The Beginning of the End of the Age of Megamammals
We are more dangerous than we seem and more potent in our ability to materialize the unexpected that is drawn from our minds.
— LOREN EISELEY, The Unexpected Universe
I hr inland from Cape Town in South Africa, the high rocky ramparts of what »■■y is known as the Great Escarpment have dried the air and created a desert. This region is now home to many sheep and a few towns. The largest of the latter is Graaf Reinett, the self-styled jewel of the Karoo. Graaf Reinett is surrounded by high "koppies" of sedimentary rock, and its outskirts are ringed by shanties and so-called game reserves, large vacant tracts of thorn and scrub. The town itself is indeed like an emerald on brown dirt; it is a green oasis surrounded by the dusty parchment of the Great Karoo Desert, a town kept verdant by an encircling river providing life-giving water. Graaf Reinett itself is now a haven for tourists, for it is a virtual museum of nineteenth-century Afrikaans architecture, a melding of Dutch, German, and Huguenot influences amid blooming gardens and staid tree-lined streets. Tree-lined, that is, in the "White" part of town. There are few trees and little green in the nearby township to which the region's blacks are relegated.
The largest hotel in town is the Drosty, a picturesque assortment of stone cottages lining a cobbled lane and two restaurants serving the best meals in the Karoo. The Drosty has been restored to the look of its glory years, the late nineteenth century; each room is filled with antiques, and the staff is dressed to match. The ancient bar is wood and memory. Old photos line the walls, scenes of the town taken in the mid- to late nineteenth century. One of the photos shows elephant tusks piled high in the street in front of the newly built hotel.
On my first visit to the bar I came upon this photo and asked the ebony and venerable barman where the tusks had come from. There must have been hundreds of them in the huge pile, and it is clear from the photo that a brisk trade of some sort was going on around them. An auction, perhaps. The old bartender looked at the photo, as if for the first time, and professed ignorance; all he knew was that there had never been elephants living around Graaf Reinett in his tribe's memory.
The Karoo is dry and dusty; it does not seem like elephant country, so I believed him. But as years went by and I learned more about elephants, I began to wonder. Elephants can and do live in places far drier than the Karoo—the Kalahari Desert, for instance. By Kalahari standards, the Karoo is a verdant paradise. Elephants are consummate imperialists, and were once found on five continents. Why not the Karoo?
Each time I returned to the Karoo I asked local people—white and black—if they had ever heard of elephants in the region. I always heard the same story: there had never been elephants in the Karoo. But never is a taboo word for a paleontologist used to dealing in millions of years, and I inquired further, coming, at last, to the door of James Kitching, a retired professor.
Kitching is a fellow paleontologist, born in the Karoo, who then went on to great fame as one of the world's most celebrated bone hunters. In the 1960s he made what might be the most important fossil find of the twentieth century. On a cold, rock-strewn slope in Antarctica he discovered a specimen of the mammal-like reptile Lystrosaurus. This same creature is perhaps the most common vertebrate fossil in the Karoo—so common, in fact, that Kitching no longer bothers to collect them. But this particular fossil, the first common animal of the Triassic period, had never been recovered in Antarctica, and its discovery there constituted a powerful geologic proof that, 250 million years ago, Antarctica and Africa lay joined. In fact, at that time, all of today's southern continents were united in a single "supercontinent" named Gondwanaland, whose components—Africa, India, South America, and Antarctica—subsequently split apart and drifted across the Earth's surface like great stately cruise ships, carrying their animals—and fossils—with them. Kitch-ing's find of Lystrosaurus in Antarctica constituted one of the proofs of what is now regarded as fact: that continents drift.
My real purpose for coming to see Kitching was to discuss Permian fossils, but I soon asked him about the elephants as well. He laughed dryly. "Of course there were elephants here. They followed the watercourses up from the coast, sticking to the rivers and finally arriving here in the Karoo. I have come across their bones many times around here. The last were killed off about the turn of the century."
I still remember that phrase, "killed off." Not surprisingly, the great pile of tusks in the photo at the Drosty Hotel in Graaf Reinett came from local elephants, hunted to extinction by the local farmers and townspeople. But what struck me was not that the local elephants had been killed off, for extinction is a fact of life, but that even the memory of their existence had been killed off in less than a century since the last one died. They were hunted to extinction and then forgotten. Where once the great elephants roamed in great herds, nothing is left of them but a fading photograph. No longer even a memory, they are now a part of a vanishing Africa, where wilderness has been transformed into farmland in a single generation.
Africa is revered for its abundance of large mammals. Nowhere else on Earth can such diversity of large herbivores and carnivores be found. Yet this animal par-adise—instead of being the exception—was once the rule: all of the world's temperate and tropical grazing regions were quite recently of African flavor. But the elephants of the Karoo are just one casualty of an extraordinary event that has depleted the Earth's biodiversity of large mammals over the past 50,000 years. Is this a mass extinction? Are the forces causing it still under way? Or was it the first cause in a multi-causal event now entering a new phase?
