Have we already spread all possible pests all around the world?
Obviously not; there are many islands still free of goats and Norway rats, and many insects and diseases to try to keep out of many countries by quarantines. The US Department of Agriculture has been trying at great expense, but apparently without success, to forestall the arrival of killer bees and Mediterranean fruit flies. In fact, what will probably prove to be the biggest extinction wave caused by an introduced predator in modern times has just started in Africa's Lake Victoria, home to hundreds of species of remarkable fishes found nowhere else in the world. A large predatory.fish called the Nile perch, intentionally introduced in a misguided effort to establish a new fishery, is now eating its way through the lake's unique fishes. Habitat destruction is the third means by which we exterminate. Most species occur in just a certain type of habitat: marsh warblers live in marshes, while pine warblers live in pine forests. If one drains marshes or cuts forests, one eliminates the species dependent on those habitats just as certainly as if one were to shoot every individual of the species. For example, when all the forest on Cebu Island in the Philippines was logged, nine of the ten birds unique to Cebu became extinct. In the case of habitat destruction, the worst is still to come because we are just starting in earnest to destory tropical rainforests, the world's most species-rich habitats. The rainforests' biological richness is legendary—over 1,500 beetle species living in a single rainforest tree species in Panama, for instance. Rainforests cover only six per cent of the Earth's surface but harbour about half of its species. Each area of rainforest has large numbers of species unique to that area. To mention only some exceptionally rich rainforests now being destroyed, the felling of Brazil's Atlantic forest and Malaysia's lowland forest is already almost complete, and those of Borneo and the Philippines will be mostly logged within the next two decades. By the middle of the next century, the only large tracts of tropical rainforest likely to be still surviving will be in parts of Zaire and the Amazon Basin.
Every species depends on other species for food and for providing its habitat. Thus, species are connected to each other like branching chains of dominoes. Just as toppling one domino in a chain will topple some others, so too the extermination of one species may lead to the loss of others, which may in turn push still others over the brink. This fourth mechanism of extinction may be described as a ripple effect. Nature consists of so many species, connected to each other in such complex ways, that it is virtually impossible to foresee where the ripple effects from the extinction of any particular species may lead.
For example, fifty years ago no one anticipated that the extinction of big predators (jaguars, pumas, and harpy eagles) on Panama's Barro Colorado Island would lead to the extinction there of little antbirds, and to massive changes in the tree species composition of the island's forest. Yet it did so, because the big predators used to eat medium-sized predators like peccaries, monkeys, and coatimundis, and medium-sized seed-eaters like agoutis and pacas. With the disappearance of the big predators, there was a population explosion of the medium- sized predators, which proceeded to eat up the antbirds and their eggs. The medium-sized seed-eaters also exploded in abundance and ate large seeds that had fallen on the ground, thereby suppressing the propagation of tree species producing large seeds and favouring instead the spread of competing tree species with small seeds. That shift in forest tree composition is expected in turn to cause an explosion of mice and rats feeding on small seeds, and then to an explosion in hawks, owls, and ocelots preying on those small rodents. Thus, the extinction of three uncommon species of big predators will have triggered a rippling series of changes in the whole plant and animal community, including the extinction of many other species.
Through these four mechanisms—overhunting, species introductions, habitat destruction, and ripple effects—probably over half of existing species will be extinct or endangered by the middle of the next century, when this year's crop of human babies reaches the age of sixty. Like many fathers today, I often wonder how I will describe to my twin sons, who are now three years old, the world that I grew up in and that they will never see. By the time they would have been old enough to come with me to New Guinea, one of the world's biological treasurehouses where I have worked for the past twenty-five years, most of New Guinea's eastern highlands will be deforested. When one adds the extinction of species we have already caused to that which we are about to cause, it is clear that the current extinction wave is surpassing the asteroid collision that may have wiped out the dinosaurs. Mammals, plants, and many other types of species survived that collision nearly unscathed, while the current wave is affecting everything from leeches and lilies to lions. Thus, the claimed extinction crisis is neither a hysterical fantasy, nor just a serious risk for the future. Instead, it is an event that has already been accelerating for 50,000 years and will start to approach completion in our children's lifetimes.
Let's finally consider two arguments that accept the reality of the extinction crisis but dismiss its significance. Firstly, is extinction not a natural process anyway? If so, why make a big deal about the wave of extinction happening now?
The answer to this first argument is that the current extinction rate caused by humans is far higher than the natural rate. If the estimate that half the world's total of thirty million species will become extinct in the next century is correct, then species are now becoming extinct at a rate of about 150,000 per year, or seventeen per hour. The world's 9,000 bird species are becoming extinct at a rate of at least two per year, but bird species under natural conditions were disappearing at a rate of less than one per century, so the present rate is at least 200 times the normal rate. Dismissing the extinction crisis on the grounds that extinction is natural would be just like dismissing genocide on the grounds that death is the natural fate of all humans.
The second argument is a simple one: so what? We care about our children, not about beetles and snail darters; who cares if ten million beetle species become extinct? The answer to this argument is equally simple. Like all species, we depend on other species for our existence, in many ways. Some of the most obvious ways are that other species produce the oxygen we breathe, absorb the carbon dioxide we exhale, decompose our sewage, provide our food, maintain the fertility of our soil, and provide our wood and paper.
Then could we not preserve only those particular species that we need, and let other species become extinct? Of course not, because the species we need also depend on other species. Just as Panama's antbirds could not have anticipated their need for jaguars, the ecological chain of dominoes is much too complex for us to have figured out which dominoes we can dispense with. For instance, could anyone please answer these three questions. Which ten tree species produce most of the world's paper pulp? For each of those ten tree species, which are the ten bird species that eat most of its insect pests, the ten insect species that pollinate most of its flowers, and the ten animal species that spread most of its seeds? Which other species do these ten birds, insects, and animals depend on? You would have to be able to answer those three impossible questions if you were the president of a timber company trying to figure out which species you could afford to allow to become extinct.
If you are trying to evaluate some proposed development project that would bring in a million dollars but might exterminate a few species, it is still tempting to prefer the certain profit over the uncertain risk. Then consider the following analogy. Suppose someone offers you a million dollars in return for the privilege of painlessly cutting out two ounces of your valuable flesh. You figure that two ounces is only one-thousandth of your body weight, so you will still have nine hundred and ninety-nine thousandths of your body left, which is plenty. That is fine if the two ounces come from your spare body fat and if they will be removed by a skilled surgeon. But what if the surgeon just hacks two ounces from any conveniently accessible part of your body, or does not know which parts are essential? You might then find that the two ounces came from your urethra. If you plan to sell off most of your body, as we now plan to sell off most of our planet's natural habitats, you are certain eventually to lose your urethra.
To conclude, let's place matters in perspective by comparing the two clouds which, as I mentioned at the outset, are hanging over our future. A nuclear holocaust is certain to prove disastrous, but it is not happening now, and it may or may not happen in the future. An environmental holocaust is equally certain to prove disastrous, but it differs in that it is already well underway. It started tens of thousands of years ago, is now causing more damage than ever before, is in fact accelerating, and will climax within about a century if unchecked. The only uncertainties are whether the resulting disaster would strike our children or our grandchildren, and whether we choose to adopt now the many obvious countermeasures.
EPILOGUE
Let's now draw together the themes of this book, by tracing our rise over the last three million years, as well as our incipient reversal of all our progress more recently.
