by Chris Clarke
No part of earth is undisrupted by humankind's acts, from the sea floor to the interior of the most remote land. Only those species that thrive on our disturbance have benefited. The rest are at least beginning to decline. Some have vanished forever. For decades, environmentalists have warned that our planet stands on the brink of a mass extinction. More and more scientists have joined the warning chorus.
Environmentalists often refer to the current frightening situation as "unprecedented," going on to predict a biologically sparse period lasting millions of years, or however long it will take for the surviving rats and starlings to evolve into antelopes and albatrosses. In the meantime, we will live in a world that is immeasurably, if intangibly, poorer for lacking wild things.
But our predicament is not unprecedented. To think it is unprecedented deprives us of the opportunity to learn from past extinctions what we may expect of this one. And one conclusion we can reasonably draw is that in the wake of this extinction, our lives will be more unpleasant in a very tangible way. The coming extinction may make us sick.
About this issue
Despite the clear threat of this wave of extinction, our species is accelerating our damage to the biosphere. As you will read in the articles that follow in this special issue of Earth Island Journal, humankind is embarked on an unprecedented project of destruction of the natural world. We are scouring the seafloor. We are systematically killing off most animals larger than a German shepherd. We are reducing the amount of habitat available to other organisms - such as the migratory songbirds and amphibians whose declines are described here - and eroding the integrity of the habitat that remains, by means ranging from climate change to the introduction of invasive species.
Some of the evidence of decline, like that in the breeding bird surveys, is ambiguous. Some damage, such as that done by clearcutting, is devastatingly obvious. Regardless of the degree of certainty in any one area, however, it is clear that the cumulative effect of our actions - wiping away habitat, poisoning the air and water, tweaking the climate and "harvesting" whole species out of existence - will (or has already) set in motion what biologists and paleontologists call a mass extinction, in which 20 to 95 percent of the species on the globe die off completely in a relatively short time.
We are not, in all likelihood, seeing that yet. Despite the dire statements of some environmentalists, we have not yet caused the level of damage seen in mass extinctions for which we have fossil evidence. Paleontologists identify five mass extinctions in the fossil record, and a host of smaller extinction events, the least of which wiped away 20 percent of living genera. The great-grandmother of all extinctions, which took place 245 million years ago at the end of the Permian, saw the disappearance of 75 to 96 percent of all species alive at the time. Sixty percent of all genera in the world didn't make it past that period. More than half of all families (the taxonomic category just above the genus) died out. We talk about critically endangered species, such as the cheetah, or subspecies, such as the Siberian tiger; imagine not just those but lions, pumas, cervals, bobcats, lynx, and Sylvester all dying out and you are still picturing an extinction event orders of magnitude less devastating than the Permian. Compared to the old-fashioned mass extinctions the planet used to have, our current unpleasantness ranks only slightly above the apparent background extinction level. We have certainly not extirpated anything near a double-digit percentage of existing genera. Our destructive proclivities may be unprecedented, but the current wave of extinctions is not - so far.
Fits and starts
As our situation isn't unprecedented, we can look at previous mass extinctions to see what we can expect if we cause another. To do so, we must take a look at the mechanics of extinction and evolution.
Unless you went to school very recently, you learned of evolution as a slow, arduous process. Minor, almost imperceptible variation enters the genetic pool of a species, through mutations. With enough time the genepool may become modified enough by accumulation of beneficial (or merely benign) mutations to constitute a new species.
Despite popular mythology, the process does not imply increasing perfection. A recent study of the famous Galapagos finches is a case in point. Researchers Peter and Rosemary Grant found that the finches are, indeed, constantly evolving, but in response to cyclically-changing environmental conditions. A dry spell might mean large, hard-shelled seeds are the only available finch food - other plants fared poorly. Thus, finches with big strong beaks reproduce more successfully. When the dry spell ends, there are more small, easily-opened seeds available, and the big-billed finches find it harder to handle this food than do their smaller-beaked cousins. Succeeding generations show a trend toward smaller snoots. Cycle after cycle, the finch populations evolve to adapt to shifting environmental conditions, at a remarkably rapid pace: one or two generations, as opposed to eons. In environmentally stable times, the goalposts of that evolution are constantly shifted by the regular cycles of the environment: drought and flood, warm and cold, perhaps even - for organisms as short-lived as bacteria - night and day.
If Galapagos finches are nothing special and other organisms evolve in basically the same fashion, one would expect to find two consequences in the paleontological record. First, that when ecological changes were cyclical and relatively gentle for long periods of time, we should find a fair amount of stability in species, that generational oscillation in a characteristic such as beak size would appear to us as a very stable range of variation in the fossils of a species.
Second, we would expect that where change is sudden, marked, and non-cyclical, as is the case during and after a mass extinction event, such change would generate dramatic evolutionary changes in the species surviving. If, as is thought, a comet smacked into what is now the Yucatan at the close of the Cretaceous, killing off the dinosaurs and ammonites and pteranodons with a global cloud of sulfur and dust, the stable cycles of environmental change would have been seriously disrupted, meaning that species would be pressed to adapt to abruptly different environmental circumstances. One would expect a sudden flourishing of diverse and innovative new species.