Although the disappearance of large animals poses a tremendous challenge to those studying extinction, one significant lesson we can take from the past is that the extinction of large animals has a far more important effect on the structure of ecosystems than does the extinction of smaller ones. The extinction at the end of the Cretaceous was significant not because so many small mammals died out, but because the dinosaurs did. It was the removal of these very large land-dwelling animals that reconfigured terrestrial environments. In similar fashion, the removal of the majority of large mammal species across most of the world over the last 50,000 years is an event whose significance is only now becoming apparent, and one that should have lasting effects for additional millions of years into the future.
In the late Pleistocene Epoch, at the end of the Ice Age, about 15,000 to 12,000 years ago, a significant proportion of the large mammals in North America went extinct. At least thirty-five genera (and thus at least that many species) disappeared from North America during this time. Six of these lived on elsewhere (such as the horse, which died out in North and South America but lived on in the Old World); the vast majority, however, died out utterly. The lost species represented a wide
The arrival of humans in North America was one of the most devastating events ever to occur on the continent.
spectrum of taxonomic groups, distributed across twenty-one families and seven orders. The only unifying characteristic of this rather diverse lot is that most (but certainly not all) were large animals.
The best-known and most iconic of these lost species were the elephant-like animals—the probisiceans. They included mastodons and gomphotheres as well as mammoths, which were closely related to the two types of still-living Old World elephants. Of these, the most widely distributed in North America was the American mastodon, which was found from coast to coast across the unglaciated parts of the continent. It was most abundant in the forests and woodlands of the eastern part of the continent, where it browsed on trees and shrubs, especially spruce trees. The gomphotheres, a bizarre group quite unlike anything now alive, are questionably recorded from deposits in Florida, but otherwise were widely distributed in South rather than North America. The last group, the elephants, was represented in North America by the mammoths, comprised of two species, the Colombian mammoth and the woolly mammoth.
The other group of large herbivores iconic of the Ice Age in North America was the giant ground sloths and their close relatives, the armadillos. Seven genera constituting this group went extinct in North America, leaving behind only the common armadillo of the American Southwest. The largest animals of this group were the ground-living sloths, ranging from the size of a black bear to the size of a mammoth. An intermediate-sized form is commonly found in the tar pits of present-day Los Angeles, while the last and best known, the Shasta ground sloth, was the size of a large bear or small elephant. Also lost at this time were the North America glyptodont, a heavily armored creature 10 feet in length, and an armadillo, a member of the genus represented today only by the common nine-banded armadillo.
Both even-toed and odd-toed ungulate animals died out as well. Among the odd-toed forms, the horse, comprising as many as ten separate species, went extinct, as did two species of tapirs. Losses were even greater among the even-toed ungulates. Thirteen genera belonging to five families went extinct in North America alone in the Pleistocene extinction, including two genera of peccaries (wild pigs), a camel and two llamas, the mountain deer, the elk-moose, three types of pronghorns, the saiga, the shrub ox, and Harlan's musk ox.
With so many herbivores going extinct, it is no surprise that many carnivores also died out. These included the American cheetah, a large cat known as the scimitar cat, the saber-toothed tiger, the giant short-faced bear, the Florida cave bear, two types of skunks, and a canid.
A few of the local fauna from Los Angeles, circa 18,000 B.C., courtesy of the La Brea tar pits.
Finally, some smaller animals round out the list, including three genera of rodents and the giant beaver. But these were exceptions—most of the animals that died out were large in size.
The animal extinction in North America coincided with a drastic change in plant community makeup. Vast regions of the Northern Hemisphere went from being made up primarily of highly nutritional willow, aspen, and birch trees to far less nutritious spruce and alder groves. Even in those areas dominated by spruce prior to the extinction, a diverse assemblage of more nutritious plants was still available. But as the number of nutritious plants began to decrease due to climate change, herbivorous mammals would have increasingly foraged on the remaining more nutritious plant types, thus exacerbating their demise. The reduction of their food supplies may, in turn, have led to reductions in size for many mammal species. As the Pleistocene ended, the more open, higher-diversity spruce forests and nourishing grass assemblages were rapidly replaced by denser forests of lower diversity and lower nutritional value. In the eastern parts of North America the spruce stands changed to large, slow-growing hardwoods such as oak, hickory, and southern pine, while in the Pacific Northwest great forests of Douglas fir began to cover the landscape. These forest types have a far lower carrying capacity for large mammals than the Pleistocene vegetation that preceded them.
It was not just North America that suffered such severe losses. Until recently, North and South America were isolated from each other, and hence their faunas underwent quite separate evolutionary histories. Many large and peculiar mammals evolved in South America, including the enormous, armadillo-like glyp-todonts as well as the giant sloths (both of which later migrated and became common in North America), giant pigs, llamas, huge rodents, and some strange marsupials. When the Isthmus of Panama formed some 2.5 million years ago, free interchange between the two continents began.
