The Extinctions Accelerate

The extinctions at Mahaulepu appear to have a single cause, namely humans. But as with mass extinctions in the past, a single cause can have many destructive effects. In Burney’s hole, he can see all of them at work. Two of the most important ways humans drove species extinct around Mahaulepu were by hunting them and by destroying their habitats.

These different kinds of destruction have emerged in a series of stages. The first animals to go extinct on Kauai were those most vulnerable to hunting. “They were slow species, they may have been particularly useful as food–a giant, flightless duck sounds great for a barbecue,” says Burney. “They may have been, in particular, species that were vulnerable simply because they were on the ground all the time. They laid their eggs on the ground, they’d never seen any ground‑based predators, like the rat, before. And so their eggs were just eaten up, by the rats and pigs and so forth.”

The flightless ducks of Kauai probably met the same fate as the elephant birds of Madagascar and the mammoths of North America: quick destruction by humans. As the people of Kauai wiped out the easiest game, they might have turned to smaller prey such as land crabs. In Burney’s hole he can watch this tragedy play out: after humans arrive, the land crab fossils get smaller and rarer, presumably as hunters were forced to take younger and younger crabs. Without enough young crabs left to reproduce, the population collapsed.

The stages of extinctions at Mahaulepu match the stages of extinction worldwide. Today, overhunting still remains a global threat to wildlife. In the deepest rain forests of central Africa, hunters are killing chimps and other primates to feed the loggers who are deforesting the region. Meanwhile, in even the remotest corner of Myanmar (formerly Burma), rare species of deer that have been discovered only in the past few years are being killed at an unsustainable rate. Hunters are killing them not to eat their meat, but in order to exchange them with Chinese traders for salt.

The second stage of extinctions at Mahaulepu–caused by the disappearance of the old habitats–came more slowly. The human population grew on Kauai, and in order to feed themselves they cleared land for crops and livestock. Without metal axes, the pioneers couldn’t bring down trees very quickly; instead, they probably killed them by girdling their trunks or poisoning their roots. In place of the forests, they planted their crops of taro and sweet potato. Once Europeans arrived at Kauai, the destruction sped up greatly. By the 1840s, plantation owners had started to clear vast estates of their trees. The sandalwood was harvested for incense, and the rest was cleared so that they could graze cattle and plant pineapple and sugarcane.

The same accelerating destruction of habitats has occurred around the world. Starting about 10,000 years ago, civilizations in Mexico, China, Africa, and the Near East brought plants and animals under their control. Agriculture provides such a reliable amount of food that farmers and herders can survive at higher densities than hunter‑gatherer peoples. To feed more mouths, more land had to be taken over. With cattle and sheep and goats people began grazing down meadows. To sow land for corn, rice, and wheat, they cleared away forests and grasslands. The English farmlands where Darwin had grown up had not always been farmlands; the forests that had once covered them were gradually cut, shrinking down to islands over the course of centuries. The animals that could live only in these islands were stranded inside them. The closest wild relative of cattle, the aurochs, survived in patches of forests in Poland, protected from hunting, until the 1600s, when it disappeared for good.

With a booming human population and the invention of better plows and saws in the last few centuries, wilderness has disappeared at a Malthusian rate. With more people come more farms and cities. More people need more firewood and lumber than many forests can sustainably supply. Technology has made it easier to build roads into those forests and haul the wood out. As a result, half of the world’s tropical forests, where an estimated two‑thirds of all species live, were logged or burned as of 2000.

As humans take over wilderness, plants and animals go extinct. In some cases–as when a dam dries up the one river where a fish species lives–the extinction is obvious. But a habitat doesn’t have to disappear completely for a species to disappear. Even fragmenting a habitat is enough to cause extinctions. These fragments are like islands, and the same rules that predict how many species can live on an island like Krakatau also apply to them.

Each fragment of a forest can support only a certain number of species, proportional to its size. If it happens to contain more than its allotment of species when it becomes cut off from the rest of the forest, those extra species will vanish. If a species should be unlucky enough to disappear from all of its fragments, it is gone for good.

