Life on the Brink
4/1/97
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RELAYED TEXT STARTS HERE:
Headline: Life on the Brink
Source: EARTH
Date: 4/1/97
Author: Karen Schimdt
This article originally appeared in the April 1997 EARTH
Copyright c1997 Kalmbach Publishing Co.
Solitaries. The last live sightings of these birds on the Indian Ocean
island of Rodriguez were made in the 1750s.
The number of shrinking species seems to be growing. Humans might be
causing a mass extinction. Or this might be the planet's normal routine.
By Karen Schmidt
The bizarre beak -- a short lower bill topped by a long, sabrelike upper
bill -- poked up at Helen James from a collection of bird bones held out to
her by a Hawaiian man. James, a paleontologist, realized she was looking at
the beak of a honeycreeper, a small Hawaiian bird. But no honeycreeper
she'd ever seen had a beak shaped like this. "I put it back in the box and
had to catch my breath," she says.
James gasped because she had stumbled upon an extinct animal, one new to
science. This was far from the only extinct bird James had come across,
however. She and husband Storrs Olson, both of the Smithsonian Institution,
have been scouring the Hawaiian Islands for avian remnants since 1976. They
have climbed down deep shafts into lava tube caves, where they have spent
long days underground, poring over the muck collected over the centuries.
In 20 years, they've discovered 34 previously unknown bird species,
including ground-dwelling ibises that strolled through the tropical forests
on stout legs, unusual gooselike waterfowl and honeycreepers with a more
spectacular array of beak shapes than Charles Darwin's famous finches.
They've also come across an ugly truth: Humans can cause mass extinctions.
Like the honeycreeper, most of these birds disappeared from the fossil
record between 900 and 500 years ago, after Polynesian settlers began
spreading across the islands. After Captain James Cooke landed in 1778 and
Europeans moved in, another 20 to 25 bird species died out. In total, about
60 land birds -- well over half of Hawaii's known birds -- have become
extinct. Says James, "It's on par with the number of large mammals lost at
the end of the Pleistocene in North America." At that time, 10,000 years
ago, species in North America faced a severe climatic shift that changed
the extent of glacier coverage on the land and brought sea level drops to
Pacific islands.
But climate change during the Pleistocene was hardly a catastrophe for
Hawaiian birds compared with the later invasion of humans. Most of these
species died out, James thinks, as the settlers converted natural bird
habitat to agriculture and brought foreign competitors such as pigs, rats
and dogs, as well as new diseases, to Paradise.
This human-caused extinction of weird and wonderful birds might be
dismissed as ancient history or as something peculiar to islands like
Hawaii. But many paleontologists believe the same story is now unfolding
everywhere. Plants and animals are disappearing from their native
landscapes in places as varied as California, central Chile and the Western
Ghats of India. "Up to this point, species on continents have survived
better than island species, but as continental land is carved up into
archipelagoes -- into habitat islands -- more extinctions will be seen
there too," says James.
Some researchers even say the number of plant, animal and microorganism
species on Earth is already heading for a catastrophic crash like the one
that occurred 65 million years ago, at the Cretaceous-Tertiary (or K-T)
boundary, when 75 percent of all species, including the reigning dinosaurs,
vanished.
This disappearing act is the so-called "biodiversity crisis." Several
popular books have advanced this theory, including The End of Evolution by
paleontologist Peter Ward, The Diversity of Life by biologist Edward O.
Wilson, The Miner's Canary by paleontologist Niles Eldridge, and The Sixth
Extinction by paleoanthropologist Richard Leakey and science writer Roger
Lewin.
But is this a real threat, or just alarmist cries? There's still
significant debate about how bad the situation really is. It turns out to
be rather difficult to calculate how many species Earth is losing and how
fast. John Terborgh, a biologist at Duke University in Durham, North
Carolina, says, "There are frequent statements that hundreds of species are
going extinct every year. It may be true, but we have no evidence -- it's
just hand-waving." And without those world-wide numbers, paleontologists
and biologists are hard pressed to prove that a global mass extinction is
actually occurring.
So Terborgh and others are looking hard for evidence. For if we are in the
midst of an extinction event, the risks are extraordinarily grave. Once a
mass extinction occurs -- it's happened five times that paleontologists
know about -- biodiversity returns only after five to ten million years,
says David Jablonski, a paleontologist at the University of Chicago. "We
don't want to experience a K-T scale extinction event," he says. "In the
blackest scenario, we could be driving ourselves toward a world of rats,
weeds and cockroaches."
