The Interaction of Humans, Megaherbivores, and Habitats in the Late Pleistocene Extinction Event

Norman Owen-Smith, University of the Witwatersrand

Real Audio Recording   

Thank you very much. I seem to be the only neontologist here among a group of paleontologists. In some sense, comparing back to the sourcebook from the previous meeting of this kind, the book by Martin and Klein on quaternary extinctions -- in that meeting, the neontologist was Jared Diamond, a very eminent person, very distinguished. And he was somehow tagged on, at least to the end of the book. I at least seem to have made it somewhere near the beginning of this meeting, but I can't hope to give you the New Guinea perspective that Jared Diamond gave to that meeting. My perspective will be an African one.

My eyes were opened up to the whole issue of Pleistocene extinctions by a visit to the University of Wisconsin by Paul Martin when I was a graduate student there, about 1967 or 1968 -- quite a long time ago now. It all seemed to fit so well. I was in the process of studying white rhinos -- white rhinoceros -- one of the species that almost went extinct in recent times. It just made it through, and the obvious vulnerability to human hunters fitted in very neatly with the story of the impact that humans had had on the Pleistocene megafauna in other parts of the world.

It all seemed very neat and simple. Until at a later stage, 1991, I went to a meeting at Helsinki, at which another eminent paleontologist, Dale Guthrie, gave a talk on the disappearance of not just the North American fauna, but the disappearance of a whole ecosystem -- the so-called "mammoth steppe." And that made me aware that the issue of the extinctions was more complex than just human overkill, and we might also need to take a look at some of the habitat changes that may have occurred at that time and may be a component of those extinctions.

Following that stimulus, I took a journey up to the far north, towards the region that was once Beringa, visiting the University of Alaska, about 1994, as a guest of David Hopkins, to air some of my thoughts from Africa, looking outwards to North America, Eurasia, and the rest of the world, about how various factors might have interacted in those extinctions. And that led to the perspective that I will present -- that even though, in summary, the humans may have been a major agent in the extinction of the very large animals -- the so- called "megaherbivores" -- perhaps people had not paid enough attention to the consequences of the elimination of those species on the habitat transformations that certainly occurred at around the same time.

There are three patterns in those Pleistocene extinctions which have to be explained by any model. The first of those patterns is the fact that species of very large body size were most strongly affected; and, in particular, all of those species weighing more than 1,000 kilograms in adult body mass went extinct everywhere in the world outside of Africa and tropical Asia. And these are the species that I will refer to as megaherbivores -- "mega" in the very strict sense, in that the adults weigh more than a megagram -- 1,000 kilograms is 1 million grams.

The severity of the generic extinctions may appear different between different people's presentations. The data that I'm presenting there were based on considering extinctions of all species above about 10 kilograms in body mass, covering large herbivores, and only concerning the herbivores and leaving out the carnivores -- because that was the focus of my interest.

Thirdly is the geographical -- in addition to the geographical pattern, there's also the temporal pattern of the extinctions that took place around the time on the left there, about the time of the transition from glacial to interglacial conditions, about between 15,000 and 10,000 years ago. And even though there have been suggestions that that climatic transition, as revealed here by change in ice volume in the Antarctic region, even though there might have been quite a radical climatic change there, the evidence indicates that that climatic change -- from glacial to interglacial conditions -- was no more extreme or rapid 11,000 years ago than it was at the previous glacial-interglacial transition about 130,000 years ago. So that is a problem for climatic interpretation.

Well, particularly as we saw earlier, the extinctions were spread out over a period of time in northern Eurasia, and very tightly clustered in time in North and South America. In Australia, they occurred in an earlier period in time, between 30,000 and 40,000 years ago, and in other parts of the world the extinctions have been much more recent -- for example, in Madagascar and other islands.

What I would like to do with an ecological perspective is to look at the extinction process. There are basically four potential causes to be considered. Working from the bottom upwards, one of them quite commonly invoked in paleoentological extinctions is competition -- the competitive replacement of one life form by a superior competitor. That is not generally invoked in the Late Pleistocene extinctions, so I won't pay any more attention to that.

Disease can also be a very potent cause of extinctions. It hasn't really been considered an effect in the last Pleistocene extinctions until recently. I'm not sure that it has much of a role back then in time, though certainly it has a very crucial role at the present time. But I'll leave other people in today's meeting to address that cause of extinction.

