James O’Donoghue (UK)
Did the destruction of forests by mammoths make the Pleistocene Ice Age even colder? It’s an extraordinary prospect. Yet, a leading fossil mammal expert thinks they did just that. Over many tens of thousands of years, mammoths and straight-tusked elephants ate their way through vast tracts of the world’s forests. Trees exert a buffering effect on global climate – take them out and face the prospect of hotter and colder extremes. Mammoths may have turned cool Ice Age periods into freezing ones. Straight-tusked elephants may have made temperatures rise during interglacials.
Both types of elephant had all but vanished by 10,000 years ago, never to return. Since then, forest cover has increased sharply while the climate has been unusually mild and stable. Could the two be linked? Humans chopping down forests are now exerting at least as profound an effect on the world’s ecosystems as the mammoths had on theirs. By comparing the destruction wrought then and now, an alarming prospect emerges. Are we in the very process of making our own climate as volatile as that of the extinct elephants?
Cores drilled from undisturbed glacial ice in Greenland and Antarctica have provided a wealth of information about almost constant shifts in the Earth’s climate over the past few hundred thousand years. Climatologists will tell you that we live in an interglacial period, in a world that is still going through an ice age that started 1.8Ma with the onset of the Quaternary period. (When I refer to the “Ice Age” in this article, I am referring broadly to the Pleistocene that encompasses both glacial and interglacial periods.)
Yet, temperatures during the past 10,000 years are a puzzle owing to their very stability. The previous interglacial, which started 135,000 years ago, lasted between 10,000 and 15,000 years and shifted wildly between periods that were warm enough to support hippos in Britain along with short, much colder periods. Why was the climate so unstable then? Indeed, what caused the Ice Age’s extremes in temperature in the ﬁrst place?
The Natural History Museum’s curator of Quaternary mammals, Andy Currant, was examining 80,000-year-old fossil bones found in Banwell Cave in Somerset, when it occurred to him that they provide a clue:
The cave was more or less filled up with bones. There is no process by which you could get such a concentration other than the physical presence of huge numbers of mammals. It’s not the only cave like it – sites of this age are frequently stuffed full of bones.”
Cave fossils suggest an extraordinary abundance in the past of what are known as “megaherbivores” – mammoth, straight-tusked elephant, woolly rhino, bison, reindeer and giant cattle. What impact did they have on the environment? South African zoologist, Norman Owen Smith, proposed in the late 1980s that megaherbivores ate their way through so much vegetation that they had a drastic impact upon their landscape. Indeed, elephants are known as “environment creators” by ecologists, so great is their role. This was graphically illustrated to Currant when he saw African elephants eating trees:
They don’t just eat the leaves, they eat the lot – wood, bark and all. If you look at an elephant tooth and think why does it need all those cusps, it becomes pretty evident. It’s a tree crusher. Elephants can live on a very wide range of things. Once the trees had gone, they had to. But, left to their own devices, they prefer trees”.
Currant then took Owen Smith’s observations a stage further and asked, “Did grazing by enormous numbers of prehistoric megaherbivores have a knock-on effect on climate?”
While global warming and cooling have always taken place, the Ice Age has seen real extremes in temperature and marked climatic instability. The underlying causes are changes to the Earth’s tilt and orbit brought about by the gravitational pull of other planets and stars. These are known as “Milankovitch cycles” and they help explain variations in solar radiation received by the Earth that are linked to global changes in temperature. However, what they do not fully explain is why Ice Age temperatures have been so extreme. During the Anglian glaciation of 450,000 years ago, arctic conditions prevailed, with ice covering all bar the south of Britain. Yet, at other times, temperatures were balmy and Mediterranean.
Currant believes that deforestation occurred on a vast scale by the grazing of mammoths during cool periods and by straight-tusked elephants during inter-glacials. Herds of bison and reindeer composed of hundreds of thousands of animals would have helped maintain open conditions. The signiﬁcance of this is that forested environments have a buffering effect on climate, providing insulation against extremes of hot and cold. Deforestation and climatic extremes are linked. Snow falling on a forest will be broken up and its effect on the climate will be limited. However, in an open environment, snow forms a continuous sheet of whiteness that reﬂects a great deal of the sun’s heat. Temperatures plummet and, if the snow extends far enough, the global climate will ultimately chill.
