On extinction

Biodiversity, a word coined only two decades ago, has become a hot topic in science and society, as humans ponder how to stem the continuing depletion of the world’s biological heritage. Over every consider­ation of biodiversity hangs the shadow of extinction, the threat that stalks all living species—the biological “end of the line.” Yet extinction has played a strangely formative role in forging the diversity of life, as Richard Holdaway, a Christchurch palaeobiologist, explains . . .

Written by      

Andrew Caldwell

Individuals die; species become extinct. That is the status quo of the natural world, as inevitable as ageing, as immutable as gravity.

Since life began on Earth 3-4 billion years ago, most species have seen their star rise, shine for a time and then disappear. In the normal course of events, they fade away when environmental conditions change over long periods, or when a more efficient competitor darkens their horizon. The slow attrition would eventually thin the ranks of living organisms were it not balanced by the evolution of new forms. In fact, the “background rate” of extinction is low enough (each species might last a million years or so) to be easily outweighed by the processes by which new species originate. The diversity of living things has tended to increase through time.

Periodically, however, the steady increase in the number of species has been dramatically reversed. There have been times in Earth’s history when the rate of extinction has rocketed above the background level, and a large portion of the panoply of life has simply vanished. Many minor extinc­tion events (as these episodes are called) and a smattering of more major ones have decimated life on Earth.

For instance, at the end of the Permian period, about 250 million years ago, more than 90 per cent of the species we know from fossils became extinct, leaving just a fraction of the accumulated eons’ worth of genetic variation available for life’s onward march. Yet each evolutionary bottle­neck has been followed by a major radiation of new forms evolving from whatever survived. So, in a sense, extinction events actually determine the course of life on Earth. The episode that is most widely known is the one that occurred at the end of the Creta­ceous, about 64 million years ago, and appears to have led to the extinction of the dinosaurs. Geologists refer to it as the K-T event (from Cretaceous-Tertiary, with a little German flavour).

Evidence of one possible cause of the cataclysm, the impact of an extra­terrestrial body or bolide (such as an asteroid or comet), has been found in a thin layer of clay marking the K-T boundary at Woodside Creek in Marlborough. Here, and at other sites throughout the world, the boundary-layer clay is enriched in the rare element iridium, which is normally associated with extraterrestrial sources.

While the demise of the dinosaurs is the aspect of the K-T event that has captured the public imagination, it is the flipside of this particular coin, the rise of mammals, that reveals the true significance of extinction. Small mam­mals had been living in the dinosaurs’ shadow for many millions of years, but they flourished and became large and dominant only after the great reptiles had been taken out of the game.

The radiation of mammals initially yielded forms that were small and generalised, with grasping hands, and eventually led to much larger, bipedal forms with grasping minds.

It is worth noting, too, that although the dinosaurs, pterosaurs, marine reptiles (including the giant plesiosaurs and mosasaurs that swam in New Zealand seas), ammonites and many marine plankton species perished in the Cretaceous, other groups survived. Indeed, the dinosaurs themselves were not totally lost—those sparrows in your backyard are distant cousins to Tyrannosaurus rex! Only the big dinosaurs became extinct. Geologists and palaeontologists refer to the end of the Permian as the Third Extinction and the K-T event as the Fourth Extinction. Throughout the post-K-T period, there have been smaller extinction events, but until very recently nothing on the scale of the K-T boundary event.

Then, in the late Quaternary (the period since the beginning of the ice ages) about 50,000 years ago, the rate of extinction started to increase. This time, because the event is so close to our time, we can begin to see patterns in the geography of the extinctions. They started in a small way in Africa, spread into Europe, had an isolated peak in Australia about 40,000 years ago, then reached a crescendo in North and South America between 12,000 and 10,000 years ago. Some scientists consider that most of the extinctions in the Americas occurred within a mere 500 years.

The wave then spread to islands—to the Mediterranean (from about 6000 B.c.), into the Pacific (about 2000 B.c.), reaching New Zealand and Madagascar in the past 2000 years, and fetching up in the Indian and South Atlantic Oceans after the arrival of European explorers. As it happens, this pattern closely follows the progress of “modern man” out of Africa and around the world, especially those groups that developed sophisticated hunting technology, such as the Clovis people of North America, with their flint-tipped spea rs.

The Fifth Extinction, as this rash of extinctions is termed, was remarkable in that on continents it affected large mammals almost exclusively. In North America, the losses included mam­moths and mastodons (we would recognise these as different forms of elephant), giant sloths, camels, horses, sabre-toothed cats, the giant short-faced bear and the American lion and cheetah. The last survivors of the Quaternary megafauna were the dwarf mammoths of Wrangel Island in the Russian Arctic, which disappeared about the time pyramids were the new architectural style in Egypt.

On islands, extinctions claimed not only the pigmy hippopotami and elephant-birds of Madagas­-car and the 11 moa of New Zealand but also a vast number of smaller birds, mammals, lizards, snails and insects. One estimate puts the number of bird species that have become extinct in Polynesia over the past 3000 years at more than 2000. As there are some 10,000 bird species alive today, that rate of extinction represents an increase of 100,000 times over the background rate. And there is no doubt that it was the human “bolide” that caused the catastrophe.