In fact, paleontology confirms exactly these conclusions. The bulk of the fossil record shows stable species, with a low background level of extinction and a similarly low background level of new species arising. But after the double handful of extinction events for which we have evidence, new species appear remarkably suddenly, diversify, and split into more new species. Then the lineages solidify and the biosphere becomes relatively stable again, until the next comet hits.
This is the heart of "punctuated equilibrium," a model of evolution first proposed by Niles Eldredge and Stephen Jay Gould. Each extinction, whether caused by comet or climate or just dumb luck on the part of the last fertile members of a species, changes the environment of its neighbors, providing both the opportunity and the impetus to evolve. The more extinctions, the more subsequent evolution. Even after the Permian extinction, when life was nearly wiped out altogether, species evolved to fill the new gaps very quickly.
Planet of plagues
So mass extinctions have, in the past, been followed by a burst of rapid evolution in which new species take advantage of whatever opportunities are available to them for survival. We face an impending mass extinction caused by our own actions. Despite the apocalyptic pronouncements of some environmental activists, our species is almost certain to weather the wave of extinctions. There are so many of us that even if nine-tenths of the people in the world died all at once, it would make little difference from an evolutionary standpoint. We can survive with bone awls and axes on glaciers; we can thrive in the bleakest, driest deserts; we breed successfully amid the rats and pigeons and carcinogenic soot of our large cities. We aren't going anywhere. On the contrary, unless we experience a species-wide shift in our approach to the rest of the world, Homo sapiens will continue to dominate the biosphere post-extinction.
Logic, then, would dictate that the majority of evolutionary job openings in the post-holocaust world will be associated, to some degree, with humans.
In the excellent article "Planet of Weeds", published in the October 1998 Harper's, David Quammen describes some of these opportunities. In the wake of ecological disruption, Quammen notes, species that are pre-adapted to take advantage of disturbed ecosystems are thriving. He forecasts a world dominated by the equivalents of rats and starlings, crows, buffel grass and kudzu, with species dependent on stable wild ecosystems increasingly displaced.
Quammen's article is relatively optimistic. Annoying and pestiferous as starlings and tumbleweeds may be, such species will generally respect the boundary of our skin, the occasional rat bite or star thistle prick notwithstanding.
A hint of a worse future hit the front pages early this year, when researchers determined that the Human Immunodeficiency Virus (HIV) originated in wild populations of chimpanzees. Chimps' habitat is shrinking, and wild bands increasingly come into proximity with human beings. Somehow, through meat-hunting or other close contact, a human contracted HIV. The subsequent human death toll may have passed 12 million as you read this.
Microorganisms rule the world, as they have since life began. The majority of biomass on the planet consists of single-celled organisms. We may notice their presence only when they make us sick or leaven our bread, but we could not live without them. They are the foundation of the nitrogen cycle: only bacteria can take nitrogen from the air and change it into usable form. It's a safe bet that there is no animal that could survive without its internal flora of microorganisms. Every moth, wildebeest, or aardvark bears a specialized community of germs, some of them so fitted to their host species that they are found nowhere else.
When these animals vanish, their internal flora either find new hosts or go extinct. Most of them will die out. Some will find a convenient alternative organism to inhabit. Humans are likely to be convenient fairly often. Many of these organisms will be relatively benign. But some, like HIV, the Ebola virus, and hantavirus, will interfere violently with the workings of the human body.
This is as bad for the pathogen as the host: if you die, most of the germs inside you die. Less-deadly forms of the pathogens survive more reliably, thus reproducing more successfully, thus being selected for in an evolutionary sense. There is some evidence that HIV is becoming less-virulent with the passing decades, and you yourself may well harbor Ebola Reston, a benign strain of that frightening virus.
As habitats are destroyed and their resident species obliterated, humans face the prospect of a series of plagues. At first, some of these diseases will kill lots of people, then they'll settle in to making life unpleasant, but not quite killing their hosts. They may take many forms: a growing range of complaints has been recently linked to infection by microorganisms. Fifteen to twenty percent of human cancers are now known to be infectious in origin. Ulcers and kidney stones have been linked to pathogens, and some postulate a schizophrenia germ. The plagues may be here already, unrecognized.
If there is an upside to this frightening trend, it is that the threat of emerging diseases provides a concrete reason for preserving biodiversity. Environmentalists have long appealed to aesthetics or morality to enlist support in protecting the wild, or dangled the elusive promise of miracle drugs from mysterious rainforest herbs. These moral and utilitarian arguments are persuasive, but incomplete.
Emerging diseases give us a compelling reason to preserve what we can of the myriad wild habitats we're now threatening: self-defense. Better to keep the next millennium's diseases where they belong: in the coral reefs and rainforest canopies and bloodstreams of limestone-cave bats. We can try to stem the next extinction, keeping the billions of strains of microorganisms in check, or we can provide those microbes with inadvertent wildlife refuges in our bodies.
Extinction and Health