For over a decade during the 1980s and 90s, I had the best job in the world. I was leading a team of researchers, and together we travelled the tropical islands of the southwest Pacific looking for marsupials, bats and rats found nowhere else on Earth. It was pioneering work. No summary of the mammal fauna of the region existed, so we had nothing to guide us apart from brief accounts scattered in the scientific literature. Many of the islands were a great blank, not having been visited by anyone interested in mammals since some pioneering naturalist dropped by during the age of sail.
Island biodiversity is exquisitely vulnerable to human disruption.
The fate of New Zealand's biodiversity is typical. Over a third of its land bird and bat species have become extinct since human settlement, and another third are threatened with extinction. Such figures made me wonder how the mammalian inhabitants of islands further north were faring in the face of introduced species and European colonization. Because nobody in recent times had gone to the islands to look, it was a question without an answer, and it became the raison d'être for our quest. It was possible, we knew, that some species had vanished before anyone even realized that they were endangered, making our adventures somewhat quixotic.
But it also seemed possible that a species or two existed among the network of islands, reefs and misty peaks that had evaded earlier visitors, and so awaited scientific discovery.
Planning for our expeditions was carried out in musty libraries and museums, for these were the days before the internet, and if one wished to consult the pages of the journal of the Museo Civico di Storia Naturale di Genova, or the obscure, Shanghai-published journal Memoires concernant l'histoire naturelle de l'Empire Chinois, one had to front up at a library that held the volumes—and oftentimes ask for help from a translator as well. But many important finds were not even mentioned in print. Perhaps their discoverer had died among the islands and his trove of treasures had been shipped home to find nobody knew enough, or cared enough, to publish an account of what was in them. And so we worked among often forgotten collections held in museums great and small, from London to Beijing, poring over stuffed skins of rats and bats for evidence. Many had somehow survived fire, war and penury, and as we chased down century-old clues to the existence of such bizarre creatures as giant rats, monkey-faced bats and piebald cuscuses, we stood in awe of those who had built the collections, as well as the curators who had tended them through decades or centuries of peril.
To open the drawer of an old museum cabinet and find the remains of a creature that lived on a distant tropic isle long since dramatically changed by European impact, and which had travelled halfway round the globe to reach its keeping place, is a magical experience—like travelling in time itself. That moth-eaten skin, or even fragment, may be all that remains of an entire species, tinting the thrill of seeing it with immense sadness. For here lies all, perhaps, that is knowable of a branch of life that may have gone its own way for a million years or more, a life form that once played an important role in an island ecosystem, but which had now winked out, never to be seen again.
Or had it? Who could really say whether it might still survive in the densest jungle or remotest peak of its island realm. The most elusive specimens lacked even a clear indication of where they'd been collected: their labels cited a whole island group, or even the voyage of discovery on which they'd been collected, rather than a precise island as their origin. Such species present a great challenge, but even the better known kinds presented us with a daunting quest. Where, and how, does one begin on an island the size of a European state to search for a fist-sized, nocturnal creature that has not been sighted for over a century? But we were young, and confident that we could make sense of the clues. Sometimes guided by no more than a single word on a specimen label, we found our way to unimagined places—to villages that had not seen a white face in living memory, or mountaintops crowned with surreal vegetation—and then we knew that the quest was at least as important as the goal.
As our work progressed, the high peaks of the Pacific Islands began to hold me in their thrall. From a biological perspective they are among the least known places on Earth, and even today some island peaks—which rival Australia's Mount Kosciusko in elevation—remain unvisited by Europeans. Often held sacred by the native people, these mist-wreathed summits are in some respects lost worlds—islands in the sky crowning islands in a tropical sea.
But reaching them was not easy. Local taboos, foul weather, dense jungle and sheer remoteness combine to make them some of the hardest places on earth in which to conduct biological research.
The great arc of islands between Sulawesi and Fiji was our stomping ground. Extending over 6000 kilometers, and crossing the Equator, it's a huge realm: stretching further than the distance from Paris to Montreal, and almost as far as Beijing to Cairo. But it's different from any similar-sized region in that it consists of thousands of islands, each distinctive in its geology, vegetation, shape, size and history of human colonization. From the picture-postcard Polynesian atoll, to some of the largest, highest, most rugged and ancient islands on Earth, the region truly is a version of the world writ small.
Among these varied lands are some that originated as slivers wrenched from ancient supercontinents a hundred million years ago. Others are continental chips torn more recently from the great island of New Guinea. Yet others formed as volcanoes that belched forth from the ocean depths, arriving above the waves as virginal lands innocent of life until drifting seeds, spores and insects arrived. Krakatoa, which blew itself apart in a paroxysm of volcanic activity in 1883 and then grew anew from the sea, gives us some idea of the process. First ferns and insects, then flowering plants, birds and lizards arrived to colonize their new-found land. Krakatoa is just a few tens of kilometers from Java and Sumatra, and less than a century old. Imagine a volcano surfacing a thousand kilometers from the nearest land, then receiving its pilgrims over a million years.
There are other ways, too, for islands to form. Some are simply heaved above the waves by the movements of continental plates, while others, known as land-bridge islands, were, just 10,000 to 20,000 years ago, attached to much larger landmasses by connections now drowned in a rising sea. Some islands originated as combinations of several of these processes. But all islands, regardless of their origins, have one thing in common: transience. True, some islands are longer-lived than others, but all, in the unfolding of geological time, are destined to sink beneath the waves or amalgamate with larger landmasses. Over just the past few centuries, dozens of islands have been born and have died, and like us ultimately all of them will die, even as new islands are born.