As in North America, a mass extinction of large mammals occurred in South America soon after the end of the Ice Age. Forty-six genera went extinct in South America between 15,000 and 10,000 years ago. In terms of the percentage of fauna affected, the mass extinction in South America was even more devastating than that in North America.
In Australia the losses were even greater. Since the Age of Dinosaurs the Australian continent had been an isolated landmass. Thus its mammals were cut off from the mainstream of the Cenozoic Era and followed their own evolutionary path, resulting in a great variety of marsupials, many of them large. During the last
50,000 years however, forty-five species of marsupials belonging to thirteen genera were killed off. Only four of the original forty-nine large species (greater than 20 pounds in weight) present on the continent 100,000 years ago survived. Of course, no new arrivals from other continents bolstered the disappearing Australian fauna. Large reptiles also disappeared, including a giant monitor lizard, a giant land tortoise, and a giant snake, as well as several species of large flightless birds. The larger creatures that did survive were those capable of speed, or with nocturnal habits.
The wave of extinctions affecting the faunas of Australia, North America, and South America coincides both with the first appearance of humanity in all three regions and with substantial climate change. Reliable evidence now shows that humans reached Australia between 35,000 and 50,000 years ago. Most of the large Australian mammals were extinct by about 30,000 to 20,000 years ago.
A different pattern emerges in the areas where humankind has had a long history, such as Africa, Asia, and Europe. In Africa, modest mammalian extinctions occurred 2.5 million years ago, but later losses, compared with those of other regions, were far less severe. The mammals of northern Africa, in particular, were devastated by the climate changes that gave rise to the Sahara Desert. In eastern Africa, few extinctions occurred, but in southern Africa, significant climate changes occurring about 12,000 to 9,000 years ago were coincident with the extinction of six species of large mammals. In Europe and Asia there were also fewer extinctions than in the Americas or Australia; the major victims were the giant mammoths, mastodons, and woolly rhinos.
The Pleistocene extinction can thus be summarized as follows:
° Large terrestrial animals were the primary victims; smaller animals and virtually all marine animals were spared.
° Large mammals survived best in Africa. The loss of large mammalian genera during the last 100,000 years in North America was 73%; in South America, 79%; in Australia, 86%; but in Africa, only 14% died out.
° The extinctions were sudden in each major group, but occurred at different times on different continents. Powerful carbon dating techniques allow very high time resolution. These techniques have shown that some species of large mammals may have gone completely extinct in periods of 300 years or less—a nanosecond in evolutionary time.
° The extinctions were not the results of invasions by new groups of animals (other than Homo sapiens). It has long been thought that many extinctions take place when new, more highly evolved or adapted creatures suddenly arrive in new environments. Such was not the case in the Ice Age extinctions, for in no case can the arrival of some new fauna be linked to extinctions among the forms already living in the given region.
This various evidence has suggested to many that humanity provoked the Pleistocene mass extinction. Others argue just as vigorously that the cause of the megamammal extinction was the changes in vegetation that occurred during the intense climate changes accompanying the end of the Pleistocene glaciation. In fact, most discussion about this extinction deals exclusively with this argument over humans versus climate as its cause.
For the sake of our arguments, however, the cause is irrelevant. No one doubts that whatever its cause, the Ice Age mass extinction resulted in a major reorganization of terrestrial ecosystems on every continent save Africa. But today Africa is making up for lost time, losing its megamammals as the large herds of game become restricted to game parks and reserves, where they become easy prey to poaching within their newly restricted habitats.
The end of the Ice Age megafauna is not a clearly defined line, like those drawn in the sand at the Permo-Triassic and Cretaceous-Tertiary boundaries. But then we are looking at it from the present, and, geologically speaking, it is just a moment away. At this distance, intervals of time lasting 10,000 years or less are insignificant and probably beyond the resolution of our technology—when viewed from tens to hundreds of millions of years away. The end of the Age of Megamammals looks protracted from our current vantage point, but it will look increasingly sudden as it disappears into the past—one of the odd aspects of time. But there may be more to the story. The megamammals still left on Earth now make up the bulk of endangered species, and many large mammalian species are now at risk. If the first phase of the modern mass extinction was the loss of megamammals, its current phase seems concentrated on plants, birds, and insects as the planet's ancient forests are turned into fields, cities, and toxic waste dumps.
As we race forward into the new millennium, powered by an Internet-fueled economy, biologists strain to look forward in time, watching for the suspected new biological onslaught to begin. In my view, it has already happened. It is visible in the rearview mirror, a roadkill already turned into geologic litter—bones not yet even petrified—the end of the Age of Megamammals.
The Norway rat, one of the few mammals as successful as humans, steps off the boat in Polynesia, circa 1767.
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