Surprisingly, researchers have only recently proven that a world with only
a few species would indeed be an unpleasant one. Ecologists had only
speculated that high species diversity protected ecosystems -- coral reefs,
mountain meadows and others -- against storms, volcanic eruptions, disease
outbreaks and similar periodic catastrophes. Species-rich ecosystems should
have built-in disaster insurance: They would be more likely to contain at
least some species that had evolved traits to survive.
This was a nice-sounding notion, but it was also a completely untested one.
Finally, in 1982, ecologist David Tilman of the University of Minnesota
began an experiment to study 207 prairie plots that contained one to 26
different species of grasses and broadleaf plants. Thanks to a terrible
drought that struck Minnesota in 1987-88, Tilman and his colleagues were
able to determine by 1994 that plots with the greatest number of species
were able to overcome adversity better than plots with the fewest species;
they regained the most vegetative cover and did so faster.
The continuing prairie experiment has since shown that biodiversity is
important for a lesser-known reason: it boosts ecosystem productivity,
which shows up as the amount of biomass, such as plant matter, that can be
generated from the raw materials in an ecosystem. After examining 147
plots, all fertilized with nitrogen and sown with one, two, four, six,
eight, 12 or 24 species of native prairie plants, Tilman's team found in
1995 that the more diverse plots were also the most lush and the most
efficient at taking up nitrogen from the soil.
Tilman and his fellow Minnesota researchers are now investigating whether
the number of plant species influences a prairie plot's susceptibility to
invasions by disease, insect pests and weeds. The newest data, says Tilman,
show that high biodiversity once again plays a positive role in all three
cases. A diverse collection of plants with a variety of defenses and
vulnerabilities creates a complex landscape that prevents insects, disease
organisms and weeds from zeroing in on their favorite victims, he explains.
Because all ecosystems rely on similar mechanisms for recovering from
disaster, controlling disease and using and recycling nutrients, Tilman
says, "I think what we've seen at our site applies equally strongly to
every ecosystem." If Tilman is right, ecosystems with low numbers of
species would be susceptible to collapse in bad times.
And ecosystem collapse has effects that range far beyond Tilman's prairie
plots. The prairie ecosystem maintains soil fertility, essential for
growing crops that people eat. Wetland ecosystems prevent flooding and
filter water used for drinking and raising seafood. Forest and marine
ecosystems regulate the global climate.
In addition, many species are valuable in their own right as sources of
food, chemicals, medicines, fibers and even genes. For instance, many of
the 232 known wild species of potatoes have tremendous resistance to pests
and diseases that plague cultivated potato crops, says David Spooner, who
works for the U.S. Department of Agriculture's Agricultural Research
Service at the University of Wisconsin in Madison. So far, plant breeders
have incorporated traits from 15 wild species into commercially grown
potato types.
But there aren't as many types of potatoes as there once were, Spooner
says. "It's a race against time. I've been to areas in Ecuador where every
tree has been cut down for agriculture, and it's very difficult to find
wild species we know used to be there."
It isn't just wild potatoes that are disappearing. Along with the tubers
and some of Hawaii's birds, the Florida panther, the Amazon river dolphin,
the primitive fish known as the coelacanth and thousands of other animals
and plants hover on the brink of extinction.
The World Conservation Union, based in Cambridge, England, has been keeping
a Red List of threatened wildlife for three decades. Its 1996 analysis,
conducted by more than 500 scientists, paints a grim picture of 5,205
species at risk. Using new criteria, such as population decline over a
10-year period, fragmentation of populations and low population numbers,
the Union found that 34 percent of fish, 25 percent of amphibians, 25
percent of mammals, 20 percent of reptiles and 11 percent of birds are
threatened with extinction.
Yet while the number of animals threatened may seem high, it does not prove
that a mass extinction event is underway. Even if all species on the Red
List did go extinct over the next hundred or thousand years, such a rate
may not be unusual.
Throughout Earth's history varieties of species have died off, including
the saber-toothed tiger, Australia's giant horned tortoises and, recently,
the ivory-billed woodpecker. Extinction has occurred for as long as there
has been life. And life has come back every time. Indeed, paleontologists
estimate that about 100 species have risen and fallen for every single
species alive today.
So how do scientists know that the global extinction rate is higher now
than in the past?
What researchers have is a patchwork view of how biodiversity has changed
during the post-human era -- a wild flower lost here, a fish lost there. In
places like Hawaii, fossils, notes from naturalists and explorers, and
surveys of wild populations show that the extinction rate for birds
skyrocketed in the short time since humans moved in.