In literature on later Quaternary extinctions, there is an arbitrary cut-off date of around 1500 A.D.—approxi­mately the time Europeans began to spread across the globe. As a result, we have tended to think of extinction events as being a thing of the past. In New Zealand, for example, people often equate the end of that wave of extinctions with the demise of the moa. But that is incorrect. We are still living with the Fifth Extinction today. Each news item concerning a successful breeding season for kakapo or the discovery of a new population of bats is a detail in the chronicle of the Fifth Extinction as it affects New Zealand.

Most of our rare and endangered birds (and, for that matter, bats, seals, lizards, tuatara, frogs, insects and snails) were brought to the edge of extinction during the first human impact. It is the tattered tapestry of that lost fauna that conservationists are trying to mend.

More than half of the bird species that greeted the first humans to make footprints on New Zealand beaches are extinct, and the figure reaches 60 per cent if you count the “living dead”—species such as the black robin, South Island saddleback, little spotted kiwi, kakapo and takahe; those without known natural populations or whose populations survive only under the most careful and dedicated management.

Some biologists are now talking about a Sixth Extinction, an event caused less by direct human interfer­ence in the form of hunting or the introduction of predators than by the overwhelming ubiquity of the human species. As the global human population grows, and technology and the demand for resources expands, natural areas are becoming fewer and smaller, and the number of living species may be undergoing its greatest contraction since the end of the Permian.

To a degree, the distinction between the Fifth and Sixth Extinctions is semantic. In New Zealand, the two overlap completely, and the underlying causes differ little. However, were we to recognise a Sixth Extinction here, it would have commenced with the loss of the New Zealand quail in the 1870s and the huia, both species of piopio and the laughing owl in the early years of the 20th century.

Some birds, such as the South Island kokako, remain in that limbo of the “probably extinct,” with little chance of rediscovery. And as we start the new millennium, we face, within 20 years, the probable loss as wild species of all the kiwi except the browns on Stewart Island, of the mainland populations of weka and kaka, and of natural populations of yellowhead and the yellow-crowned parakeet. Although birds are the most studied group in the country, there are still plenty of gaps in our knowledge about them. When we come to other groups, we know so little that we cannot even guess at how many species are being lost.

Recent work on the herpetofauna (amphibians and lizards), coinciding with the introduction of biochemical techniques, has shown that there are many more species of skink and maybe even native frog and tuatara than we once thought. A major project to identify and describe the fishes of New Zealand’s Exclusive Economic Zone regularly turns up new species. And, for the terrestrial invertebrates, who knows? We are losing species all the time—weevils, flies, moths, beetles, snails. Many of them we never even knew existed.

Does all this matter? Well, even at the merely commercial level, yes. New Zealand sells itself on its environmental integrity, yet within the scientific community we are already a byword for extinction in the recent past. Our flora and fauna contain an unusually high percentage of endemic species. All of our indigenous terrestrial non-avian vertebrates are endemic, 90 per cent of our insects and marine molluscs are endemic, as well as 80 per cent of our vascular plants. Great Britain, of similar size to New Zealand, has one species of endemic plant, and one endemic mammal.

But in the longer, more important, view, the maintenance of biological diversity is a prerequisite for our continued existence on the planet. Although we are still profoundly ignorant of the way that ecosys­tems operate, it is clear that there is a threshold of loss beyond which irrecoverable changes can occur. Whether the threshold is crossed as a result of the loss of a “keystone” species on which the stability of the system depends, or because a level is reached beyond which the “fabric” cannot be maintained, depends on the system.

In New Zealand, we already live in an environment which has been brutalised by 750 years of human occupation. The “clean, green” environment we all cherish is almost totally man-induced—there is no part of mainland New Zealand that is as it was when people arrived. But there is sufficient left—just—to allow us to halt the slide.

Eminent biologist Edward 0. Wilson, originator of the science of sociobiology and co-originator of the theory of island biogeography that did much to stimulate the growth of ecology in the 1960s and 1970s, views the Sixth Extinction as the greatest threat to human survival. In his best­selling book, The Diversity of Life,  he catalogues the ways that we are sawing through the branch that supports us all, from reducing our options for crops and pharmaceuticals to threatening the sources of the oxygen we breathe.

Above all, he empha­sises our current lack of knowledge of the ways things work, and the fact that we are, for better or worse, part of and a product of our environ­ment. To destroy the diversity of life around us is to damage our own life-support system. And, contrary to the dreams of technophiles, there is no substitute for that.

Not least, Wilson points to the emotional needs of people: one of the most damaging of punishments is to remove someone from natural sights and sounds. It is dehumanising. Wilson’s chilling message is that by destroying nature we are signing our own death warrant.

And he is not alone in sounding dire warnings. More and more biologists have been calling attention to the importance of maintaining viable samples of ecosystems to preserve the health of the planet, and our own future. It is our responsibility to ourselves not to add to the catalogue of icons of extinction.

More by