Basically we will be comparing the contrasting arguments between protagonists of either predation by a human overkill as the cause of the extinctions, and where there's a habitat change as a result of climatic transition or other factors.

How does predation cause extinction of a species? Three things have to follow. The first is a demographic situation, where the rate of loss of individuals from a population to predators exceeds the rate of recruitment to that population. If that is the case, that population is on its decline downwards towards extinction. But, generally, that kind of situation does not lead to extinction, because when the prey species gets rare, the predators switch to other prey. Or the predator population itself is not able to persist because of the paucity of their prey base. So for extinction to occur, one also needs to have predation maintained even when the prey is rare. There must be no gain or compensation in predation rate when prey abundance declines.

And thirdly, there's also a geographic aspect which has to apply, as well, and that is that, to use current ecological jargon, there must be no "predator-free space." In other words, the pressure of that predation must be pervasive in all the areas where that species occurs. In most cases, predator and prey happen to persist in complex ecosystems, because the prey is able to keep finding space that the predator hasn't yet occupied, and there's a coexistence, a dynamic coexistence, in space between predator and prey.

A model favored by ecologists is the so-called "predator pit model." What you see here is a recruitment curve, as prey population density increases. In the absence of predation, you'd have a situation where the recruitment rate of the prey population would be higher at low density, declining towards high density through food-resource limitations to an equilibrium point of zero net growth rate, a replacement rate of one. Next year's population is the same as this year's population, and that's the point we call the "carrying capacity," when, in this case, the carrying capacity is set by resources. What this diagram suggests is that you can have a situation where, as prey density increases from low levels, you have density-dependent predation producing a situation where population growth becomes zero at a level well below the limit produced by food resources.

Now, this predator-pit diagram is, in fact, misleading. It's presented as a demographic model -- but that can't be a demographic model for those of us who remember the logistic population growth model that is the basis of maximum sustained yield estimates for fisheries. Because according to the classical sustained yield models, you get your maximum yield at a density of about half of the carrying capacity. And this model suggests that the predator equilibrium is well below -- about a tenth of the level set by food resources. And that implies that there's the crossing of time frames between the geographic scale and the demographic scale. Because what we have here is a situation where resources must be unavailable to the population for many habitats because of the presence of predators. So the prey eventually persists in a habitat from which it is relatively well adapted to resist predation.

The example, the well-known example in North America, for a kind of predator-pit situation applies to the wolf-moose, or wolf-moose-bear, situation in northern areas -- where, in the presence of wolves, and in some cases human hunters and bears as well, the moose exist at a density of about .4 per square kilometer, compared to a density of about 2 per square kilometer that they can attain in other situations.

And here's another example of a predator pit applied to Africa. This is applied to the migratory wildebeest population in the Serengeti, suggesting that two populations -- migratory populations -- that it does not go into a predator pit, because the predation rate does not produce zero growth at low numbers. The migrant population escapes predation and moves up to a food equilibrium which, in the Serengeti, is a population total of about one and one-third million wildebeest. And then, in addition, in the same ecosystem, you have wildebeest populations that do not migrate, and they exist at a density of about one-thirtieth of that of the migratory population. So if they're being subjected to predation year-round, they are held at a much lower density. That is a geographic limitation, and not just a demographic one.

So, is this the way it happened with the Pleistocene fauna? Here is an imaginative diagram that I was able to find in a book, where the early humans in North America used pits to trap mammoths, like this, is imaginary. But the question is: Were they the agents responsible for the deaths of these species? And was the predation pressure the result of the ultimate extinction of these species throughout much of the world? The suggestion is that humans became very skilled as predators in hunting large mammals and were able to break through the defense of these species through their large body size -- which had made them almost invulnerable to nonhuman predation as adults. Humans had the skill, through the use of weapons, to be able to dispatch these animals one by one, until none were left.

And I was very much aware, from having worked with white rhinos, just how vulnerable every large mammal of this kind can be to human predators. The white rhino study was easy in the field. You can walk right up to these animals, as long as you approach them from downwind; and they may not even be aware that you're there until you take your picture and the camera clicks. And they get a frightened and they run away.

The typical response of a white rhino to nonhuman predators is to adopt this hedgehog formation, also shown by musk oxen and, one presumes, by many of the extinct megafauna. They simply stand with their rear ends together, poke their horns outwards, and in that way they were quite successful at warding off attacks by lions, hyenas, and the other mammalian predators. For humans that doesn't work. Humans just stand and keep throwing spears in till the animals lose enough blood to die. There's only one species for which fleeing is the appropriate response for white rhinos, and that is against humans. And white rhinos, to this present day, waver. They can't decide, when there's something in the environment that they might be scared of, whether they should stand or run. In other words, behavioral adaptations have not taken place, despite a long period of coexistence with humans, to give them an effective antipredator response to this novel predator.

