PLANET OF WEEDS
Tallying the losses of Earth's animals and plants
by David Quammen
Hope is a duty from which paleontologists are exempt. Their job is to take
the long view, the cold and stony view, of triumphs and catastrophes in the
history of life. They study teeth, tree trunks, leaves, pollen, and other
biological relics, and from it they attempt to discern the lost secrets of
time, the big patterns of stasis and change, the trends of innovation and
adaptation and refinement and decline that have blown like sea winds among
ancient creatures in ancient ecosystems. Although life is their subject,
death and burial supply all their data. They're the coroners of biology.
This gives to paleontologists a certain distance, a hyperopic perspective
beyond the reach of anxiety over outcomes of the struggles they chronicle.
If hope is the thing with feathers, as Emily Dickinson said, then it's good
to remember that feathers don't generally fossilize well. In lieu of hope
and despair, paleontologists have a highly developed sense of cyclicity.
That's why I recently went to Chicago, with a handful of urgently grim questions,
and called on a paleontologist named David Jablonski. I wanted answers unvarnished
with obligatory hope.
Jablonski is a big-pattern man, a macroevolutionist, who works fastidiously
from the particular to the very broad. He's an expert on the morphology and
distribution of marine bivalves and gastropods--or clams and snails, as he
calls them when speaking casually. He sifts through the record of those mollusk
lineages, preserved in rock and later harvested into museum drawers, to extract
ideas about the origin of novelty. His attention roams back through 600 million
years of time. His special skill involves framing large, resonant questions
that can be answered with small, lithified clamshells. For instance: By what
combinations of causal factor and sheer chance have the great evolutionary
innovations arisen? How quickly have those innovations taken hold? How long
have they abided? He's also interested in extinction, the converse of abidance,
the yang to evolution's yin. Why do some species survive for a long time,
he wonders, whereas others die out much sooner? And why has the rate of extinction--low
throughout most of Earth's history--spiked upward cataclysmically on just
a few occasions? How do those cataclysmic episodes, known in the trade as
mass extinctions, differ in kind as well as degree from the gradual process
of species extinction during the millions of years between? Can what struck
in the past strike again?
The concept of mass extinction implies a biological crisis that spanned large
parts of the planet and, in a relatively short time, eradicated a sizable
number of species from a variety of groups. There's no absolute threshold
of magnitude, and dozens of different episodes in geologic history might
qualify, but five big ones stand out: Ordovician, Devonian, Permian, Triassic,
Cretaceous. The Ordovician extinction, 439 million years ago, entailed the
disappearance of roughly 85 percent of marine animal species--and that was
before there were any animals on land. The Devonian extinction, 367 million
years ago, seems to have been almost as severe. About 245 million years ago
came the Permian extinction, the worst ever, claiming 95 percent of all known
animal species and therefore almost wiping out the animal kingdom altogether.
The Triassic, 208 million years ago, was bad again, though not nearly so
bad as the Permian. The most recent was the Cretaceous extinction (sometimes
called the K-T event because it defines the boundary between two geologic
periods, with K for Cretaceous, never mind why, and T for Tertiary), familiar
even to schoolchildren because it ended the age of dinosaurs. Less familiarly,
the K-T event also brought extinction of the marine reptiles and the ammonites,
as well as major losses of species among fish, mammals, amphibians, sea urchins,
and other groups, totaling 76 percent of all species. In between these five
episodes occurred some lesser mass extinctions, and throughout the intervening
lulls extinction continued, too--but at a much slower pace, known as the
background rate, claiming only about one species in any major group every
million years. At the background rate, extinction is infrequent enough to
be counterbalanced by the evolution of new species. Each of the five major
episodes, in contrast, represents a drastic net loss of species diversity,
a deep trough of biological impoverishment from which Earth only slowly recovered.
How slowly? How long is the lag between a nadir of impoverishment and a recovery
to ecological fullness? That's another of Jablonski's research interests.
His rough estimates run to 5 or 10 million years. What drew me to this man's
work, and then to his doorstep, were his special competence on mass extinctions
and his willingness to discuss the notion that a sixth one is in progress
now.
Some people will tell you that we as a species, Homo sapiens, the savvy ape,
all 5.9 billion of us in our collective impact, are destroying the world.
Me, I won't tell you that, because "the world" is so vague, whereas what
we are or aren't destroying is quite specific. Some people will tell you
that we are rampaging suicidally toward a degree of global wreckage that
will result in our own extinction. I won't tell you that either. Some people
say that the environment will be the paramount political and social concern
of the twenty-first century, but what they mean by "the environment" is anyone's
guess. Polluted air? Polluted water? Acid rain? A frayed skein of ozone over
Antarctica? Greenhouse gases emitted by smokestacks and cars? Toxic wastes?
None of these concerns is the big one, paleontological in scope, though some
are more closely entangled with it than others. If the world's air is clean
for humans to breathe but supports no birds or butterflies, if the world's
waters are pure for humans to drink but contain no fish or crustaceans or
diatoms, have we solved our environmental problems? Well, I suppose so, at
least as environmentalism is commonly construed. That clumsy, confused, and
presumptuous formulation "the environment" implies viewing air, water, soil,
forests, rivers, swamps, deserts, and oceans as merely a milieu within which
something important is set: human life, human history. But what's at issue
in fact is not an environment; it's a living world.
