The Third Chimpanzee: The Evolution and Future of the Human Animal Page 18

  CHAPTER 16

  THE SECOND CLOUD

  MY GENERATION IS THE FIRST IN HUMAN HISTORY to worry whether our children and grandchildren will survive, or will have a planet worth living on. That’s because we as a species have two clouds hanging over us. These clouds could lead to similar results, but we view them very differently.

  One cloud is the risk of a nuclear war that could destroy us all. That risk first revealed itself in the mushroom-shaped cloud over the Japanese city of Hiroshima, where the first atom bomb was dropped in 1945, during World War II . Everyone agrees that the nuclear risk is real. Nations have stockpiles of weapons, and politicians throughout history have occasionally made dumb mistakes. The nuclear risk shapes much of world diplomacy today.

  The second cloud is the risk of an environmental collapse. One potential cause of such a collapse is the gradual extinction of most of the world’s species. But while everyone agrees that a nuclear holocaust would be bad, there is great disagreement about whether the risk of a mass extinction is real—and about whether it would do much harm if it happened.

  Is an Environmental Holocaust Under Way?

  Figures from the International Council for Bird Preservation (ICBP) suggest that humans have caused about 1 percent of the world’s bird species to go extinct within the last few centuries. What do people think about that?

  At one extreme, many thoughtful people— especially economists and industrial leaders, but also some biologists and ordinary citizens—think that 1 percent is an overestimate and that the true loss has been smaller. But, they add, if it happened, losing 1 percent of bird species wouldn’t matter.

  At the opposite extreme, many other thoughtful people—especially conservation biologists and people who belong to environmental groups—think that 1 percent is an underestimate and that the true loss has been larger. They also think that mass extinction would greatly undermine the quality, or even the possibility, of human life. It will make a big difference to future generations which of these extreme views is closer to the truth.

  How many species have humans driven into extinction? How many more are likely to go extinct in your lifetime, or your children’s lifetimes? And what does it matter if they do? Aren’t all species going to become extinct sometime or other? Is mass extinction a fantasy, a real risk for the future, or a crisis that’s already well under way?

  To answer these questions we first need realistic numbers. We need to find out how many species have gone extinct in modern times— that is, since 1600, when scientific naming and classification of species was just beginning. Then we need to estimate how many extinctions were caused by humans before 1600, and how many might happen in the future. At that point we can ask what difference it all makes to us anyway.

  Modern Extinctions

  Where did the figure of 1 percent of bird species lost since 1600 come from? The ICBP lists 108 species of bird, plus many additional subspecies, as having become extinct since 1600. Nearly all these extinctions were caused in some way by humans— more on that later in this chapter. Approximately 9,000 species of birds exist today. The 108 extinct species is almost 1 percent of 9,000.

  But the ICBP calls a species extinct only after it has been specifically searched for in a place where it was once known to exist, or where it might turn up. What about birds that haven’t been deliberately searched for? In Europe and North America, hundreds of thousands of fanatical birdwatchers monitor the status of bird species every year. Unfortunately, this does not apply to plants and animals. It doesn’t even apply to birds in many parts of the world.

  Most countries in the tropics, where the overwhelming majority of species live, have few birdwatchers. The status of many tropical species is unknown because no one has seen them or specifically looked for them since they were discovered. One example from New Guinea is Brass’s friarbird. It is known only from eighteen specimens found at a single lagoon in 1939. No scientist has gone back to that lagoon, so we know nothing about the status of Brass’s Friarbird today.

  At least we know where to look for that friarbird. Many other species are known from collections made by nineteenth-century explorers. Sometimes they gave vague information about where they collected the birds. Try settling the status of a species when you have only “South America” to tell you where to look! So one problem in answering questions about extinction is that we do not know whether or not many named species still exist. But could species have gone extinct before they were even named?

  Of course they could. Scientists think the total number of the world’s species is around thirty million, but fewer than two million have been identified and named. An example from the plant world shows why we can be sure that many species vanished before being named. Botanist Alwyn Gentry surveyed the plants of Centinela, an isolated ridge in the South American nation of Ecuador. He found thirty-eight new plant species that grew on that ridge and nowhere else. Soon afterward, the ridge was logged and those plants were exterminated.

  It was pure accident that Gentry visited Centinela before it was logged. Thousands of species of plants, snails, and other creatures must have existed on countless other ridges now cleared. We exterminated those species before we even discovered them.

  A cyprinid, native to Malaysia.

  MALAYSIA’S MISSING FISH

  TROPICAL COUNTRIES TEND TO BE RICH IN SPECIES. But between growing populations and economic demands, many of them face pressure on their environments and resources. The Southeast Asian nation of Malaysia is typical. The case of its missing fish shows how this pressure can lead to extinction. Biological explorers had identified 266 species of fish in the forest rivers of Malaysia. But after most of the country’s lowland forest was cut down, a search that lasted four years was able to find only 122 of those species—fewer than half. The other 144 Malaysian freshwater fish species must either be very rare and limited to small areas, or extinct. They reached that status before anyone noticed it. If Malaysia has already or almost lost half its freshwater fish species, this gives us a reasonable ballpark figure for the status of plants, fish, and many other kinds of species elsewhere in the tropics.

