determine the background extinction rate, scientists look to the
fossil record and to genetic material, or DNA, which accumulates
small changes in its sequence as it is copied and passed down from
generation to generation. Because these minor copying mistakes occur
at a known rate, they can act as "molecular clocks" to
help establish how long ago closely related species diverged and
to track other aspects of species history. This evidence suggests
that under normal circumstances species survive for one million
to 10 million years. If species typically lived for only one million
years, then we ought to see one extinction per million species-years,
or one per million species per year." And so what that tells
us is that the rate of bird extinctions is a hundred times greater
than it should be," says Pimm.
In
their 1995 paper, Pimm and his colleagues also performed similar
analyses of mammals, reptiles, frogs and toads, and freshwater clams
(dividing the number of extinctions witnessed over the past century
by the total number of known species in each of these groups) and
came up with similar results: current extinction rates are two orders
of magnitude above normal. But the real body count is likely to
be even higher, because species usually don't go extinct immediately
when their habitats are destroyed, exotic predators arrive, or they
otherwise come to ecological harm. Instead, they often hang on for
decades or even longer before disappearing forever.
Habitat
destruction is a major cause of species loss and has accelerated
rapidly in recent years, especially in the world's most species-rich
environments -- about half the original extent of tropical moist
forest has been lost, for example, most of it in the last 50 years
-- so it's likely that many extinctions have not yet had time to
occur. That means the number of threatened and endangered species
(those that are likely to go extinct in the next few decades without
human intervention to save them) might be a better assessment ofthe
probable toll than simply the number of recent extinctions.
The
World Conservation Union (IUCN) currently lists 1,213 birds as threatened,
about 12 percent of all avian species. "Which means that by
the end of the century we could expect that probably a thousand
species of birds might disappear," Pimm says. That would be
10 extinctions per 10,000 species per year, or 1,000 times the background
extinction rate. The numbers for other well-known groups are similar,
if not worse: 20 percent of the world's mammal species appear on
the IUCN Red List of threatened species.
Not
all scientists agree that hundred-to thousand-fold increased extinction
rates among birds and a few other well-known groups mean that all
kinds of species are disappearing at the same rates. "There
are just so many differences among, even within, taxa, how species
respond to the kinds of forces that are causing extinction,"
says Daniel Simberloff, an ecologist at the University of Tennessee
in Knoxville. Do the patterns we see in birds, which make up just
0.6 percent of known species, necessarily apply to insects, which
account for 54 percent?
Pimm
and Simberloff were once colleagues at the University of Tennessee
and know each other's work well. (Pimm even likes to joke that one
of Simberloff's primary research interests is "pointing out
Stuart Pimm's mistakes.") "There are certainly lots of
credible scientists who don't like the extrapolation methods and
would argue with aspects of them," Simberloff says.
Yet
within the scientific community, the debate over extinction rates
is about details, like just how much extrapolation is appropriate,
rather than the big picture." I don't know of any credible
environmental scientist that doesn't think that extinctions are
happening at greatly increased rates," Simberloff says. To
him, high extinction rates among birds and other well-known groups
are evidence enough of a biodiversity crisis -- regardless of what
the exact patterns might prove to be among other kinds of species.
Moreover,
as researchers have begun to look more closely at those other groups
of species, all evidence suggests that things are just as bad, if
not worse, than studies of birds and mammals indicate. According
to a Nature Conservancy study, dragonflies and beetles are more
highly threatened than birds in North America. In the sea, where
many scientists had long believed that species would be relatively
shielded from extinction risk, more than 40 percent of a subfamily
of groupers meet IUCN criteria for imperilment, says Callum Roberts,
a marine conservation biologist at the University of York in the
United Kingdom. Most marine species have not yet been assessed.
In
a study comparing population trends among butterflies, birds, and
plants in Britain, a group of ecologists led by Jeremy Thomas of
the National Environment Research Council in Dorchester found that
butterflies fared the worst in recent decades. Seventy-one percent
of butterfly species declined over the course of the study, compared
to 54 percent of birds and 28 percent of plants.The group's analyses
of other types of insects, while less detailed, suggested similar
patterns. Their study involved over 20,000 volunteers who submitted
more than 15 million records of species sightings -- an enormous
amount of effort to analyze just a few groups of organisms on a
relatively small, species-poor island with a well-characterized
biota, and a good illustration of why sampling is necessary, and
probably always will be.
It's
just about inconceivable that the precise status of every species
on Earth can be known, and there has to be some point at which reasonable
people decide that what is known is enough. Admittedly, what we
know are still only scattered details woven into a much grander,
and still largely mysterious, tapestry. Many of the largest groups
of organisms, and the most unexplored. Tropical moist forests, for
example, are thought to contain half the Earth's species, and if
that's true only about one in 20 of the species living there have
been catalogued.