Here instead is what I'd like to tell you: The consensus among conscientious
biologists is that we're headed into another mass extinction, a vale of biological
impoverishment commensurate with the big five. Many experts remain hopeful
that we can brake that descent, but my own view is that we're likely to go
all the way down. I visited David Jablonski to ask what we might see at the
bottom.
On a hot summer morning, Jablonski is busy in his office on the second floor
of the Hinds Geophysical Laboratory at the University of Chicago. It's a
large open room furnished in tall bookshelves, tables piled high with books,
stacks of paper standing knee-high off the floor. The walls are mostly bare,
aside from a chart of the geologic time scale, a clipped cartoon of dancing
tyrannosaurs in red sneakers, and a poster from a Rodin exhibition, quietly
appropriate to the overall theme of eloquent stone. Jablonski is a lean forty-five-year-old
man with a dark full beard. Educated at Columbia and Yale, he came to Chicago
in 1985 and has helped make its paleontology program perhaps the country's
best. Although in not many hours he'll be leaving on a trip to Alaska, he
has been cordial about agreeing to this chat. Stepping carefully, we move
among the piled journals, reprints, and photocopies. Every pile represents
a different research question, he tells me. "I juggle a lot of these things
all at once because they feed into one another." That's exactly why I've
come: for a little rigorous intellectual synergy.
Let's talk about mass extinctions, I say. When did someone first realize
that the concept might apply to current events, not just to the Permian or
the Cretaceous?
He begins sorting through memory, back to the early 1970s, when the full
scope of the current extinction problem was barely recognized. Before then,
some writers warned about "vanishing wildlife" and "endangered species,"
but generally the warnings were framed around individual species with popular
appeal, such as the whooping crane, the tiger, the blue whale, the peregrine
falcon. During the 1970s a new form of concern broke forth--call it wholesale
concern--from the awareness that unnumbered millions of narrowly endemic
(that is, unique and localized) species inhabit the tropical forests and
that those forests were quickly being cut. In 1976, a Nairobi-based biologist
named Norman Myers published a paper in Science on that subject; in passing,
he also compared current extinctions with the rate during what he loosely
called "the 'great dying' of the dinosaurs." David Jablonski, then a graduate
student, read Myers's paper and tucked a copy into his files. This was the
first time, as Jablonski recalls, that anyone tried to quantify the rate
of present-day extinctions. "Norman was a pretty lonely guy, for a long time,
on that," he says. In 1979, Myers published The Sinking Ark, explaining the
problem and offering some rough projections. Between the years 1600 and 1900,
by his tally, humanity had caused the extinction of about 75 known species,
almost all of them mammals and birds. Between 1900 and 1979, humans had extinguished
about another 75 known species, representing a rate well above the rate of
known losses during the Cretaceous extinction. But even more worrisome was
the inferable rate of unrecorded extinctions, recent and now impending, among
plants and animals still unidentified by science. Myers guessed that 25,000
plant species presently stood jeopardized, and maybe hundreds of thousands
of insects. "By the time human communities establish ecologically sound life-styles,
the fallout of species could total several million." Rereading that sentence
now, I'm struck by the reckless optimism of his assumption that human communities
eventually will establish "ecologically sound life-styles."
Although this early stab at quantification helped to galvanize public concern,
it also became a target for a handful of critics, who used the inexactitude
of the numbers to cast doubt on the reality of the problem. Most conspicuous
of the naysayers was Julian Simon, an economist at the University of Maryland,
who argued bullishly that human resourcefulness would solve all problems
worth solving, of which a decline in diversity of tropical insects wasn't
one.
In a 1986 issue of New Scientist, Simon rebutted Norman Myers, arguing from
his own construal of select data that there was "no obvious recent downward
trend in world forests--no obvious 'losses' at all, and certainly no 'near
catastrophic' loss." He later co-authored an op-ed piece in the New York
Times under the headline "Facts, Not Species, Are Periled." Again he went
after Myers, asserting a complete absence of evidence for the claim that
the extinction of species is going up rapidly--or even going up at all."
Simon's worst disservice to logic in that statement and others was the denial
that inferential evidence of wholesale extinction counts for anything. Of
inferential evidence there was an abundance--for example, from the Centinela
Ridge in a cloud-forest zone of western Ecuador, where in 1978 the botanist
Alwyn Gentry and a colleague found thirty-eight species of narrowly endemic
plants, including several with mysteriously black leaves. Before Gentry could
get back, Centinela Ridge had been completely deforested, the native plants
replaced by cacao and other crops. As for inferential evidence generally,
we might do well to remember what it contributes to our conviction that approximately
105,000 Japanese civilians died in the atomic bombing of Hiroshima. The city's
population fell abruptly on August 6, 1945, but there was no one-by-one identification
of 105,000 bodies.
Nowadays a few younger writers have taken Simon's line, pooh-poohing the
concern over extinction. As for Simon himself, who died earlier this year,
perhaps the truest sentence he left behind was, "We must also try to get
more reliable information about the number of species that might be lost
with various changes in the forests." No one could argue.
But it isn't easy to get such information. Field biologists tend to avoid
investing their precious research time in doomed tracts of forest. Beyond
that, our culture offers little institutional support for the study of narrowly
endemic species in order to register their existence before their habitats
are destroyed. Despite these obstacles, recent efforts to quantify rates
of extinction have supplanted the old warnings. These new estimates use satellite
imaging and improved on-the-ground data about deforestation, records of the
many human-caused extinctions on islands, and a branch of ecological theory
called island biogeography, which connects documented island cases with the
mainland problem of forest fragmentation. These efforts differ in particulars,
reflecting how much uncertainty is still involved, but their varied tones
form a chorus of consensus. I'll mention three of the most credible.
