by Colin Tudge
But there is one idea at least that really does seem to hold water. There is indeed a very great deal of life in general in the tropics. There are millions of creatures milling about, with billions of possible interactions between them. Not all of those interactions are antagonistic – cooperation is an important fact of life – but some definitely are. Predator–prey relationships are antagonistic: creatures that kill, versus those that are killed. So, too, are parasite–host relationships. All trees suffer both kinds of attack in abundance: battalions of sap-suckers, leaf-eaters, bark-borers, root-miners, fruit-stealers and seed nibblers, from viruses through bacteria and fungi to weevils, toucans and orang-utans.
Of particular interest in this context are the many disease-causing parasites of trees, including viruses, bacteria and fungi. In general, these small parasites like and need large and dense populations to work on. Unlike big tree predators such as orang-utans and toucans, they cannot travel easily from host to host. They prefer close contact. But most such parasites are highly specialist. They do not leap easily from species to species. So they thrive best in large, close-set populations of the same kind of tree.
So a tree that seeks to avoid parasites is advised to stay as far away as possible from others of its own kind. Thus it is commonly observed (though not always!) that when seedling trees grow up close to their parent tree they die more quickly than those that are set further away. Young trees in general are more vulnerable than older ones, and the ones that stay close to home are killed off by their parents’ parasites. If trees are killed off when they grow too close to own kin, then those of the same kind will finish up growing a long way apart. The gaps in between will be filled by trees of different species – each of which is anxious to put as much distance as possible between itself and others of its own kind. Thus we finish up with enormous diversity and enormous distance – half a kilometre is commonplace – between any two of the same species.
So the secret of tropical diversity – or least, of the diversity of the trees themselves – may lie with parasites. This may seem humbling: that such magnificence, sheer variety, should have such a squalid cause. But then, the great English biologist W.D. (Bill) Hamilton proposed that it was the need to avoid parasites that prompted the evolution of sex – for sex produces the generation-by-generation variation that makes life difficult for parasites, which tend to be highly specialized, to get a hold.
This simple if unsavoury idea seems to stand up more firmly than most, and perhaps is indeed the key. But it immediately raises two obvious problems. The first is how trees in tropical forests manage to find breeding partners if the nearest individual of the same kind is half a kilometre away. This is indeed a difficulty – which, as discussed in the next chapter, has led to some of the most ingenious and extraordinary evolutionary exercises in all of nature.
But also – if disease causes tropical forests to be so diverse, why doesn’t it have the same effect on temperate forest? Temperate trees have plenty of diseases too: Dutch elm disease in elms, chestnut blight in North America, and so on.
The answer is again uncertain (of course), but two grand ideas seem very definitely to be pertinent. One emerged in the 1950s, again from Theodosius Dobzhansky. He pointed out that in the tropics, where life in general is so easy, and so many different creatures can find a niche, the real pressures are biological: in other words, the pressure on any one creature comes from all the other creatures around it. By contrast, in temperate climates, the main problems are posed by the elements – cold winters, whether wet or dry; late frosts; and lack of adequate warmth in summer to stimulate growth. Only the hardy few that can hack such vicissitudes need apply at all. We might further speculate that the cold zaps the parasites too, and so relieves the pressure from them. After all, temperate fruit trees seem to suffer more damage from parasites after a mild winter, when more of the pests survive.
Many a case history offers good support for Dobzhansky’s idea that northern creatures must cope first and foremost with the sheer violence of the physical conditions. I will round off this chapter with three examples from North America. But before we look at them we should acknowledge one final joker in the pack. The strongest reason of all why the tropics are so much more varied than the north may be a matter of history.
History
History marches on an infinity of timescales simultaneously. Every living creature or the ancestors that gave rise to it has been influenced by events that happened yesterday, decades ago, thousands of years ago or hundreds of millions of years ago and by the same token, everything that happens in any one moment affects the next second, the next year, and so on into the indefinite future. On the short scale (of years and decades and centuries) all trees everywhere (or their ancestors) have been affected by storms, floods, landslips and fires. On the very grandest timescale, all trees have been affected by the movement of continents, as already outlined. On the intermediate timescale – from centuries to tens of millions of years – the world may experience vast changes of climate.