  Past Extinctions

  We know that species have become extinct since 1600 because the world’s human population has grown in numbers, moved into uninhabited areas, and invented increasingly destructive technologies. What about human-caused extinctions before 1600? Is there a way to estimate them?

  Fifty thousand years ago our species was confined to Africa and the warmer parts of Europe and Asia. Between then and 1600, we expanded to occupy the other continents and most oceanic islands. We also underwent a massive expansion in numbers, from perhaps a few million people fifty thousand years ago to about half a billion in 1600. We’ve already seen in chapters 14 and 15 that in every part of the world paleontologists have studied, and where humans arrived within the last fifty thousand years, waves of extinctions happened around the same time the humans arrived.

  Ever since scientists realized this, they’ve argued over whether humans caused the extinctions or just happened to arrive at a bad time, when species were dying out because of changes in climate. There is no reasonable doubt that human activities caused the wave of bird extinctions on Polynesian islands and on Madagascar. As for earlier extinctions, especially those in Australia and the Americas, the cause is still being debated. But I doubt that climate did it. Climate swings did occur, but they did not cause waves of extinction every time they occurred, or everywhere they occurred. The extinction waves match up more closely with human arrival than with climate change.

  Prehistoric people probably exterminated species not just in newly colonized lands but also in places where they had lived for a long time. Within the past twenty thousand years, Eurasia lost its mammoths, giant deer, and woolly rhinos. Africa lost its giant buffalo and giant horse. These big beasts may have been wiped out by humans who had been hunting them for a long time and who suddenly developed better weapons than ever before. The big animals d
isappeared for the same reasons that California’s grizzly bears and Britain’s bears, wolves, and beavers disappeared in modern times, after being hunted for thousands of years. Those reasons were more people and better weapons.

  No one has ever tried to guess how many plant, lizard, or insect species were exterminated by prehistoric humans. Studies of bird extinction on islands, though, suggest that prehistoric humans on islands killed off about two thousand species—a fifth of all the bird species that existed a few thousand years ago. That doesn’t count bird species that may have been exterminated on continents in prehistoric times. As for large mammals, scientists have looked at the disappearance not just of species but of genera, which are groups of related species. In North America, 73 percent of the genera of large mammals became extinct at the time humans arrived or afterward. For South America and Australia, the figures are 80 and 86 percent.

  Future Extinctions

  Is the peak of the human-caused extinction wave already past, or is most still to come? There are a couple of ways to think about this question.

  One way to predict future extinctions is to think that tomorrow’s extinct species will come from the list of species that are endangered today. How many species that still exist have had their numbers reduced to dangerously low levels? The International Council for Bird Preservation estimates that at least 1,666 bird species are either endangered or at risk of becoming extinct soon. That’s almost 20 percent, or one-fifth, of the world’s surviving birds. I said “at least 1,666” because this number is a low estimate. It is based just on species whose status caught scientists’ attention, not on a survey of the status of all bird species.

  Birds are not the only species at risk, of course. Numerous species of mammals, fish, reptiles and amphibians, insects and other small creatures, and plants are also known to be on the brink of extinction.

  Another way to think about future extinctions is to understand the ways we exterminate species. Our growing population drives species to extinction in four main ways: overhunting, species introduction, habitat destruction, and ripple effects. Let’s see if any of these has leveled off.

  Overhunting means killing animals faster than they can maintain their numbers by breeding. It’s the main way we’ve exterminated big animals. Have we already killed off all the big animals we might kill off? No. After overhunting drove down the number of whales, most countries signed an international ban on hunting whales for commercial purposes. Japan, however, tripled the number of whales it allowed to be killed for “scientific reasons.” Africa’s rhinos and elephants are being slaughtered at an increasing rate for their horns and ivory. At these rates, not just rhinos and elephants but most other large mammals of Southeast Asia and Africa—that is, those outside zoos and game parks—will be extinct within a few decades.

  Species introduction means introducing species to parts of the world, either accidentally or on purpose, where they didn’t previously live. In the United States, for example, the introduced species that are now firmly established include Norway rats, European starlings, and the fungi that are damaging our Dutch elm and chestnut trees. None of these or other introduced species is native to North America. All were brought there, accidentally or on purpose, by humans.

  When species are introduced to a new area, they often exterminate some native species, either by eating them or by causing diseases. New Zealand bird species that nest on the ground, for example, are threatened by nonnative rats that eat eggs and young birds. American chestnut trees are another example. They have been practically exterminated by chestnut blight, an Asian fungus. That fungus doesn’t harm chestnut trees in Asia because they had time to evolve defenses against it.

  We are still spreading pests around the world, although many islands remain free of goats and Norway rats, and countries try to keep insects and diseases from entering. Good intentions do not guarantee good outcomes. What might be the biggest extinction wave in modern times was started a few decades ago in Africa’s Lake Victoria, home to hundreds of fish species found nowhere else. People deliberately introduced a large fish called the Nile perch to the lake, thinking it would make a good commercial food source. The Nile perch is a predator that is now eating its way through Lake Victoria’s unique species.