W.V. Reid, of the World Resources Institute, in 1992 gathered numbers on
the average annual deforestation in each of sixty-three tropical countries
during the 1980s and from them charted three different scenarios (low, middle,
high) of presumable forest loss by the year 2040. He chose a standard mathematical
model of the relationship between decreasing habitat area and decreasing
species diversity, made conservative assumptions about the crucial constant,
and ran his various deforestation estimates through the model. Reid's calculations
suggest that by the year 2040, between 17 and 35 percent of tropical forest
species will be extinct or doomed to be. Either at the high or the low end
of this range, it would amount to a bad loss, though not as bad as the K-T
event. Then again, 2040 won't mark the end of human pressures on biological
diversity or landscape.
Robert M. May, an ecologist at Oxford, co-authored a similar effort in 1995.
May and his colleagues noted the five causal factors that account for most
extinctions: habitat destruction, habitat fragmentation, overkill, invasive
species, and secondary effects cascading through an ecosystem from other
extinctions. Each of those five is more intricate than it sounds. For instance,
habitat fragmentation dooms species by consigning them to small, island-like
parcels of habitat surrounded by an ocean of human impact and by then subjecting
them to the same jeopardies (small population size, acted upon by environmental
fluctuation, catastrophe, inbreeding, bad luck, and cascading effects) that
make island species especially vulnerable to extinction. May's team concluded
that most extant bird and mammal species can expect average life spans of
between 200 and 400 years. That's equivalent to saying that about a third
of one percent will go extinct each year until some unimaginable end point
is reached. "Much of the diversity we inherited," May and his co-authors
wrote, "will be gone before humanity sorts itself out."
The most recent estimate comes from Stuart L. Pimm and Thomas M. Brooks,
ecologists at the University of Tennessee. Using a combination of published
data on bird species lost from forest fragments and field data gathered themselves,
Pimm and Brooks concluded that 50 percent of the world's forest-bird species
will be doomed to extinction by deforestation occurring over the next half
century. And birds won't be the sole victims. "How many species will be lost
if current trends continue?" the two scientists asked. "Somewhere between
one third and two thirds of all species--easily making this event as large
as the previous five mass extinctions the planet has experienced."
Jablonski, who started down this line of thought in 1978, offers me a reminder
about the conceptual machinery behind such estimates. "All mathematical models,"
he says cheerily, "are wrong. They are approximations. And the question is:
Are they usefully wrong, or are they meaninglessly wrong?" Models projecting
present and future species loss are useful, he suggests, if they help people
realize that Homo sapiens is perturbing Earth's biosphere to a degree it
hasn't often been perturbed before. In other words, that this is a drastic
experiment in biological drawdown we're engaged in, not a continuation of
routine.
Behind the projections of species loss lurk a number of crucial but hard-to-plot
variables, among which two are especially weighty: continuing landscape conversion
and the growth curve of human population.
Landscape conversion can mean many things: draining wetlands to build roads
and airports, turning tallgrass prairies under the plow, fencing savanna
and overgrazing it with domestic stock, cutting second-growth forest in Vermont
and consigning the land to ski resorts or vacation suburbs, slash-and-burn
clearing of Madagascar's rain forest to grow rice on wet hillsides, industrial
logging in Borneo to meet Japanese plywood demands. The ecologist John Terborgh
and a colleague, Carel P. van Schaik, have described a four-stage process
of landscape conversion that they call the land-use cascade. The successive
stages are: 1) wildlands, encompassing native floral and faunal communities
altered little or not at all by human impact; 2) extensively used areas,
such as natural grasslands lightly grazed, savanna kept open for prey animals
by infrequent human-set fires, or forests sparsely worked by slash-and-burn
farmers at low density; 3) intensively used areas, meaning crop fields, plantations,
village commons, travel corridors, urban and industrial zones; and finally
4) degraded land, formerly useful but now abused beyond value to anybody.
Madagascar, again, would be a good place to see all four stages, especially
the terminal one. Along a thin road that leads inland from a town called
Mahajanga, on the west coast, you can gaze out over a vista of degraded land--chalky
red hills and gullies, bare of forest, burned too often by grazers wanting
a short-term burst of pasturage, sparsely covered in dry grass and scrubby
fan palms, eroded starkly, draining red mud into the Betsiboka River, supporting
almost no human presence. Another showcase of degraded land--attributable
to fuelwood gathering, overgrazing, population density, and decades of apartheid--is
the Ciskei homeland in South Africa. Or you might look at overirrigated crop
fields left ruinously salinized in the Central Valley of California.