Consider also that in the past 3,000 years, the island of Madagascar has
lost 17 of about 49 lemur species, including some that were larger than
gorillas. In the last century, North America has lost 40 of approximately
950 fish species. Of the 8,500 known plant species unique to Southern
Africa's floral region, 36 have recently died out, and 618 more are
threatened with extinction. In these cases, species are disappearing far
faster than they could be created through evolution, signaling a rapid
decline in biodiversity.
On a world-wide scale, however, the picture grows murky, because no one
knows how many species the Earth now houses. This lack of basic information
is "one of the most heartbreaking problems of all," says Jablonski. It
makes it virtually impossible to interpret the real impact of extinctions.
"If you lose a hundred beetle species, what does that mean for beetles and
what does it mean for the so-called biodiversity crisis?" You need to have
some idea of the big picture, he says, before you can understand how much
of that picture is being erased.
Getting that big picture, however, is far from easy. It's difficult enough
to identify all the plants and animals in a single small wetland, patch of
forest or coral reef, so imagine trying to tally up all the species in the
world. So far, 1.4 million species have been named and described, but
scientists are continually discovering more, particularly in tropical rain
forests and the deep sea.
New species are still found right under our noses. David Wake, an expert on
amphibians at the University of California at Berkeley, says he just
discovered a striking new species of salamander in densely populated Los
Angeles County. "That tells you something about the nature of this
problem," says Wake.
Even large mammals still turn up. Seven new species of monkeys have been
found in Brazil since 1990, says Russell Mittermeier, chairman of the World
Conservation Centre's primate specialist group and president of
Conservation International. Scientists estimate the true number of species
on Earth to be between 10 million and 100 million. "It is staggering what
we don't know!" says Mittermeier, who favors the high estimate.
Realizing that the "total species number" obstacle may indeed be too high a
hurdle, some scientists have tried to work around it. Their approach is to
calculate and compare the average extinction rates for well-known groups of
organisms in the past and at present.
To estimate the "normal" background rate of extinction in the past,
paleontologists have looked at the fossil record. They have found that on
average one to 10 species per million became extinct every year -- with
mammals on the high end of extinction rates and invertebrates, such as
clams, at the low end. But the fossil record is a tricky thing to read.
Organisms that are preserved are those that occurred in large numbers and
were relatively extinction-resistant, so they provide a minimum extinction
rate. The overall past extinction rate might actually be higher, says
Jablonski. But it wouldn't be higher than the rate at which new species
appeared in the record, since biodiversity rose overall during the past 600
million years. So the estimate appears to be in the ballpark, he says.
Still, getting a handle on total species number is only half of the battle.
Researchers also have to cope with the messy problem of calculating how
fast species are becoming extinct now. Stuart Pimm, an ecologist at the
University of Tennessee at Knoxville, says it's possible to do this using
-- as true canaries in the global coal mine -- birds. Biologists know where
to find nearly all of the 10,000 named birds, and new species are rarely
discovered. At present, about two or three bird species become extinct
every year, Pimm says. That translates to an average global rate of
extinction for birds that's about 200 to 300 times higher than the one- to
10-per-million background rate indicated by the fossils, he says. (In the
Pacific islands, the rate is about 1,000 times the normal rate.)
But Jablonski points out that using such calculations to estimate global
extinction rates for all species requires researchers to make some unlikely
apples-and-oranges comparisons. After all, would an extinction rate
calculated for birds hold true for butterflies, pine trees or sea turtles?
Probably not, says Jablonski, "Different groups in different areas are
punished to different degrees -- a single number can't capture what is
going on."
Pimm, however, contends that the extinction rate for birds is likely to be
an underestimate of the extinction rate for other groups, even if it's not
a perfect number. For evidence, he points to the Nature Conservancy's 1996
Annual Report Card for U.S. Plant and Animal Species. The Conservancy
gathered and analyzed data collected by state natural heritage programs on
the status of 13 groups of organisms. Of the groups they looked at, the
most threatened was freshwater mussels, with two-thirds of species extinct
or at risk of extinction. Crayfish, amphibians and freshwater fish were
right behind. Birds were the most robust group of all, with 14 percent at
risk of extinction. Similarly, the 1996 Red List also identifies birds as
the least threatened group worldwide.
In tracking the status of groups of organisms and calculating present-day
extinction rates, scientists run into other sticky problems, such as how to
define "threatened" and when to proclaim a species extinct. Says Terborgh,
"Rumors of sightings leak out for decades because there are thousands of
hopeful people. Even in extremely well-studied groups of organisms, such as
birds, it takes decades to document extinction."
