 April 2003 |
Date with Extinction
For a thousand years before people settled in
New Zealand, a small alien predator may have been
undermining the islands’ seabird populations.
By Laura Sessions
Our yellow Zodiac bobbed across the choppy sea and made its way slowly through the clouds of seabirds that wheeled and soared around us. Albatross, cape pigeons, diving petrels, mollymawks, mottled petrels, and sooty shearwaters all took their turns skimming our bow wave for fish. In the distance my boat mates and I could see the final stop on our sub-Antarctic tour: the Snares Islands, about 130 miles south of New Zealand’s South Island. The chorus of screeching birds drowned out our rumbling boat motor, and even from several miles away we could smell the acrid white guano that coats much of the Snares’s rocky coasts. During the summer breeding season the Snares, whose entire area totals not much more than one and a quarter square miles, are home to more than 6 million seabirds—as many as nest along the coasts of Great Britain and Ireland combined.
Today in the New Zealand archipelago, such dense seabird colonies persist only on small offshore islands, but at one time much of the coastline of the North and South Islands (by far New Zealand’s two largest islands, commonly called the mainland) would have been equally pungent and raucous. New Zealand once supported one of the most diverse seabird faunas in the world; the country was particularly rich in species of petrels. Nowadays those populations have crashed, and many species have been extirpated on the mainland. One can only imagine what it must have been like for ancient Polynesian seafarers reaching the shores of uninhabited New Zealand. The archipelago, no doubt a welcome sight after months of arduous ocean sailing in a double-hulled canoe, would also have presented a far different scene from that of most of New Zealand today.
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According to the standard account of the invasion, the rats arrived in New Zealand between 800 and 1,000 years ago, in the canoes of the first Polynesian settlers. |
But did these colonizers encounter a truly pristine environment? It would be easy to “round up the usual suspects” and blame the loss of so many species from the mainland on the encroachments of civilization. But in reality, the early Polynesian settlers were not responsible for the destruction of many of the seabird populations. Even before people settled this southern land, other visitors may have already irrevocably altered the New Zealand environment.
Those earlier arrivals on the New Zealand mainland were Pacific rats (Rattus exulans), or kiore, as they are called in the Maori language. It has been known for almost a decade that these small stowaways helped drive some of the native bird species from the mainland, or, in some cases, to outright extinction. According to the standard account of the invasion, the rats arrived in New Zealand between 800 and 1,000 years ago, in the canoes of the first Polynesian settlers. But in 1996, Richard Holdaway, an independent extinction biologist, presented evidence that the rodents first made landfall perhaps a thousand years earlier. That date has called into question the entire sequence of prehistoric events that shaped New Zealand—and, not surprisingly, has fueled much debate in New Zealand about the strength and validity of Holdaway’s evidence.
| A rat-generated crash in island bird populations could have led to "a cascade of damage" and even to a change in the nearshore oceanic food web. |
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But even more, Holdaway has hypothesized that a rat-generated crash in island bird populations could have led to “a cascade of damage” and even to a change in the nearshore oceanic food web: seabird colonies generate a prodigious quantity of guano, which can form a kind of organic bridge between sea and shore, enriching soil and promoting plant growth. If the seabird populations crashed, Holdaway argues, so did this bridge. The islands would have lost a major source of nutrients. If Holdaway is right, the rats had accidentally landed on a choke point of the ecosystem, causing a ripple effect that went far beyond the destruction of seabirds.
Thanks to their remoteness—New Zealand lies 1,200 miles east of its nearest neighbor, Australia—the North and South Islands faced the onslaught of invaders considerably later than did many other islands around the globe. But just as they have on Hawaii and Guam, alien species that were suddenly introduced onto the islands have had devastating effects. New Zealand birds were particularly at risk, because they had evolved for millennia in the absence of mammalian predators (indeed, the only land mammals of prehistoric New Zealand were three species of ground-feeding bats). Many of the native birds were flightless and seminocturnal, making them easy prey for the rats and the eleven other introduced species of predatory mammals that eventually prospered in the archipelago. Even seabirds were vulnerable; though they can spend months of each year at sea, many of them nest in ground burrows and are helpless against terrestrial threats.