Also typical of animals weighing more than 1,000 kilograms is the fact their reproduction is decoupled from the annual seasonal cycle, so that their birth intervals are extended over longer than one year -- typically, two to four years. Four years for elephants, two to three years for rhinos, hippos and other species. The result of that is, of course, a limitation on the potential population growth rates. They can easily be estimated roughly on that birth interval. For example, for an elephant population with an intercalf interval of four years, if you allow for the fact that that gives a breeding rate of one-quarter of a calf per adult female per year -- but half of the population of females are immature, below breeding age. Another half of the total population are males, not producing babies. You simply divide .25 by a quarter, and you get a potential population growth rate of about 6.5% per annum, not allowing for any additional mortality. So that's about the maximum population growth rate of an elephant population. For white rhinos, with their shorter breeding interval, their population growth rate is about 9%.

So those are the typical maximum rates of recruitment possible for those populations. Here's the exact estimate of the population growth rate of white rhinos, upon the situation that I studied. This part of the curve here, where the population growth rate is constant, indicates an annual growth increment of just over 9% per annum. But what I'd also like to point out in this diagram is the fact that the censuses indicate that when that population was at a low level in the early period, its population growth rate was less than after density had increased. So there's some suggestion of a situation that some people have called the "Allee effect," where, when populations get rare, their population growth rate is less than what could be achieved at a higher density. What the mechanisms are we can speculate. It may be differences in finding mates. It may also be that, when densities get high, they modify habitat conditions to make them more suitable rather than less suitable.

Here's another important bit of evidence relating to white rhinos. This is the historical distribution of the white rhinos in Africa that survive in 92 parts of the continent. Here, in southern Africa, where they were abundant and widespread south of the Zambezi River, but didn't occur north of the Zambezi. And Umfolozi Game Reserve, where I studied them, was right at the southern limit of their distribution.

They also occur in a region of northern East Africa, to the west of the Nile River -- in parts of Uganda, Zaire and Sudan. This population is almost extinct now. The surprising thing here is the absence from most of eastern central Africa. Why are white rhinos absent there? All the rest of the fauna that occurs with them in Umfolozi -- the wildebeest, the zebra, buffalo -- everything else occurs in East Africa. And the grasses that they feed upon in Umfolozi are also present in East Africa. What could have made the species go extinct there, excepting for human hunters? And, of course, East Africa is the center for the development of the Bantu culture that led to the first Iron Age humans in Africa, which, by the time they had spread into South Africa, had large herds of livestock -- cattle, sheep and goats -- with them. And when they entered South Africa, they did not need to hunt for a living, so they did not exert an impact upon the large mammals in South Africa.

But that situation may have been different in East Africa. White rhinos are extremely abundant, right through the Pleistocene, in fossil deposits at Olduvai Gorge. And there are some records of teeth being found on the surface in East Africa, which seem to have a Holocene age of just a few thousand years ago. So my suggestion is that the white rhino was a species that went locally extinct in Africa. It was just fortunate that it happened to survive in the two extreme areas that were not colonized by humans as efficient predators until very recently, when they did not need to hunt by then.

It's very hard to find modern examples of mammalian extinctions, so the best I can present is the case of near extinction that is very recent, recorded in the Kruger National Park, of the decline in the population of roan antelope, from a level of about, at a peak of about 450 a few years ago, to a current situation where the population exists at about 50 animals -- and, of course, in the really acute danger zone for extinction through small-population processes. An implication from the evidence, that I won't go into in detail, that this extinction was largely driven by predation by lions. The story is that during the drought period -- these indicate three drought periods . . . the drought period in 1982-1983 -- zebras moved into the range where roan occurred in large numbers. The roan went through that drought period without any effects of the drought itself on the population levels. And then, in the next drought period, in 1987, the population plummeted down to less than half of its previous level in over two years, sort of plateauing out, and then has drifted down to this much lower level. And the evidence indicates that what happened here is that following the increase in zebra numbers, lions moved in and settled in the roan range; and the roan were not well-adapted to resist that predation, so lions were the agents that caused that population crash. And this is under conditions where roan antelope are not the major prey species of lions -- zebra and wildebeest are.