"I think we've got biased information and even worse, I don't know which
way it's biased. For a scientist it's very annoying," says Berkeley's Wake,
who's lobbying to get a worldwide database on amphibians established.
Anecdotal reports suggest that frogs around the world are disappearing. For
instance, in Costa Rica's Monteverde Cloud Forest Preserve, 20 out of 50
once abundant species of frogs have not been seen at all in the past six
years, says Wake.
But other frog species appear to be doing just fine. So what does the
global snapshot look like for amphibians? It's difficult to bring it into
focus. There are 4,500 known amphibian species, Wake points out. Scientists
know a lot about amphibians that live in some geographic areas such as
Costa Rica, Oregon and South Carolina but virtually nothing about
amphibians in other regions, such as Africa. Moreover, about 10 percent of
all amphibian species are now known only from one specimen in a museum
collection.
Again, to get around this lack of detailed information, scientists have
used another shortcut to arrive at an estimate for current extinction
rates. They use habitat destruction -- generally deforestation -- as a
proxy. There's a direct mathematical relationship between the number of
species that dwell in a place and the size of the area. Scientists can
estimate conversely how many species overall are lost when a forested area
is reduced. As a rule of thumb, when a habitat is cut by 90 percent, half
of its plants, animals, insects and microscopic life forms are expected to
become extinct.
But these kinds of estimates are fraught with uncertainties, since species
are not evenly distributed. Some will migrate and adapt their ranges. And
the calculation says nothing about how long it takes for the extinctions to
occur -- ten years or 100 years, says Daniel Simberloff, a conservation
biologist at Florida State University in Tallahassee. "The species-area
relationship provides a rough guide," he says. "It can tell you that if you
have a big loss of area, you will lose species, but it's not useful in
predicting how many."
In the end, the fundamental gap in knowledge about how many species exist
and where they are located makes Simberloff, Jablonski, Terborgh and many
other scientists skeptical of estimated species losses and extinction rates
-- and thus, they say, it is probably too early to conclude that an era of
global mass extinction has begun. They do not, however, dispute the
seriousness of the many local extinction events that could amount to a
global disaster. And there is a consensus among scientists that species
seem to be disappearing at an unnaturally high rate. In November 1995,
1,000 scientists from more than 50 countries concluded in a report called
Global Biodiversity Assessment that species have recently become extinct at
50 to 100 times the average expected natural rate (a relatively
conservative estimate).
Even if the calculated species losses are inaccurate, other signs still
point to a serious biodiversity crisis, says Simberloff. "If we spend a lot
of time looking at the precise status and range of species, we find big
problems," he explains. "Talk to any systematic biologist who works on a
single group of organisms and he or she will tell you of species that have
disappeared and of places where only one population exists. I'm certain
that when you add all of this up, we're seeing a mass extinction. But I
couldn't prove it in a court of law."
Indeed, many biologists say it's time to move beyond number crunching,
since the total number of species on Earth will remain a mystery. "There's
no way to fill that gap, so we'll never have evidence beyond the shadow of
a doubt," says Terborgh. "The biodiversity crisis is based on a
plausibility argument." Measurements of the rate of destruction and
degradation of habitat are enough to convince him that the biodiversity
crisis is real, he says. "Extinction does not happen all at once -- it
comes in a big bang at the end," says Terborgh, who studies the effects of
fragmentation on tropical forest ecosystems. "There will be an enormous
surge of extinctions as the last hectare of forest falls to chain saws."
Pimm also agrees: "The reality is that whatever numbers we come up with --
and those sorts of details we love to debate -- it doesn't alter the big
picture. Many habitats are disappearing incredibly rapidly and they will be
gone in 50 to 100 years."
Habitat destruction is likely be a major focus of scientific discussion
later this year. In October, many of the scientists who drew attention to
the biodiversity crisis a decade ago will meet again in Washington, D.C.,
for a conference sponsored by the National Research Council, Smithsonian
Institution, Library of Congress and American Association for the
Advancement of Science. As Conservation International's Mittermeier sums
up, "There's a much better awareness of the need to protect biodiversity
than there was 10 years ago, but meanwhile, the destruction of habitat
continues. We still have a long way to go."
And Norman Myers, a consultant in environment and international development
based in Oxford, England, doesn't think it's going to be a particularly
easy journey. "The forces of habitat destruction have built up such
momentum that turning it around will be like turning around a supertanker
," he says. "It will take a while to get facing the other direction."