As an ecologist and environmental writer, one of my main interests is to understand the damage that relatively recent introductions—from stoats to wasps to possums—have wreaked on native plants and animals. Often enough, I have discovered, even one such invader can set in motion a spiral of ill effects [see “A Floral Twist of Fate,” September 2000, and “New Zealand Sweet Stakes,” May 2001]. It was through those interests that I first met Holdaway, and became acquainted with his work, in 2001. Holdaway not only shares my concerns about the current state of New Zealand’s ecosystems; he also adds perspective by looking to the past. He hopes to document the species of prehistoric New Zealand, how its ecosystems worked, and what they can say about the islands today.
From 1991 until 1995 Holdaway took part in a collaborative study to examine fossils for signs that early Polynesian settlement had led to changes in animal populations and the mix of animal species. When rat bones were discovered at some of the fossil sites, he decided to test the assumption that the rats had arrived in New Zealand with the people who settled in the islands about 800 years ago. (Archaeologists have established the dates of those settlements on independent grounds.)
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The early date suggested that rats had not only occupied New Zealand before any other introduced land mammal, but they had appeared on the scene more than a millennium before any human settlers. |
Holdaway measured the ages of the fossil rat bones discovered at non-archaeological sites, that is, sites without evidence of human settlement, with accelerator mass spectrometry (AMS)—a relatively new and particularly sensitive method of radiocarbon dating. To his astonishment, the AMS readings showed that some of the bones were as old as 2,000 years. That may sound recent by the standards of North American or European archaeology, but it is ancient history for New Zealand, the last large habitable landmass to be settled by people. The early date suggested that rats had not only occupied New Zealand before any other introduced land mammal, but that they had appeared on the scene more than a millennium before any human settlers.
How firm are Holdaway’s findings? How likely are they to stand up to further scientific scrutiny? The late astronomer Carl Sagan once remarked that “extraordinary claims require extraordinary evidence.” The implications of Holdaway’s theory of early rat arrival are far-reaching, both for ecology and for the protohistory of Polynesian migrations. And sure enough, his findings have been hotly contested by some archaeologists, who have questioned the accuracy of his radiocarbon dates.
Atholl Anderson, an archaeologist at the Australian National University in Canberra, argues vigorously that dates earlier than 800 years ago, as determined by AMS measurement, can be explained by contamination of bones in the deposits. Recently, however, Holdaway published the results of a comparison between radiocarbon dating and a second technique, known as optical dating, carried out by geochronologist Bert Roberts of the University of Wollongong in Australia. Optical dating determines when the quartz grains in the sediments containing the fossilized bones were last exposed to sunlight. The technique made it possible to estimate how long the sediments and the rat bones had been buried. The results of those tests confirmed the radiocarbon chronology for the Pacific rat fossils.
Archaeologists also point to the lack of evidence for human colonization of New Zealand before about 800 years ago. Known for their intrepid voyages throughout the Pacific, the people who first permanently settled New Zealand were eastern Polynesians in language and culture, and they were to become the direct ancestors of the Maori people. Holdaway believes that any population of more than about fifty people occupying New Zealand for any length of time would have left clues to their presence, such as bones of large flightless birds killed for food, or the remains of cleared forests—as the first settlers did. Yet no evidence for settlements more than 800 years old has ever come to light.
So if the colonizers didn’t bring the rats, how could the animals have come to New Zealand? Pacific rats originated in southeastern Asia. Could they have made the trip on their own? The answer is surely no. The animals are reluctant swimmers, unable to cross water barriers more than 200 yards wide, even in the tropics. And it is unlikely that they rafted on vegetation to New Zealand from some far-off South Pacific isle. In spite of their relatively long tenure in the North and South Islands, they did not reach some of the offshore islands of New Zealand until quite recently.