However,
two important pieces of information about these environments are
available. First, it's often possible to determine how much of a
habitat has been destroyed, by means of forest surveys or satellite
photos.
Second,
it's known that larger areas of habitat can support more species,
and by contrast smaller areas contain not only fewer numbers of
creatures but fewer species -- a principle called the species-area
relation. Specifically, an area of habitat half the size of another
area doesn't host half the number of species as the larger area,
but about 85 percent. Thus, say Pimm and many other ecologists,
the 50 percent of tropical moist forest that's been lost so far
is expected to lead to the extinction of 15 percent of tropical
moist forest species.
Scientists
also use the species-area relation to predict future extinctions
as habitat destruction continues. Peter Raven, president of the
Missouri Botanical Garden in St. Louis, estimates that tropical
moist forests will be reduced to about five percent of their original
extent by mid-century. According to the species-area relation, that
would commit more than half of the species they hold to extinction."
If you put that together with habitat destruction in temperate regions,"
Raven says, "you come up with something like half to two-thirds
of all the species in the world becoming extinct during the course
of this century" -- or at least set on an inexorable path to
that fate. Callum Roberts, who has been working on similar calculations
for coral reefs, reports that "the species-area relationships
suggest that marine species will be lost as a consequence of habitat
destruction almost as fast as terrestrial species will."
Not
all species are equally vulnerable to ecological threats. "It's
easier to destroy a species with a small range than a big one,"
Pimm says, simply because it's easier to wipe out the entire area
where it lives. In fact, a large proportion of species have small
ranges, and they're not evenly distributed over the planet. For
reasons scientists are still debating, they are clumped together
in particular spots, most of which are in the tropics. Habitat destruction
in those areas could be particularly devastating, as British ecologist
Norman Myers has pointed out. Myers pioneered the concept of "biodiversity
hotspots," and in 2000, with input from scientists from Conservation
International, he defined 25 hotspots covering just 1.4 percent
of the planet's land area. The hotspots include 15 tropical forests
but also places like the Mediterranean basin and the Cape Floristic
Region at the southern tip of Africa. Destroying these habitats
could wipe out 44 percent of all plant species, as well as 35 percent
of birds, mammals, reptiles, and amphibians. In a similar analysis
of coral reef environments, which are among the most diverse parts
of the sea, Roberts and a group of colleagues identified 10 marine
biodiversity hotspots, representing just 0.012 percent of the ocean
but containing a large proportion of small-range species. Moreover,
since that analysis was published in 2002, researchers have been
gathering evidence of unexpectedly rich concentrations of biodiversity
in other parts of the ocean, such as deep-ocean seamounts and cold-water
coral reefs, which are being destroyed at a rapid clip by factory
trawlers. "The habitats on them are literally being clear-cut
as effectively as any forest cutting in the Amazon," Roberts
says.
Some
scientists have objected to this use of the species-area relation,
arguing that it's a tool for predicting the total number of species
you'll find in an area if you sample a smaller portion of it --
not for predicting the number of species you'll lose by destroying
a portion of habitat. In other words, you can use the equation to
make predictions about going from a smaller area to a bigger one,
but not from a bigger area to a smaller one.
Yet
in several different environments around the world, researchers
have found that predictions of species loss based on the species-area
relation align pretty well with reality. In the eastern U.S. forests,
which were reduced by about 50 percent at their smallest extent
(around 1870), the species-area relation predicts a loss of 15 percent
of species. In fact, of 28 bird species restricted to the forest,
four (or 14.3 percent) had gone extinct and a fifth was critically
endangered as of 1995, according to Stuart Pimm and Robert Askins.
(One of the extinct species was the ivory-billed woodpecker, so
the best-case scenario now stands at three extinct and two critically
endangered.) Likewise, in tropical forests such as the Atlantic
Forest of Brazil and the island chains of Indonesia and the Philippines,
where deforestation is more recent, the species-area relation accurately
predicts or underestimates the number of threatened bird species
-- an expected result, says Pimm, because in many areas other threats
such as invasive species and over-hunting also contribute to species
endangerment.
Still,
Simberloff says that these numbers should be taken with a grain
of salt, because the species-area relation is "a very blunt
tool." A great deal of habitat loss will surely lead to substantial
species loss, but there are many other factors besides area that
influence how many species live in a certain place, and the species-area
relation doesn't say how fast species will go extinct. "All
[the analyses] can say is at some point in the future there are
going to be fewer species," he emphasizes.
"It's
a glass-half-empty/glass-half-full situation," Pimm responds.
Even if these analyses don't yield a precise number of species destined
for extinction, they do give us a good sense of the magnitude of
the problem. A loss of half to two-thirds of all species, as Peter
Raven predicts is possible, puts the present era on par with the
five previous mass extinctions in the history of life on Earth.