So it was not that lions are dependent upon roan antelope -- the predation was incidental. But it is also the case that, through roan being a very large antelope species, they are quite vulnerable to being predated. In fact, they might even have persisted in the area where they occurred in the north because of the lack of other prey species for lions -- such that lions were present, but not present in large enough numbers to have impacted their population.

What I'd also like to point out here is that this part of the story is not simple. The zebra were attracted into the northern area because of the provision of water points, supposedly to help rare antelope species survive droughts. And the actual population declines of roan antelope -- both in 1986-1987 and 1991-1992 -- were associated with droughts. So it's hard to separate the impact of predation as a cause of extinction from the interaction of that predation with other species in the system, and with climatic stresses or effects of drought periods.

It seems to me that, at least for the very large megaherbivores, there's no need to consider any other explanation beyond direct human predation as a cause of those extinctions, for the simple reason that if humans were dependent upon them as food, as soon as a predator becomes dependent upon a species with a population growth of under 10%, that species is doomed -- if it has no refuge at low density. So, for the megaherbivores, starting off with the straight-tusked elephant, Palaeoloxodon in southern Europe, about 40,000 years ago; the steppe rhinos about 20,000 years ago; carrying on through the woolly mammoth in the far north as recently as 10,000 to 12,000 years ago. Extinctions -- woolly rhinos, mastodons, the giant ground sloth, not these gomphotheres, but their descendants in South America -- the Columbian mammoth, the Toxodon, in Australia, Diprotodon -- it's very easy to accept that humans were the sole cause of the extinctions. Climatic factors may have made them a bit more vulnerable, but they were doomed as soon as a predator cracked their defenses.

What about the other smaller species lurking in the background behind the mammoth and mastodon -- the equids, the camelids -- which were not only fleeter, they had higher potential population growth rates, probably of the order of 15% to 25% per annum? Why were those species also so vulnerable to being wiped out by humans?

And let's look at other factors -- habitat-related factors -- that may have contributed to their demise. How do habitat changes bring about extinctions? The thing to emphasize here is that habitat-related extinctions are geographic. To use ecological jargon, what was formerly a source habitat, producing a population surplus, must become a sink habitat, with a net population growth rate of zero, through changes in resources or other factors. Eventually those patches of source habitat become isolated; and it must also be the case that the core-habitat refugia, where large enough populations can keep surviving despite the changes that have made source habitat become sink.

Here's a diagrammatic presentation of the habitat model. What I've suggested is that what were formerly quite large patches of source habitat have now had changes in vegetation or whatever, such that only a small core area still persists as a source habitat, and the surrounding area becomes a sink. As a result of that, populations become isolated in these source habitats in a sea of nonhabitat or temporarily occupied sink habitat. And, eventually, these small populations may go extinct through various processes -- and, in particular, if you consider that the end of the Pleistocene, or the time of climatic change, what may have been a suitable habitat in this source habitat here may become nonhabitat because of climatic influences upon vegetation. And the source habitat may happen to reappear somewhere much further north. Populations have to disperse to recolonize that.

How do megaherbivores bring about habitat changes? The white rhino's an important species, because it is a grazer, like many of the extinct megafauna -- unlike many of the persisting species. White rhinos are grazers. They can transform grassland. This is Umfolozi Game Reserve in the early days, with the extensive cover of tall themeda grassland. Through white rhino grazing pressure, the tall grass cover gets grazed down to stubble. The tall grasses get replaced by short-grass species to form this productive lawn supporting white rhinos and many other species very well. And the transformation there is not just in grass height, but the species outside that grazing enclosure are quite different to the species growing inside it. So there's a radical transformation of the grass cover through the grazing pressure.

The effect on many other species of grazers is not negative -- wildebeest, zebra, impala, warthogs -- all the species that are favored by short grass thrive, along with white rhinos. Only those species that depend upon tall grass may suffer restriction in their distribution.