Holdaway agrees with that scientific consensus: people brought rats to New Zealand. In fact, the Polynesians are known to have sometimes transported Pacific rats—perhaps at times for food—throughout the vast Pacific triangle, from Hawaii to Easter Island to New Zealand. But Holdaway thinks the people who brought the rats to New Zealand were earlier seafarers known as Lapita, who were probably the ancestors of the Polynesians. The Lapita left no archaeological evidence of their presence in New Zealand, Holdaway argues, simply because their visits to such southern latitudes were so transient. They may have touched on New Zealand shores intermittently for centuries before they, or other early Polynesians, decided to stay. And for rats to have arrived, become feral, and established themselves in the new habitat, they need not have “jumped ship” or been purposefully introduced in great numbers. Pacific rats, after all, are rodents, and so they are prolific reproducers; arriving at the right time of year, one pregnant rat could eventually have populated an entire island.
| The rodents’ mitochondrial DNA could
serve as an independent record of prehistoric human migrations. |
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The idea that intermittent visitors carried rats to the islands several times is borne out by a study of the mitochondrial DNA of Pacific rats by Lisa Matisoo-Smith, an anthropologist at Auckland University in Auckland, New Zealand. Matisoo-Smith compared genetic sequences in Pacific rats from New Zealand with the same sequences in rats of the same species from other Pacific islands. Random mutations in mitochondrial DNA accumulate at a slow but relatively constant rate.
Matisoo-Smith reasoned that the mutations could serve as a clock for determining the arrival dates of rats on various islands across the Pacific. Assuming the rats arrived with early Polynesians (or perhaps with the Lapita), the rodents’ mitochondrial DNA could serve as an independent record of prehistoric human migrations. Her data show that the rats living in New Zealand today carry genetic heritages from various lineages of R. exulans, suggesting that they were introduced to New Zealand more than once, from various geographic sources.
Ecologists have only recently come to recognize that the rodents could have fundamentally altered New Zealand’s ecosystems. Until a decade ago, most biologists thought that Pacific rats, unlike Norway rats (R. norvegicus) and black, or ship, rats (R. rattus), were vegetarians. Pacific rats do feed frequently on plants and insects, but they are also avid meat eaters when the opportunity arises. Moreover, because rats are nocturnal, few people had actually seen them in the act of predation until infrared video captured them at their gruesome tasks.
But by now biologists have observed them attacking adult saddlebacks (a native songbird whose numbers are dwindling) and devouring eggs of the little shearwater (a native petrel). Petrel chicks are sometimes skinned alive and their eyes eaten out. And Pacific rats are voracious. In New Zealand they weigh in at less than half a pound but can devour any prey as large as they are and eat eggs two-and-a-half inches long. They can even threaten the eggs of such large birds as the kakapo, the heaviest parrot in the world and now one of the rarest.
The Polynesians themselves are usually blamed for irrevocably changing the landscape of a pristine New Zealand. Indeed, Holdaway and his colleagues have found evidence that within a hundred years or so of their arrival, Polynesian settlers had hunted the giant flightless birds known as moas to extinction. But if Holdaway’s work is on the mark, New Zealand was far from pristine when the Polynesians came to stay. Rats that arrived 1,000 years before any permanent human occupation would have had more than twice as long as people had to alter their adopted ecosystem.
Although Polynesians might occasionally have killed and eaten lizards and the small flightless wrens that were once widespread throughout the islands, they would hardly have hunted such smaller prey to extinction. The Pacific rat was the only small feral mammalian predator in New Zealand before the European era began with the arrival of Captain James Cook in 1769. (Archaeologists think that Polynesian dogs, also introduced by Polynesian settlers and kept for food, had little impact on native species.) The blame for those extinctions clearly rests with the rat.
When Pacific rats arrived in New Zealand, they found themselves in a land of plenty. Millions of seabirds nested in ground burrows. Other birds, such as the flightless wrens, emerged at dusk to forage, the rats’ favorite time to hunt. The rats simply did what any hunter would do. Their victims were well-adapted to avoiding birds of prey, which had always occupied the islands. But predatory birds attack from above, relying on their excellent vision to sight prey during daylight. The rats hunted by night, on the ground, with their keen sense of smell.