Before the northward spread of mammoths, there would not have been anything that could break up the forest on the kind of scale seen during the Ice Age. When mammoths became extinct in Britain and North West Europe, around 10 to 12,000 years ago, there was a return of large-scale forest cover and climatic stability ensued.
Danielle Schreve, a lecturer in Quaternary studies at the Royal Holloway College, agrees that ancient grazing mammals may have been a prime factor in climatic instability. As Schreve says:
The importance of megaherbivores as major agents of habitat modification should not be underestimated. Admittedly, it isn’t easy for us to envisage the lowlands of Britain filled with bison, wild horses and rhinos. The sheer biomass is almost impossible to image.”
However, not everyone is convinced by Currant’s interpretation. Adrian Lister is professor of palaeobiology at University College, London and the author of a book on mammoths. He points out that mammoth teeth are at least as well adapted for eating grass as they are for eating trees. Half of the food consumed by modern elephants is grass even when they live in habitats with plenty of trees. What’s more, trees constantly re-grow in forests where elephants live – an ecological balance is achieved. Lister ﬁnds it unlikely that mammals could have held back millions of square miles of forest if the climate suited its growth.
Professor Lister argues that “various climatic regimes produce treeless habitat without any help from the megaherbivores”, citing northern tundra and the great plains of North America as examples. His explanation for the severity of the Ice Age is that it was down to a combination of plate tectonics and Milankovitch cycles. The Earth has become cooler as tectonic forces have pushed much of the world’s landmass northwards away from the equator. In turn, during cold stages of the Ice Age, areas of treeless habitat expanded greatly.
So how can the dispute be resolved? Currant’s theory would be strengthened if he could ﬁnd wide scale evidence that trees had been destroyed in Ice Age environments. Ancient pollen preserved in, of all things, hyaena droppings, could provide him with the data he’s looking for. Fossil faeces are treasure troves for palaeontologists, providing not only evidence of diet but also a microscopic snapshot of the types of plants that were around when the meal was consumed. Currant has examined 100,000-year-old pollen samples from caves in South Devon, which he believes show signs of unexpected climatic instability. Interglacial pollen dug up in the heart of London’s Trafalgar Square also shows anomalies.
The pollen samples could be interpreted like this. During the very coldest periods, arctic conditions would have prevented trees from growing. Both trees and mammoths relocated south until conditions improved, perhaps dozens of centuries later. When the climate did warm up again, the woodlands reappeared. Were that sequence observed in pollen but culminated in the tree pollen disappearing – even though the climate was warming – then that would be compelling evidence that the trees were being removed. By what? By grazing megaherbivores repopulating formerly inhospitable and frozen lands, says Currant.
Only by further in-depth research will the conﬂicting theories be resolved. Currant emphasizes that:
Mammoths didn’t cause the Ice Age but the reduction of forest on the kind of scale they seem to have been capable of would have been a global phenomenon”.
Open conditions wreaked havoc, exposing the Earth to the full extremes of the Milankovitch cycles. Ice core evidence shows that before the disappearance of mammoths 10,000 years ago, dramatic shifts in temperature occurred in remarkably short periods of time – sometimes over as little as ten years. Devastating freezing periods followed hard on the heels of global warming.
The ecological role of humans has been to replace the megaherbivores as the forest clearers. There’s the rub. In recent decades, humans have been responsible for global deforestation on a massive scale. How much do we really know about the consequences of our actions? While few would dispute that greenhouse gas emissions are the primary cause of global warming, much remains poorly understood. With this example, the fossil record provides clues to help us better understand our own increasingly unstable climate.
For 10,000 years, global temperatures have been unusually mild and stable. As Currant says:
We tend to regard what was around when we were little as being what it has always been like. So, if we see change in our own lifetimes that begins to frighten us a bit. Yet, over the past two million years, the climate was always changing and it could change again very rapidly indeed.”
We could be in for a rough ride, in a world dominated by hot and cold extremes. The mammoths are trying to tell us something, if only we would listen.