Species with small ranges are most vulnerable to extinction by fragmentation. Imagine that loggers cut down most of the forests along a mountain range. A salamander that lives on a single mountainside might end up in only three forest fragments, whereas a salamander that lived along the entire range might end up in 100 fragments. It’s much more likely that the widespread salamander will cling to existence in at least some of its refuges than the one with a limited range. And if the widespread salamander can then travel between fragments, it may even be able to recolonize some of its former range. The salamander on the single mountainside meanwhile becomes extinct.

Most of the songbirds of the eastern United States have managed to survive forest fragmentation thanks to their big ranges. The range of most of the 200 species that lived in the forests before European contact extends beyond the eastern United States. By the twentieth century, 95 percent of the eastern U.S. forests had been cut. But they had not been logged all at once; the destruction spread out from the Northeast like a wave. By the time Ohio’s forests were disappearing, New England’s were beginning to recover. At any particular moment, birds could find refuges where they could survive. In this century people in the eastern United States have generally abandoned farming and the forests have recovered enough for widespread songbird species to return.

But the 28 species that lived only in the eastern United States have not been so fortunate. Their odds of survival were worse, because they had smaller ranges than other birds. As their habitat broke up into forest islands, they survived in fewer fragments, raising their odds of going extinct. Four of these birds–the passenger pigeon, Bachman’s warbler, the ivory‑billed woodpecker, and the Carolina parakeet–are now gone.

Many animals and plants around the world are now going the way of the Carolina parakeet. Their small ranges have been destroyed or fragmented by farming or logging. Many of those that are not yet extinct are almost certainly doomed. Research by Stuart Pimm, a biologist now at Columbia University, and his colleagues have put a time scale to this dwindling. In one study, Pimm surveyed a western region of Kenya that is known for its rich diversity of forest birds. Over the past century heavy farming and woodcutting has chopped it up into small parcels. His team studied aerial photographs dating back 50 years to figure out when these forest fragments first became isolated. They then looked through museum collections to see how many species were originally there. (Unlike insects, of which we probably know only a small fraction of the total species, birds are easy to spot. Ornithologists have identified the vast majority of the estimated 10,000 species of birds on Earth.) And Pimm visited the forest fragments to tally the species in each one.

Pimm found that the older the fragments were, the closer they were to their predicted diversity. Younger fragments still held a surplus of species, because extinctions had not yet brought them down to their equilibrium level. By comparing the older fragments to the younger ones, Pimm’s team concluded that extinctions progress in a way that’s similar to the decay of radioactive elements: they have a half‑life, a period in which 50 percent of their species will disappear. Half of the remaining species will then disappear in the next half‑life period, and so forth. The half‑life of the birds Pimm studied in Kenya is roughly 50 years, a result that Pimin’s colleague Thomas Brooks has also found holds for Southeast Asian birds. In other words, the damage that has already been done to the world’s forests still needs a few decades to come fully into view.

Alien Invasion

In addition to destroying forests and hunting animals, humans caused the extinctions at Mahaulepu in a third way: by bringing new species along with them, such as rats, chickens, dogs, and goats. Biological invaders, as these newcomers are known, are turning out to be one of the most powerful agents of global extinction at work today. Unlike hunting or deforestation, biological invasion is pretty much irreversible. If people stop cutting down forests, the trees can in time grow back. But once a biological invader has settled successfully in its new home, it is usually impossible to get it out.

Biological invasion is nothing new in the history of life. The mammals that marched from North America to South America 3 million years ago were biological invaders, suddenly confronting an isolated group of animals and driving many of them extinct. As Darwin showed, biological invasions could happen when eggs and seeds stuck to the feet of birds and traveled thousands of miles. But before humans, biological invasions were a rare event. Continents take millions of years to collide. Traveling by ocean or hitchhiking on a bird’s foot is not easy: before the arrival of humans, paleontologists estimate that a new species managed to take hold in Hawaii once every 35,000 years. And those colonists were birds and bats and various small invertebrates. There was no way for a dog to ride to Hawaii on a bird’s foot.