Among all forms of landscape conversion, pushing tropical forest from the
wildlands category to the intensively used category has the greatest impact
on biological diversity. You can see it in western India, where a spectacular
deciduous ecosystem known as the Gir forest (home to the last surviving population
of the Asiatic lion, Panthera leo persica) is yielding along its ragged edges
to new mango orchards, peanut fields, and lime quarries for cement. You can
see it in the central Amazon, where big tracts of rain forest have been felled
and burned, in a largely futile attempt (encouraged by misguided government
incentives, now revoked) to pasture cattle on sun-hardened clay. According
to the United Nations Food and Agriculture Organization, the rate of deforestation
in tropical countries has increased (contrary to Julian Simon's claim) since
the 1970s, when Myers made his estimates. During the 1980s, as the FAO reported
in 1993, that rate reached 15.4 million hectares (a hectare being the metric
equivalent of 2.5 acres) annually. South America was losing 6.2 million hectares
a year. Southeast Asia was losing less in area but more proportionally: 1.6
percent of its forests yearly. In terms of cumulative loss, as reported by
other observers, the Atlantic coastal forest of Brazil is at least 95 percent
gone. The Philippines, once nearly covered with rain forest, has lost 92
percent. Costa Rica has continued to lose forest, despite that country's
famous concern for its biological resources. The richest of old-growth lowland
forests in West Africa, India, the Greater Antilles, Madagascar, and elsewhere
have been reduced to less than a tenth of their original areas. By the middle
of the next century, if those trends continue, tropical forest will exist
virtually nowhere outside of protected areas--that is, national parks, wildlife
refuges, and other official reserves.
How many protected areas will there be? The present worldwide total is about
9,800, encompassing 6.3 percent of the planet's land area. Will those parks
and reserves retain their full biological diversity? No. Species with large
territorial needs will be unable to maintain viable population levels within
small reserves, and as those species die away their absence will affect others.
The disappearance of big predators, for instance, can release limits on medium-size
predators and scavengers, whose overabundance can drive still other species
(such as ground-nesting birds) to extinction. This has already happened in
some habitat fragments, such as Panama's Barro Colorado Island, and been
well documented in the literature of island biogeography. The lesson of fragmented
habitats is Yeatsian: Things fall apart.
Human population growth will make a bad situation worse by putting ever more
pressure on all available land.
Population growth rates have declined in many countries within the past several
decades, it's true. But world population is still increasing, and even if
average fertility suddenly, magically, dropped to 2.0 children per female,
population would continue to increase (on the momentum of birth rate exceeding
death rate among a generally younger and healthier populace) for some time.
The annual increase is now 80 million people, with most of that increment
coming in less developed countries. The latest long-range projections from
the Population Division of the United Nations, released earlier this year,
are slightly down from previous long-term projections in 1992 but still point
toward a problematic future. According to the U.N's middle estimate (and
most probable? hard to know) among seven fertility scenarios, human population
will rise from the present 5.9 billion to 9.4 billion by the year 2050, then
to 10.8 billion by 2150, before leveling off there at the end of the twenty-second
century. If it happens that way, about 9.7 billion people will inhabit the
countries included within Africa, Latin America, the Caribbean, and Asia.
The total population of those countries--most of which are in the low latitudes,
many of which are less developed, and which together encompass a large portion
of Earth's remaining tropical forest--will be more than twice what it is
today. Those 9.7 billion people, crowded together in hot places, forming
the ocean within which tropical nature reserves are insularized, will constitute
90 percent of humanity. Anyone interested in the future of biological diversity
needs to think about the pressures these people will face, and the pressures
they will exert in return.
We also need to remember that the impact of Homo sapiens on the biosphere
can't be measured simply in population figures. As the population expert
Paul Harrison pointed out in his book The Third Revolution, that impact is
a product of three variables: population size, consumption level, and technology.
Although population growth is highest in less-developed countries, consumption
levels are generally far higher in the developed world (for instance, the
average American consumes about ten times as much energy as the average Chilean,
and about a hundred times as much as the average Angolan), and also higher
among the affluent minority in any country than among the rural poor. High
consumption exacerbates the impact of a given population, whereas technological
developments may either exacerbate it further (think of the automobile, the
air conditioner, the chainsaw) or mitigate it (as when a technological innovation
improves efficiency for an established function). All three variables play
a role in every case, but a directional change in one form of human impact--upon
air pollution from fossil-fuel burning, say, or fish harvest form the seas--can
be mainly attributable to a change in one variable, with only minor influence
from the other two. Sulfur-dioxide emissions in developed countries fell
dramatically during the 1970s and 80s, due to technological improvements
in papermaking and other industrial processes; those emissions would have
fallen still farther if not for increased population (accounting for 25 percent
of the upward vector) and increased consumption (accounting for 75 percent).
Deforestation, in contrast, is a directional change that has been mostly
attributable to population growth.
According to Harrison's calculations, population growth accounted for 79
percent of the deforestation in less-developed countries between 1973 and
1988. Some experts would argue with those calculations, no doubt, and insist
on redirecting our concern toward the role that distant consumers, wood-products
buyers among slow-growing but affluent populations of the developed nations,
play in driving the destruction of Borneo's dipterocarp forests or the hardwoods
of West Africa. Still, Harrison's figures point toward an undeniable reality:
more total people will need more total land. By his estimate, the minimum
land necessary for food growing and other human needs (such as water supply
and waste dumping) amounts to one fifth of a hectare per person. Given the
U.N.'s projected increase of 4.9 billion souls before the human population
finally levels off, that comes to another billion hectares of human-claimed
landscape, a billion hectares less forest--even without allowing for any
further deforestation by the current human population, or for any further
loss of agricultural land to degradation. A billion hectares--in other words,
10 million square kilometers--is, by a conservative estimate, well more than
half the remaining forest area in Africa, Latin America, and Asia. This raises
the vision of a very exigent human population pressing snugly around whatever
patches of natural landscape remain.