  Habitat destruction is the third way we exterminate species. Most species live in a particular type of habitat and that habitat only. Marsh warblers are bird that live in marshes, while pine warblers live in pine forests. When the marshes are drained or the forests cut, the species that depend on those habitats are eliminated. Cebu Island in the Philippines once had ten species of birds that lived only on that island. When all the forest on Cebu Island was logged, nine of those species became extinct.

  The worst habitat destruction is still to come. We are beginning to destroy the world’s tropical rain forests, which cover only 6 percent of the earth’s surface but are home to more than half its species. Brazil’s Atlantic forest and Malaysia’s lowland forest are almost completely gone, and those of Borneo and the Philippines are going. By the middle of the twenty-first century, the only large tracts of tropical rain forest likely to be surviving will be in parts of the Amazon Basin of South America and the Democratic Republic of Congo in Africa.

  The ripple effect, the fourth form of habitat destruction, occurs when an action has unexpected results. Every species depends on other species for food and habitat. Species are connected to one another like branching rows of dominoes. Toppling one domino causes others to fall. Removing one species may lead to the loss of others, which in turn can push still other species to the brink.

  THE JAGUAR AND THE ANTBIRD

  NATURE CONSISTS OF SO MANY SPECIES, connected to one another in such complex ways, that it’s impossible to foresee where the ripple effects from the loss of one species may lead. The fate of the little antbird on Barro Colorado Island in Panama shows the ripple effect in action.

  In the middle of the twentieth century, Barro Colorado had three big predators: jaguars, pumas, and harpy eagles. No one expected that the removal of those predators would lead to the extinction on the island of the little antbird, and to massive changes in the island’s forests. But it did.

  The three big predators used to eat mediumsize predators, such as monkeys, peccaries (a type of wild pig), and coatimundis (a relative of raccoons). The big predators also ate mediumsize seed eaters, such as agoutis and pacas (two types of rodents). With the disappearance of the big predators, there was a population explosion of the medium-size predators, who gobbled up the antbirds and their eggs. There was also an explosion of medium-size seed eaters, who ate large seeds that had fallen on the ground. This made it harder for trees with large seeds to replace themselves and spread, and easier for trees with small seeds to do so.

  As the forests of Barro Colorado shift toward having more of the small-seeded trees and fewer large-seeded ones, other changes will occur. Species that feed on small seeds, such as mice and rats, will have their own population explosion, and this in turn will lead to greater numbers of the hawks, owls, and ocelots (small jungle cats) that feed on mice and rats.

  Jaguars, pumas, and harpy eagles were never common on Barro Colorado. But their complete removal from the island had a ripple effect on the whole plant and animal community, including the extinction of many other species.

  Barro Colorado Island.

  Why Does Extinction Matter?

  Isn’t extinction a natural process? If so, why should we worry about the extinctions that are happening now?

  Yes, every species will eventually go extinct. But the current human-caused rate of extinction is much higher than the natural rate. We know from the fossil record the average rate at which species became extinct over long periods of geological time. For birds, for example, the natural extinction rate averages less than one species each century. Today, though, we are losing at least two bird species each year—two hundred times the natural extinction rate. Not worrying about today’s extinction wave because
extinction is natural would be like not worrying about mass murder because death is the natural fate of all humans.

  As to why we should worry about mass extinction, remember the ripple effect. The species we depend on depend on other species. Can you say which ten tree species produce most of the world’s paper? For each of those ten trees, which are the ten bird species that eat most of the tree’s insect pests, the ten insect species that pollinate most of its flowers, and the ten animal species that spread most of its seeds? Which species do those birds, insects, and animals depend on? You’d have to be able to answer those questions if you were the president of a timber company trying to figure out which species you could afford to let go extinct.

  Compare the two clouds I mentioned at the beginning of this chapter, the ones hanging over our future. A nuclear holocaust is certain to be a disaster for us, but it isn’t happening now, and it may never happen. An environmental holocaust is equally certain to be a disaster, but it is already well under way. It started tens of thousands of years ago and it is currently causing more damage than ever before—and the rate of destruction is increasing. Will we now choose to stop it?

  AFTERWORD

  NOTHING LEARNED, EVERYTHING FORGOTTEN?

  THE THEMES OF THIS BOOK COME TOGETHER IN our rise over the past three million years— and also in the way we now stand on the verge of reversing all our progress.

  The first signs that our ancestors were unusual among animals were the crude stone tools that appeared in Africa 2.5 million years ago. Although tools were becoming a regular, important part of our livelihood, they did not trigger a big jump in our development as a species.

  For another 1.5 million years we remained in Africa. Around a million years ago we spread to the warm areas of Europe and Asia. This made us the most widespread of the three chimpanzee species—but still much less widespread than lions. By 100,000 years ago, at least, Neanderthal humans were using fire, but in other ways we were still just another big mammal. We had not developed a trace of art, architecture, or high technology. No one knows whether we had developed language, drug addictions, or our mating habits and life cycle.