When Polynesian settlers arrived at Kauai, a host of new species arrived with them. Rats established themselves on the island and devoured bird eggs and land snails. The chickens and pigs the Polynesian settlers brought as livestock rooted up saplings and ate seeds, and may have caused even more damage to the native forests than the settlers themselves. The birds and land snails that could live only in those forests retreated from human settlements.

New species arrived at a much faster rate with Europeans. As the first truly global traders, Europeans could ferry species around the entire world in their ships. Some of the introductions were intentional–the gift of Captain Cook’s goats, for example–but many were accidental. In 1826 whaling ships brought mosquitoes carrying a form of malaria lethal to Hawaiian birds. The feral pigs dug pools of standing water where the mosquitoes could breed, and the insects began biting the native birds. The malaria probably killed a large number of them; today many bird species survive only at high elevations, where the cold air kills the mosquitoes.

Biological invasions have accelerated over the past 200 years, not just at Kauai but all over the world. As sailing ships have been replaced by cargo ships and airplanes, it gets easier for plants and animals and microbes to shuttle between distant continents in staggering numbers. In one study of the ships coming into Chesapeake Bay, scientists found crabs, mullet fish, and hundreds of other species of animals in the ballast water of each ship − 2,000 animals in every cubic meter. And each year 100 million metric tons of ballast water comes to the United States. Meanwhile, insects and seeds can be carried into the country in crops and lumber. There are 50,000 alien species in the United States alone, and more coming fast. Between 1850 and 1960 San Francisco Bay received a new invader about once a year. Since then, a new species has established itself every three months. In Hawaii, a dozen new species of insects and other invertebrates establish themselves every year.

Only a few alien species become successful invaders. Weedy plants and animals can do well in human‑dominated habitats because they can survive in unstable ecosystems and spread aggressively. Some alien predators also do well because their diet can include many different kinds of prey. Before World War II, for example, Guam was free of snakes. As the United States began moving military equipment onto the island, it also brought brown tree snakes, which had stowed away in the planes. When the snakes arrived in Guam they began eating any small animal they could find. Of the 13 species of native forest birds on Guam, only 3 still exist. Of the 12 native species of lizards, only 3 have survived.

Another way an invader can succeed is by escaping the restraints that kept them in check where they originally evolved. In 1935 the enormous cane toad Bufo marinus was brought to Australia to destroy sugarcane‑eating beetles. They have since spread across northern Australia, expanding their range 30 kilometers every year. They blanket the countryside, reaching densities 10 times greater than they can manage in their native habitats. Cane toads do better in Australia than they did in Latin America, it seems, because they have escaped the forces that normally rein them in. Australian predators that try to eat the toads are killed by a venom that the toads produce in glands on their back; their New World predators have evolved an antidote to it. Meanwhile, the viruses and other pathogens that help keep the toad’s numbers in check back in Latin America aren’t found in Australia. The explosion of cane toads might have been tolerable if they had gotten rid of the beetles, but they showed no interest in them at all. Instead, they have eaten practically anything else they can get in their mouths, including rare marsupials and lizards.

Aliens can also succeed by changing the rules of the ecosystems where they arrive. Hawaii, for example, is an unusual place because it hasn’t experienced much fire. To get fire, you need lightning, and to get lightning you need thunderstorms, which form when big landmasses heat up, churning the atmosphere above them. On most continents fire is a part of life, and plants and animals have evolved ways to defend themselves against it. But because Hawaii is a string of islands surrounded by a vast ocean, it is only rarely hit by lightning, so its plants and animals are unaccustomed to fire.