Add to that vision the extra, incendiary aggravation of poverty. According
to a recent World Bank estimate, about 30 percent of the total population
of less-developed countries lives in poverty. Alan Durning, in his 1992 book
How Much Is Enough? The Consumer Society and the Fate of the Earth, puts
it in a broader perspective when he says that the world's human population
is divided among three "ecological classes": the consumers, the middle-income,
and the poor. His consumer class includes those 1.1 billion fortunate people
whose annual income per family member is more than $7,500. At the other extreme,
the world's poor also number about 1.1 billion people--all from households
with less than $700 annually per member. "They are mostly rural Africans,
Indians, and other South Asians," Durning writes. "They eat almost exclusively
grains, root crops, beans, and other legumes, and they drink mostly unclean
water. They live in huts and shanties, they travel by foot, and most of their
possessions are constructed of stone, wood, and other substances available
from the local environment." He calls them the "absolute poor." It's only
reasonable to assume that another billion people will be added to that class,
mostly in what are now the less-developed countries, before population growth
stabilizes. How will those additional billion, deprived of education and
other advantages, interact with the tropical landscape? Not likely by entering
information-intensive jobs in the service sector of the new global economy.
Julian Simon argued that human ingenuity--and by extension, human population
itself--is "the ultimate resource" for solving Earth's problems, transcending
Earth's limits, and turning scarcity into abundance. But if all the bright
ideas generated by a human population of 5.9 billion haven't yet relieved
the desperate needfulness of 1.1 billion absolute poor, why should we expect
that human ingenuity will do any better for roughly 2 billion poor in the
future?
Other writers besides Durning have warned about this deepening class rift.
Tom Athanasiou, in Divided Planet: The Ecology of Rich and Poor, sees population
growth only exacerbating the division, and notes that governments often promote
destructive schemes of transmigration and rain-forest colonization as safety
valves for the pressures of land hunger and discontent. A young Canadian
policy analyst named Thomas F. Homer-Dixon, author of several calm-voiced
but frightening articles on the linkage between what he terms "environmental
scarcity" and global sociopolitical instability, reports that the amount
of cropland available per person is falling in the less-developed countries
because of population growth and because millions of hectares "are being
lost each year to a combination of problems, including encroachment by cities,
erosion, depletion of nutrients, acidification, compacting and salinization
and waterlogging from overirrigation." In the cropland pinch and other forms
of environmental scarcity, Homer-Dixon foresees potential for "a widening
gap" of two sorts--between demands on the state and its ability to deliver,
and more basically between rich and poor. In conversation with the journalist
Robert D. Kaplan, as quoted in Kaplan's book The Ends of the Earth, Homer-Dixon
said it more vividly: "Think of a stretch limo in the potholed streets of
New York City, where homeless beggars live. Inside the limo are the air-conditioned
post-industrial regions of North America, Europe, the merging Pacific Rim,
and a few other isolated places, with their trade summitry and computer information
highways. Outside is the rest of mankind, going in a completely different
direction."
That direction, necessarily, will be toward ever more desperate exploitation
of landscape. When you think of Homer-Dixon's stretch limo on those potholed
urban streets, don't assume there will be room inside for tropical forests.
Even Noah's ark only managed to rescue paired animals, not large parcels
of habitat. The jeopardy of the ecological fragments that we presently cherish
as parks, refuges, and reserves is already severe, due to both internal and
external forces: internal, because insularity itself leads to ecological
unraveling; and external, because those areas are still under siege by needy
and covetous people. Projected forward into a future of 10.8 billion humans,
of which perhaps 2 billion are starving at the periphery of those areas,
while another 2 billion are living in a fool's paradise maintained by unremitting
exploitation of whatever resources remain, that jeopardy increases to the
point of impossibility. In addition, any form of climate change in the mid-term
future, whether caused by greenhouse gases or by a natural flip-flop of climatic
forces, is liable to change habitat conditions within a given protected area
beyond the tolerance range for many species. If such creatures can't migrate
beyond the park or reserve boundaries in order to chase their habitat needs,
they may be "protected" from guns and chainsaws within their little island,
but they'll still die.
We shouldn't take comfort in assuming that at least Yellowstone National
Park will still harbor grizzly bears in the year 2150, that at least Royal
Chitwan in Nepal will still harbor tigers, that at least Serengeti in Tanzania
and Gir in India will still harbor lions. Those predator populations, and
other species down the cascade, are likely to disappear. "Wildness" will
be a word applicable only to urban turmoil. Lions, tigers, and bears will
exist in zoos, period. Nature won't come to and end, but it will look very
different.
The most obvious differences will be those I've already mentioned: tropical
forests and other terrestrial ecosystems will be drastically reduced in area,
and the fragmented remnants will stand tiny and isolated. Because of those
two factors, plus the cascading secondary effects, plus an additional dire
factor I'll mention in a moment, much of Earth's biological diversity will
be gone. How much? That's impossible to predict confidently, but the careful
guesses of Robert May, Stuart Pimm, and other biologists suggest losses reaching
half to two thirds of all species. In the oceans, deepwater fish and shellfish
populations will be drastically depleted by overharvesting, if not to the
point of extinction then at least enough to cause more cascading consequences.
Coral reefs and other shallow-water ecosystems will be badly stressed, if
not devastated, by erosion and chemical runoff from the land. The additional
dire factor is invasive species, fifth of the five factors contributing to
our current experiment in mass extinction.