A series of inadvertent natural experiments, involving the mix of people, rats, and wildlife on some of New Zealand’s offshore islands, suggests in even greater detail how rats may have affected native fauna over time. For example, on off-shore islands such as Aorangi and Stephens—which became home to Polynesians but never to Pacific rats—petrels, other small birds, invertebrates, frogs, and lizards still abound. Those species have largely disappeared from islands inhabited for long periods by Pacific rats.
It is the petrels, though, that most dramatically illustrate the magnitude of the damage that rats probably inflicted in New Zealand. Thirteen species of petrel once bred on the South Island. Today only six still breed there, and only one, Hutton’s shearwater, remains on the island in great numbers. The seven extirpated species certainly disappeared before the Europeans arrived and may have been gone even before the Polynesians settled in New Zealand.
The petrel species that became extinct were precisely the ones whose size and habitat made them most accessible to the Pacific rats. Petrel species that were too big a mouthful for rats persisted on the mainland, even in lowlands, where rats were common. In contrast, the smaller petrel species all disappeared, even where their breeding habitats remained intact. Scarlett’s shearwater, for instance, disappeared from the west coast of the South Island, even though the area retains some of the largest tracts of relatively undisturbed forest in the country. The only small petrel species that survived were the ones that nested on rat-free offshore islands or in cold mountainous regions, inhospitable to subtropical rats. Many of those refuges were later invaded by other, even feistier rat species and by stoats introduced by Europeans.
If rats were responsible for killing off petrels and other native species, what additional effects might their depredations have had? By eliminating huge colonies of seabirds, for instance, Pacific rats could have generated ecological damage far beyond the extinction of particular species.
Holdaway has drawn particular attention to the amount of organic waste once generated by the seabird colonies—mainly guano, but also lost eggs, dead birds, spilled food, and molted feathers. That waste would have constituted a bonanza of nutrients that flowed continuously from the sea to the land. (Miners on other oceanic islets, such as Nauru and Christmas Island, have come across guano-derived phosphate rock deposits as thick as seventy-five feet.)
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The Pacific rat may be the only mammal in the world, besides our own species, that has fundamentally altered an ecosystem on a continental scale. |
The massive wastes of the seabird colonies on the mainland would have added phosphorus, nitrogen, and carbon to relatively nutrient-poor soils, and lowered the pH of the soil. The birds would also have aerated the soil as they burrowed to shape their nests. The nutrients would have fostered the growth of plants that sustained invertebrates, lizards, birds, bats, and other herbivores. Take the case of Hutton’s shearwater. Holdaway estimates that a remnant colony of these birds on the South Island still supplies more than 1,000 pounds of guano per acre in each breeding season. Extrapolating from that estimate, he maintains that before the appearance of the rats, seabird colonies could have supplied two million tons of fertilizer a year.
The possible implications of the loss of such a nutrient flow are astonishing. Pacific rats have spread to hundreds of islands and have eliminated hundreds of seabird populations. If those changes have equally disrupted hundreds of food webs, it could be that these small rodents have altered island wildlife across the entire Pacific. Holdaway’s next step is to collaborate with investigators from a variety of disciplines to examine the possible connections between petrel disappearances and changes in those food webs. The Pacific rat may be the only mammal in the world, besides our own species, that has fundamentally altered an ecosystem on a continental scale.
A transplanted Virginian, Laura Sessions has lived in New Zealand since 1996. After earning her master’s degree in botany at the University of Canterbury in Christchurch, she began working on a doctorate in science communication, which she expects to complete this year. Sessions lectures on ecology and coordinates and leads tours for groups of American ecology students. “I was first introduced to New Zealand through one of these programs,” she says, “so it is a great pleasure to introduce other students to such a beautiful place.” Her article in this issue is her third contribution to Natural History.
Copyright © Natural History Magazine, Inc., 2003