In the 1960s, two fire‑adapted plants arrived in Hawaii the bush beard‑grass (Schizachyrium condensatum) of Central America, and molasses grass (Melinis minutiflora) from Africa. Their dry stems and leaves created blankets of tinder waiting for a spark. Humans obliged, with cigarettes and campfires, and fires began breaking out. The fires devastated native plants, and afterward the aliens took over the charred ground. As they spread, the fires became more intense. In some spots in Hawaii, fires now scorch 1,000 times more land every year than before the invasions. In such an inferno, native grasses have no chance of taking back their old territory.

In other ecosystems, the arrival of many aliens at once can break down a healthy ecosystem’s resistance. The Great Lakes are a victim of such an invasional meltdown. Before 1900 most ships that visited the Great Lakes used rock, sand, or mud as ballast, which could carry only a few animals and seeds. In the early twentieth century, ships switched to water for ballast, and in 1959, when the Saint Lawrence Seaway was finally opened for deep‑draft shipping, foreign ships began bringing a regular supply of alien species into the lakes.

Although ships come to the Great Lakes from many parts of the world, most of the successful alien species have arrived from the same region: the Black Sea and the Caspian Sea. Animals that live in those waters are adapted for handling the unexpected. The levels of the Black Sea and the Caspian Sea have risen and fallen more than 600 feet over the past few thousand years, and their waters have swung between salty and fresh. The animals that live there have evolved under these wild fluctuations, and they can now survive in a broad range of conditions. They are rugged enough to endure the long journey from Europe to North America in the ballast water, and they can multiply quickly in the freshwater of the Great Lakes.

In the mid‑1980s, a small shellfish called the zebra mussel made the journey from southern Russia to Lake Saint Clair. The zebra mussel produces sticky threads that can anchor it to just about any hard surface, and it feeds by pumping water through its body to filter plankton. It can multiply until it cakes dams, water‑intake pipes, and riverbeds. The sharp shells litter lake bottoms, cutting the feet of swimmers.

The zebra mussel has spread throughout the Great Lakes and into the surrounding watershed, and everywhere it goes it has turned the native ecology upside down. It covers over the shells of endangered native mussels, sealing them shut and killing them. As a rule, native mussel species disappear four to eight years after the zebra mussel arrives in a lake or a river. They are so good at filtering water that they are sucking out most of the plankton that small crustaceans depend on, starving them out of the lakes, and starving the fish that depend on them as well.

Thanks to the trail‑blazing zebra mussel, other invaders from the Caspian and Black Seas are having an easier time becoming established in North America. The round goby, a major predator of the zebra mussel in Europe, was discovered in the Great Lakes in 1990. A crustacean called Echinogammarus that feeds on the waste of zebra mussels settled in the Great Lakes in 1995 and has increased twenty‑fold since then, displacing a native crustacean in the process. Echinogammarus is the food of choice for young round gobies back in the old country, so they have helped the alien fish increase its numbers even more. Before the zebra mussel arrived, another immigrant–a tiny colonial animal called a hydroid–had already been in the Great Lakes for decades but was still a rare species in its new home. In Europe the hydroid feeds on zebra mussel larvae, and the advent of zebra mussels in the Great Lakes triggered a burst of hydroids, which now blanket the new mussel beds. As the zebra mussels filter dirt out of the lakes, more sunlight can penetrate their waters, encouraging the growth of underwater plants. These plants offer the zebra mussels surfaces that they can attach to. In other words, by helping the plants grow, the zebra mussels are raising their own numbers. It’s not just a few individual species that are alien in the Great Lakes; an entire alien ecosystem is assembling itself.

At the rate at which biological invasions are taking place today, scientists suspect they are becoming one of the most important threats to biodiversity worldwide, ranking close to habitat destruction in their deadliness. Some islands risk losing most of their native life. Native species on the island of Mauritius have declined from 765 to 685, while 730 alien species have made it their home. Half of the imperiled species in the United States are at risk thanks to biological invaders.

In the history of life, this onslaught of biological invasions is a completely new experience. Sudden catastrophes have wiped out tropical forests or coral reefs before humans evolved, but never have so many species jumped so far around the world. Biological invasions may do more than help create a mass extinction; they may leave nature altered long after we’re gone.