That factor, even more than habitat destruction and fragmentation, is a symptom
of modernity. Maybe you haven't heard much about invasive species, but in
coming years you will. The ecologist Daniel Simberloff takes it so seriously
that he recently committed himself to founding an institute on invasive biology
at the University of Tennessee, and Interior Secretary Bruce Babbitt sounded
the alarm last April in a speech to a weed-management symposium in Denver.
The spectacle of a cabinet secretary denouncing an alien plant called purple
loosestrife struck some observers as droll, but it wasn't as silly as it
seemed. Forty years ago, the British ecologist Charles Elton warned prophetically
in a little book titled The Ecology of Invasions by Animals and Plants that
"we are living in a period of the world's history when the mingling of thousands
of kinds of organisms from different parts of the world is setting up terrific
dislocations in nature." Elton's word "dislocations" was nicely chosen to
ring with a double meaning: species are being moved from one location to
another, and as a result ecosystems are being thrown into disorder.
The problem dates back to when people began using ingenious new modes of
conveyance (the horse, the camel, the canoe) to travel quickly across mountains,
deserts and oceans, bringing with them rats, lice, disease microbes, burrs,
dogs, pigs, goats, cats, cows, and other forms of parasitic, commensal, or
domesticated creature. One immediate result of those travels was a wave of
island-bird extinctions, claiming more than a thousand species, that followed
oceangoing canoes across the Pacific and elsewhere. Having evolved in insular
ecosystems free of predators, many of those species were flightless, unequipped
to defend themselves or their eggs against ravenous mammals. Raphus cucullatus,
a giant cousin of the pigeon lineage, endemic to Mauritius in the Indian
Ocean and better known as the dodo, was only the most easily caricatured
representative of this much larger pattern. Dutch sailors killed and ate
dodos during the seventeenth century, but probably what guaranteed the extinction
of Raphus cucullatus is that the European ships put ashore rats, pigs, and
Macaca fascicularis, an opportunistic species of Asian monkey. Although commonly
known as the crab-eating macaque, M. fascicularis will eat almost anything.
The monkeys are still pestilential on Mauritius, hungry and daring and always
ready to grab what they can, including raw eggs. But the dodo hasn't been
seen since 1662.
The european age of discovery and conquest was also the great age of biogeography--that
is the study of what creatures live where, a branch of biology practiced
by attentive travelers such as Carolus Linnaeus, Alexander von Humboldt,
Charles Darwin, and Alfred Russel Wallace. Darwin and Wallace even made biogeography
the basis of their discovery that species, rather that being created and
plopped onto Earth by divine magic, evolve in particular locales by the process
of natural selection. Ironically, the same trend of far-flung human travel
that gave biogeographers their data also began to muddle and nullify those
data, by transplanting the most ready and roguish species to new places and
thereby delivering misery unto death for many other species. Rats and cats
went everywhere, causing havoc in what for millions of years had been sheltered,
less competitive ecosystems. The Asiatic chestnut blight and the European
starling came to America; the American muskrat and the Chinese mitten crab
got to Europe. Sometimes these human-mediated transfers were unintentional,
sometimes merely shortsighted. Nostalgic sportsmen in New Zealand imported
British red deer; European brown trout and Coastal rainbows were planted
in disregard of the native cutthroats of Rocky Mountain rivers. Prickly-pear
cactus, rabbits, and cane toads were inadvisedly welcomed to Australia. Goats
went wild in the Galapagos. The bacterium that causes bubonic plague journeyed
from China to California by way of a flea, a rat, and a ship. The Atlantic
sea lamprey found its own way up into Lake Erie, but only after the Welland
Canal gave it a bypass around Niagara Falls. Unintentional or otherwise,
all these transfers had unforseen consequences, which in many cases included
the extinction of less competitive, less opportunistic native species. The
rosy wolfsnail, a small creature introduced onto Oahu for the purpose of
controlling a larger and more obviously noxious species of snail, which was
itself invasive, proved to be medicine worse than the disease; it became
a fearsome predator upon native snails, of which twenty species are now gone.
The Nile perch, a big predatory fish introduced into Lake Victoria in 1962
because it promised good eating, seems to have exterminated at least eighty
species of smaller cichlid fishes that were native to the lake's Mwanza Gulf.
The problem is vastly amplified by modern shipping and air transport, which
are quick and capacious enough to allow many more kinds of organism to get
themselves transplanted into zones of habitat they never could have reached
on their own. The brown tree snake, having hitchhiked aboard military planes
from the New Guinea region near the end of World War II, has eaten most of
the native forest birds of Guam. Hanta virus, first identified in Korea,
burbles quietly in the deer mice of Arizona. Ebola will next appear who knows
where. Apart from the frightening epidemiological possibilities, agricultural
damages are the most conspicuous form of impact. One study, by the congressional
Office of Technology Assessment, reports that in the United States 4,500
nonnative species have established free-living populations, of which about
15 percent cause severe harm; looking at just 79 of those species, the OTA
documented $97 billion in damages. The lost value in Hawaiian snail species
or cichlid diversity is harder to measure. But another report, from the U.N.
Environmental Program, declares that almost 20 percent of the world's endangered
vertebrates suffer from pressures (competition, predation, habitat transformation)
created by exotic interlopers. Michael Soule, a biologist much respected
for his work on landscape conversion and extinction, has said that invasive
species may soon surpass habitat loss and fragmentation as the major cause
of "ecological disintegration." Having exterminated Guam's avifauna, the
brown tree snake has lately been spotted in Hawaii.