What impact would the vast herds of mammoths have had in North America? Because today, even in Africa, this is a rare photograph -- the photograph in Dick Law's book -- of what conditions were like in Uganda when he first studied elephants there. These are the kinds of concentrations which can potentially develop in the absence of human predation. What would the ecosystem and vegetation impacts of these numbers of very large megaherbivores have been? Modern-day elephants are, of course, mainly browsers, and the impacts of the grazers are mainly through browsing. They opened up woodlands. Where there were woodlands you get open areas. Other areas of woodland and savanna get transformed to shrublands. I want to also illustrate here, there's a second influence, and that is that of fire. Fire has also burned through this area -- that's why it looks so bleak. The fact is, as soon as elephants open up woodlands, you get enough grass cover developing to support fire. And it's an interesting fact that in Africa we don't seem to find a whole ecosystem that is mapped on maps of the biomes of the world. The dry deciduous forest biome is replaced in Africa by what we call "moist dystrophic savanna." It's called savanna because those areas support fire. Why that difference? By the time areas become open grasslands -- there's a photograph taken in the Masai Mara area of Kenya -- the elephants work hard at the remaining woody plants, to keep them from growing back to woodlands.

And just to try to illustrate the kind of dramatic landscape-level changes that elephants can induce -- I'm sorry about the quality of the picture; I took it out of an old journal -- it's a picture taken in eastern Zaire, from one area that formerly had fairly intact fauna, a fauna of large animals. This picture was taken in the 1930s, and this picture was taken in the 1960s. Exactly the same point -- everything matches exactly in the background and foreground. Complete conversion of a woodland to an open grassland with few relict trees -- with an elephant density of about 2 per square kilometer. And despite that habitat transformation, other species of grazer -- including topi, buffalo, kob -- were present in even larger numbers at the end than they were in the beginning. So those are the kinds of ecosystem changes that can be induced by megaherbivores.

One final example -- Huluhluwe Game Reserve forest patches. Elephants were eliminated there about 1870. Everything was fine. Fire maintained open glades until animal populations built up increasing grazing pressure, suppressing fire. The woody canopy cover increased from about 50% to 75%. And where there'd been open glades, you had thickets developing. And in those thickets, nyala and impala thrived -- those two species that were not represented in that Game Reserve when it was created; they were novel species. This is what happened to the grazers, which had formerly been prevalent.

Wildebeest -- peak population of several thousand -- under 50 left. Water buck -- from a few hundred down to a couple of score. Zebra -- down from several thousand to a couple of hundred. Buffalo, much less affected by those changes. None of these species has gone extinct, but the population levels are now in the danger zone. A couple of species have, in fact, gone extinct, but they were species that were originally rare in the area. These are local extinctions, of course.

A final example -- looking at the other side of the world. You can, of course, get open grassy areas persisting in the absence of megaherbivore impacts. This is the salt plain in the northern part of Wood Buffalo National Park in Canada. You don't need megaherbivores to create this grassland -- it's due to edaphic causes. You get similar grassy plains in the flood plains of the Athabaska. And the bison survive in this boreal forest region on those grassy plains -- that's all you'll really find. But if your grassy habitat is moving its location because of climatic change, how do the species get from one grassy plain to another? If you're a bison it's okay -- you can penetrate the forest. Somewhere in that forest there's a bison peering out. If you're not a species that can move through that forest, though, and of nonhabitat, then you undergo extinction.

The mammoth steppe hasn't completely disappeared. Here's another photograph from a book, the Beringa book, showing the persistence of that mammoth steppe in part of Siberia to the present day. This is a localized area of open vegetation. The extent of that would have been much greater if you had ecosystem engineers in the presence of mammoths and other large mammals present.

I want to finish off now. The final point is to emphasize that in intact African ecosystems, megaherbivores, in the form of elephants in this band, rhinos and hippos in that band, make up more than 50% of total large mammalian biomass. You can't remove that component from the ecosystem and not expect to have major changes occurring. Almost every African ecosystem, apart from the Serengeti, has a situation where megaherbivores -- today it's only elephants, rhinos and hippos -- combine to make up more than 50% of total large herbivore biomass.

So, my final message, for those people who think that humans as predators were the main agent of the Late Pleistocene extinctions -- you can't ignore the fact that once those species, the megaherbivores, were eliminated, there might have been habitat changes occurring which might have had some effect upon making the other species in the ecosystem somewhat more vulnerable to extinction than they might have been. For those people who support a climatic cause of those extinctions, you can't just describe the habitat reorganization that took place at the end of the Pleistocene to climatic change alone. The elimination of those megaherbivores from those systems must have also had a dramatic impact on the kinds of habitat changes which seem to have occurred as evident from biological records. And we know today that even though humans might be directly responsible for eliminating elephants through hunting, that a much more important and more insidious factor has been the preemption of elephant habitat by expanding human populations.

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