The most recent one, 65 million years ago, wiped out the dinosaurs
along with about two-thirds of all forms of life on land.
While
habitat destruction was the focus of most work on global extinction
rates throughout the 1990s, recently scientists have begun to consider
the biodiversity impacts of climate change. A group of researchers
presented perhaps the most comprehensive effort to date to quantify
these possible effects in a 2004 paper in Nature.
Led
by biologist Chris Thomas (then at the University of Leeds in the
United Kingdom), the group assessed the present distributions of
1,103 animal and plant species and projected how the habitat available
to them would change under conditions predicted by the most commonly
used computer model of climate change. As the Earth warms, boreal
forest is expected to shrink toward the poles, for example, and
alpine habitat will retreat up the sides of mountains.
Reasoning
that habitat loss is habitat loss whether it's caused by chainsaws
or the greenhouse effect, Thomas's team calculated the proportion
of habitat that species are likely to lose as the climate warms,
then used the species-area relation to predict the number of extinctions
likely to result. They found that, depending on the assumptions
of the model, 15-37 percent of the species would be on their way
to extinction by 2050. The paper generated an uproar almost immediately.
Daniel Botkin, of the University of California at Santa Barbara,
says the analysis makes inappropriate use of the species-area relation
and is based on weak underlying data.
"I've
shown that we don't even know the area that is boreal forest very
well," he says, pointing out that calculating the future loss
of a certain habitat is pretty meaningless when we don't know its
present extent.
Yet
the gloomy predictions don't depend on the species-area relation,
Thomas and his coauthors explained in an online follow-up to their
article. The computer model predicted that eight percent of species
would have no suitable habitat left at all by 2050. Moreover, warming
isn't likely to stop in 2050 -- in fact, the maximum temperature
increase predicted for 2050 is pretty close to the minimum increase
predicted for 2100. So for species that lose most of their habitat
by 2050, "it doesn't take much extrapolation in the mind to
realize it's not going to be more than a few decades before they've
lost the rest," Thomas says. Although he views the 2004 analysis
as only "a first step" to understanding the effects of
global warming on biodiversity, Thomas still sees it as a pretty
good indicator of the magnitude of extinctions that are likely to
result from climate change: "It looks like it's going to be
in the tens of percents of species."
How
do older predictions of species loss from habitat destruction line
up with newer ones about extinction from climate change?
No
one has done a formal analysis, and Thomas says no one knows yet
how much the two groups of species at risk will overlap. But Pimm
reluctantly ventures the conclusion that the losses may prove to
be additive, because habitats likely to shrink most as the planet
warms, like those on mountaintops and in the polar regions, also
tend to be remote and thus relatively unaffected by habitat destruction.
"Global warming is going to start knocking off the species
that we thought might survive," Pimm says.
Of
course, nature is full of surprises, and could turn out to be more
resilient than we think. Maybe species will be able to adapt to
a warmer climate, disperse to newly suitable areas, or hang on in
human-altered habitats. The Brazilian maroon-bellied parakeet survives
in Rio de Janeiro's city parks and gardens, despite the fact that
over 90 percent of its native coastal-forest habitat has been wiped
out.
But
most ecologists agree that while a few species here and there will
be able to make a go of it in a changed world, such species will
be part of a small minority. Chris Thomas points out that species
trying to adapt to a warmer climate will have to compete with heat-loving
species that will arrive from warmer climes. And Stuart Pimm has
found that those tropical forest species able to survive in human-altered
habitats like cow pastures are relatively widely distributed generalist
species that are not at high risk of extinction anyway. In other
words, the maroon-bellied parakeet is also a lonely bird, having
once been part of a teeming avian community in the Atlantic Forest;
200 species of birds with which the parakeet shared its lost habitat
are on the brink of extinction.
If
there is any real cause for optimism, it lies in the time lag before
extinction. If species can hang on for 50 or 100 years, we humans
may be able to organize a system of protected areas and alter our
own activities to ensure their long-term survival in the wild. The
ivory-billed woodpecker was decimated when the mature bottomland
forests it depends on were razed in the 19th and early 20th centuries,
but these forests are now coming back. If the bird has managed to
survive for this long, its chances will only be better in the future
as the big trees continue to grow. Similarly, Thomas says that if
global temperatures peak at a relatively low level sometime late
this century and then decline towards pre-industrial levels 150
or 200 years from now, about half of the extinctions predicted by
his group's analysis could be avoided.
Perhaps
we humans are not yet fated to be lonely.
This
article appears in the June/July issue of World
Watch.
Sarah
DeWeerdt is a Seattle-based science writer specializing in biology
and the environment.
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