Is there a larger pattern to these invasions? What do fire ants, zebra mussels,
Asian gypsy moths, tamarisk trees, maleleuca trees, kudzu, Mediterranean
fruit flies, boll weevils and water hyacinths have in common with crab-eating
macaques or Nile perch? Answer: They're weedy species, in the sense that
animals as well as plants can be weedy. What that implies is a constellation
of characteristics: They reproduce quickly, disperse widely when given a
chance, tolerate a fairly broad range of habitat conditions, take hold in
strange places, succeed especially in disturbed ecosystems, and resist eradication
once they're established. They are scrappers, generalists, opportunists.
They tend to thrive in human-dominated terrain because in crucial ways they
resemble Homo sapiens: aggressive, versatile, prolific, and ready to travel.
The city pigeon, a cosmopolitan creature derived from wild ancestry as a
Eurasian rock dove (Columba livia) by way of centuries of pigeon fanciers
whose coop-bred birds occasionally went AWOL, is a weed. So are those species
that, benefiting from human impacts upon landscape, have increased grossly
in abundance or expanded in their geographical scope without having to cross
an ocean by plane or by boat--for instance, the coyote in New York, the raccoon
in Montana, the white-tailed deer in northern Wisconsin or western Connecticut.
The brown-headed cowbird, also weedy, has enlarged its range from the eastern
United States into the agricultural Midwest at the expense of migratory songbirds.
In gardening usage the word "weed" may be utterly subjective, indicating
any plant you don't happen to like, but in ecological usage it has these
firmer meanings. Biologists frequently talk of weedy species, meaning animals
as well as plants.
Paleontologists, too, embrace the idea and even the term. Jablonski himself,
in a 1991 paper published in Science, extrapolated from past mass extinctions
to our current one and suggested that human activities are likely to take
their heaviest toll on narrowly endemic species, while causing fewer extinctions
among those species that are broadly adapted and broadly distributed. "In
the face of ongoing habitat alteration and fragmentation," he wrote, "this
implies a biota increasingly enriched in widespread, weedy species--rats,
ragweed, and cockroaches--relative to the larger number of species that are
more vulnerable and potentially more useful to humans as food, medicines,
and genetic resources." Now, as we sit in his office, he repeats: "It's just
a question of how much the world becomes enriched in these weedy species."
Both in print and in talk he uses "enriched" somewhat caustically, knowing
that the actual direction of the trend is toward impoverishment.
Regarding impoverishment, let's note another dark, interesting irony: that
the two converse trends I've described--partitioning the world's landscape
by habitat fragmentation, and unifying the world's landscape by global transport
of weedy species--produce not converse results but one redoubled result,
the further loss of biological diversity. Immersing myself in the literature
of extinctions, and making dilettantish excursions across India, Madagascar,
New Guinea, Indonesia, Brazil, Guam, Australia, New Zealand, Wyoming, the
hills of Burbank, and other semi-wild places over the past decade, I've seen
those redoubling trends everywhere, portending a near-term future in which
Earth's landscape is threadbare, leached of diversity, heavy with humans,
and "enriched" in weedy species. That's an ugly vision, but I find it vivid.
Wildlife will consist of the pigeons and the coyotes and the white-tails,
the black rats (Rattus rattus) and the brown rats (Rattus norvegicus) and
a few other species of worldly rodent, the crab-eating macaques and the cockroaches
(though, as with the rats, not every species--some are narrowly endemic,
like the giant Madagascar hissing cockroach) and the mongooses, the house
sparrows and the house geckos and the houseflies and the barn cats and the
skinny brown feral dogs and a short list of additional species that play
by our rules. Forests will be tiny insular patches existing on bare sufferance,
much of their biological diversity (the big predators, the migratory birds,
the shy creatures that can't tolerate edges, and many other species linked
inextricably with those) long since decayed away. They'll essentially be
tall woody gardens, not forests in the richer sense. Elsewhere the landscape
will have its strips and swatches of green, but except on much-poisoned lawns
and golf courses the foliage will be infested with cheatgrass and European
buckthorn and spotted knapweed and Russian thistle and leafy spurge and salt
meadow cordgrass and Bruce Babbitt's purple loosestrife. Having recently
passed the great age of biogeography, we will have entered the age after
biogeography, in that virtually everything will live virtually everywhere,
though the list of species that constitute "everything" will be small. I
see this world implicitly foretold in the U.N. population projections, the
FAO reports on deforestation, the northward advance into Texas of Africanized
honeybees, the rhesus monkeys that haunt the parapets of public buildings
in New Delhi, and every fat gray squirrel on a bird feeder in England. Earth
will be a different sort of place--soon, in just five or six human generations.
My label for that place, that time, that apparently unavoidable prospect,
is the Planet of Weeds. Its main consoling felicity, as far as I can imagine,
is that there will be no shortage of crows.
Now we come to the question of human survival, a matter of some interest
to many. We come to a certain fretful leap of logic that otherwise thoughtful
observers seem willing, even eager to make: that the ultimate consequence
will be the extinction of us. By seizing such a huge share of Earth's landscape,
by imposing so wantonly on its providence and presuming so recklessly on
its forgivingness, by killing off so many species, they say, we will doom
our own species to extinction. This is a commonplace among the environmentally
exercised. My quibbles with the idea are that it seems ecologically improbable
and too optimistic. But it bears examining, because it's frequently offered
as the ultimate argument against proceeding as we are.