On islands, evolution can be slowed down or speeded up. It can also take unlikely directions, fashioning novel creatures adapted to the particular conditions of the island. Why do islands have such peculiar powers over the evolutionary process? Imagine taking a species from the complex, rich continental ecosystem in which it has evolved, then releasing one or two randomly chosen individuals on an island where nothing like it has previously existed. If they survive, the individuals that begat the island population will have just a small subset of the species' genetic diversity, and this alone will wield an influence. To understand how, just imagine choosing two humans—say a red-head and a very tall person—and leaving them on a desert isle, then returning in a million years to examine the characteristics of their descendants.
But genetics is only the start, for when any creature reaches an island, it has effectively been transplanted into a new world. Its predators, competitors, diseases and even favored foods may not exist in its new home, and, instead of the boundless habitat of a continent, it finds itself part of a tiny population hemmed in on each side by sea. Such circumstances can greatly speed the evolutionary process. In the beginning the species is likely to breed up rapidly, for in the absence of predators and disease there is no check on its rate of increase. But soon it faces overpopulation: most individuals will die and only those with a particular advantage will survive. Perhaps those lucky few can utilize some food inaccessible to the rest, or perhaps they can conserve energy because they don't fly much, or perhaps they are smaller than average and can subsist on the slender resources the island affords them. Because the population is small and the selection of the survivors so rigorous, the evolutionary process is greatly accelerated. The influence of such powerful evolutionary pressure can be profound. As we see in the dodo, sometimes it creates beings which don't appear to belong here on Earth. Hardly anybody studies mammals on islands: it's those able colonizers the birds that usually get the attention. Yet some island mammals have, like the dodo, been as remarkably transformed. On islands, bats have been known to take on some of the characteristics of apes, and rats those of badgers, shrews or possums. Even humans and their behaviors are shaped by island life, and as a result island cultures have become as varied and novel as any on Earth.
Not all island life is established by vagrants arriving by raft, wind or wing. Islands that originate as slices of continents, severed from the mainland by powerful geological forces, carry with them a subset of continental life. When that happens, entire ecosystems are set adrift to adjust over millions of years to life in a small, isolated sphere. Inevitably, some species become extinct, unable to adjust to the limited circumstances they find themselves in. At the same time new species, which evolved in other parts of the continent, are unable to invade the now-isolated island. The result, for the island survivors, is often a slowing of evolutionary change. Because competition drives evolution, fewer competing species means less change, and in these circumstances evolution can come to a near halt. So it is that islands can become arks full of 'living fossils'—species whose relatives elsewhere are long extinct or transformed by evolution into very different kinds of creatures.
Evolution on islands also plays with the size of creatures. The world's islands are (or rather were) full of giant rats and tortoises, and oversized flightless birds. But there are island dwarves too. Before humans arrived, elephants, mammoths and hippos the size of Shetland ponies abounded on some islands in the Mediterranean and Arctic. There was even a miniature hominid, the hobbit, on the island of Flores in the Indonesian archipelago. Such gigantism among the tiny, and dwarfing of the great, means islands are great levellers, the species isolated on them converging on an ideal size.
Whatever their size or origins, the species and the ecosystems of islands are exquisitely sensitive to invasion. Whether arriving by themselves or by human agency, invaders can be fatal to the original inhabitants, and virtually no island survived into the last century unscathed. After humans the most common invaders are introduced rats—though cats, snakes and even snails have time and again devastated entire islands of unique species.
Why should the long-isolated islanders be so vulnerable? Island species often have only a limited range of predators, competitors and diseases. Under such circumstances, any species that invests too much energy in being able to flee swiftly or in producing toxins that deter predators will, prior to the arrival of new invaders, be outcompeted by those that put less effort into these faculties and more into reproduction. This is why so many island birds are flightless and why the nuts and leaves of so many island trees are edible, even without cooking—trees that invest in toxins where there are no predators, rather than making more nuts and fruit, are disadvantaged in the evolutionary race for survival. But island species also lose their fear. Fearful creatures use a great deal of energy in fleeing danger, both imagined and real. Where the danger is almost entirely imagined, evolution selects for individuals that conserve their energy for reproduction. Island birds have been known to sit on their nests even while being eaten alive by rats. Many will not flee from cat or human, even when attacked.
As colonial history has shown, the native cultures of islands are also vulnerable to change from outside. The power structures of Hawaii were transformed by a dozen or so iron blades fashioned by a ship's cooper during Captain Cook's voyage of discovery. So armed, the few fortunate chiefs who received them went on to forge empires. What is remarkable, however, is how much traditional island culture still survives in spite of the constant waves of challenge arriving from the larger world. Change may be integral to island life—as old as the first island and as pervasive as the sea that surrounds them all—but such examples convince me that, given half a chance, much that is unique to the islands can persist.
The continent of Australia has cast a long shadow over the extensive archipelago we explored—a shadow composed of living things that, over the aeons, have drifted, flown or made their way some other way to the islands. Among them are curious marsupials, unique birds of paradise and countless other life-forms which have their points of origin millions of years ago on the wide brown land. But what a sea change was wrought on their descendants as they settled into island living. Such species are utterly irresistible to biologists because their myriad modifications reveal the secret workings of evolution. When the animals' ancestors left, Australia's rainforests were extensive, and a vestige of ancestral forms long vanished from the larger landmasses can be found on some islands. Examining such plants and animals can reveal living clues to a now vanished and very different Australia.
We twentieth-century biologists travelled through the vast island realm that was our field of research by whatever means were at hand—sometimes by air, at others by ocean liner, interisland ferry or dugout canoe. Then we would set about collecting, documenting and exploring a world of nature that in some cases had never before been entered by a biologist. And the excitement of setting up a mist-net and laying a trap-line on an island which had not yielded a single record of a mammal was just about the most exciting thing you could do.