Jablonski also has his doubts. Do you see Homo sapiens as a likely survivor,
I ask him or as a casualty? "Oh, we've got to be one of the most bomb-proof
species on the planet," he says. "We're geographically widespread, we have
a pretty remarkable reproductive rate, we're incredibly good at co-opting
and monopolizing resources. I think it would take really serious, concerted
effort to wipe out the human species." The point he's making is one that
has probably already dawned on you: Homo sapiens itself is the consummate
weed. Why shouldn't we survive, then, on the Planet of Weeds? But there's
a wide range of possible circumstances, Jablonski reminds me, between the
extinction of our species and the continued growth of human population, consumption,
and comfort. "I think we'll be one of the survivors," he says, "sort of picking
through the rubble." Besides losing all the pharmaceutical and genetic resources
that lay hidden within those extinguished species, and all the spiritual
and aesthetic values they offered, he foresees unpredictable levels of loss
in many physical and biochemical functions that ordinarily come as benefits
from diverse, robust ecosystems--functions such as cleaning and recirculating
air and water, mitigating droughts and floods, decomposing wastes, controlling
erosion, creating new soil, pollinating crops, capturing and transporting
nutrients, damping short-term temperature extremes and longer-term fluctuations
of climate, restraining outbreaks of pestiferous species, and shielding Earth's
surface from the full brunt of ultraviolet radiation. Strip away the ecosystems
that perform those services, Jablonski says, and you can expect grievous
detriment to the reality we inhabit. "A lot of things are going to happen
that will make this a crummier place to live--a more stressful place to live,
a more difficult place to live, a less resilient place to live--before the
human species is at any risk at all." And maybe some of the new difficulties,
he adds will serve as incentive for major changes in the trajectory along
which we pursue our aggregate self-interests. Maybe we'll pull back before
our current episode matches the Triassic extinction or the K-T event. Maybe
it will turn out to be no worse than the Eocene extinction, with a 35 percent
loss of species. "Are you hopeful?" I ask. Given that hope is a duty from
which paleontologists are exempt, I'm surprised when he answers, "Yes, I
am."
I'm not. My own guess about the mid-term future, excused by no exemption,
is that our Planet of Weeds will indeed be a crummier place, a lonelier and
uglier place, and a particularly wretched place for the 2 billion people
comprising Alan Durning's absolute poor. What will increase most dramatically
as time proceeds, I suspect, won't be generalized misery or futuristic modes
of consumption but the gulf between two global classes experiencing those
extremes. Progressive failure of ecosystem functions? Yes, but human resourcefulness
of the sort Julian Simon so admired will probably find stopgap technological
remedies, to be available for a price. So the world's privileged class--that's
your class and my class--will probably still manage to maintain themselves
inside Homer-Dixon's stretch limo, drinking bottled water and breathing bottled
air and eating reasonably healthy food that has become incredibly precious,
while the potholes on the road outside grow ever deeper. Eventually the limo
will look more like a lunar rover. Ragtag mobs of desperate souls will cling
to its bumpers, like groupies on Elvis's final Cadillac. The absolute poor
will suffer their lack of ecological privilege in the form of lowered life
expectancy, bad health, absence of education, corrosive want, and anger.
Maybe in time they'll find ways to gather themselves in localized revolt
against the affluent class. Not likely, though, as long as affluence buys
guns. In any case, well before that they will have burned the last stick
of Bornean dipterocarp for firewood and roasted the last lemur, the last
grizzly bear, the last elephant left unprotected outside a zoo.
Jablonski has a hundred things to do before leaving for Alaska, so after
two hours I clear out. The heat on the sidewalk is fierce, though not nearly
as fierce as this summer's heat in New Delhi or Dallas, where people are
dying. Since my flight doesn't leave until early evening, I cab downtown
and take refuge in a nouveau-Cajun restaurant near the river. Over a beer
and jambalaya, I glance again at Jablonski's Science paper, titled "Extinctions:
A Paleontological Perspective." I also play back the tape of our conversation,
pressing my ear against the little recorder to hear it over the lunch-crowd
noise.
Among the last questions I asked Jablonski was, What will happen after this
mass extinction, assuming it proceeds to a worst-case scenario? If we destroy
half or two thirds of all living species, how long will it take for evolution
to fill the planet back up? "I don't know the answer to that," he said. "I'd
rather not bottom out and see what happens next." In the journal paper he
had hazarded that, based on fossil evidence in rock laid down atop the K-T
event and others, the time required for full recovery might be 5 or 10 million
years. From a paleontological perspective, that's fast. "Biotic recoveries
after mass extinctions are geologically rapid but immensely prolonged on
human time scales," he wrote. There was also the proviso, cited from another
expert, that recovery might not begin until after the extinction-causing
circumstances have disappeared. But in this case, of course, the circumstances
won't likely disappear until we do.
Still, evolution never rests. It's happening right now, in weed patches all
over the planet. I'm not presuming to alert you to the end of the world,
the end of evolution, or the end of nature. What I've tried to describe here
is not an absolute end but a very deep dip, a repeat point within a long,
violent cycle. Species die, species arise. The relative pace of those two
processes is what matters. Even rats and cockroaches are capable--given the
requisite conditions; namely, habitat diversity and time--of speciation.
And speciation brings new diversity. So we might reasonably imagine an Earth
upon which, 10 million years after the extinction (or, alteratively, the
drastic transformation) of Homo sapiens, wondrous forests are again filled
with wondrous beasts. That's the good news.
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