In particular, the world has grown steadily cooler over the past 40 million years or so, albeit interrupted by a few warm spells, culminating in the ice ages of the past 2 million years. This climatic shift altered the course of all evolution – indeed, it apparently brought our own species into being as the forests of East Africa shrank during a series of cold spells a few million years ago and forced our arboreal ancestors to the ground. More pertinent here, is that the cooling of the earth in general and the ice ages in particular may largely account both for the variety of trees in the tropics, and for the impoverishment of the north.
The reason for the cooling lies ultimately with greenhouse gases in the atmosphere and in particular with carbon dioxide. All kinds of evidence – including cores drawn from extremely ancient ice in Greenland, which still holds bubbles of ancient atmosphere – attest that when carbon dioxide levels are high, then the surface temperature goes up; and this is what seems to be happening now, and is causing global warming. Contrariwise, when carbon dioxide levels are low, the earth cools. Physics theory supports this idea. The basic reason is that greenhouse gases (like carbon dioxide) are relatively impervious to infra-red radiation. The earth is warmed in the day by sunlight, and loses the heat again at night in the form of infrared. But carbon dioxide inhibits the loss of infra-red, and so reduces cooling. This is how the glass in a greenhouse works, which is why carbon dioxide is said to be a greenhouse gas (and so are a number of other gases, such as methane, but carbon dioxide is the one that counts in this context). The earth as a whole has cooled this past 40 million years or so because the concentration of atmospheric carbon dioxide has steadily gone down.
Why should this be so? The explanation was provided most ingeniously by Maureen Raymo, from the Massachusetts Institute of Technology, in the 1980s. It is linked to continental drift. For, as we have seen, about 60 million years ago the tectonic plate that bore the vast island land mass that is now India began to crunch into the south of Asia, and puckered the land in front of it to produce the Tibetan Plateau, with the Kunlun Shan and the Himalayas to the north and south. The plateau and the mountain ranges form, as Raymo puts it, ‘a giant boulder’. The wind sweeps over the Pacific to the south and east, picking up water along the way; and as it hits the high land at the top of India it rises, cools, and releases its water, which falls in the rainstorms known as the monsoons, and runs away in eight great rivers that include the Ganges, Brahmaputra, Indus, Yangstze and Mekong. The mass of water feeds both the forests and the farms of Asia (although the land left in the rain shadow includes the Gobi, the Mediterranean and the Sahara).
But the water that falls in the monsoon rains is not pure. Rainwater always contains gases dissolved from the atmosphere, and among the most soluble is carbon dioxide. Thus rain – all rain, everywhere – is a weak solution of carbon dioxide, otherwise known as carbonic acid. As the carbonic acid falls on to the Himalayas and the Tibetan Plateau, it reacts with calcium and
magnesium in the rock to form a weak solution of bicarbonates. This is washed away in the rivers and into the ocean, where the bicarbonate salts become incorporated with the ocean bed and eventually (thanks to plate tectonics) are thrust down into the magma beneath. The net result is that carbon dioxide is steadily leached out of the atmosphere – and has been leached, storm by storm, for the past 40 million years. The dwindling carbon dioxide causes what is sometimes known as an ‘icebox effect’: opposite to a greenhouse effect. This may sound fantastical, but sober calculations based on elementary chemistry and the topography of the mountains suggest that it is all eminently plausible.
There is one more variable. In the early twentieth century the Yugoslav mathematician Milutin Milankovitch sought to find a relationship between the fluctuations in climate and the changes in the Earth’s orbit as it circles the Sun. In general the Earth’s orbit is almost circular – but at times it becomes more elliptical. Ellipses are like flattened circles, pointed at both ends; and when the Earth is at the points, it is further from the Sun than it ever is when the orbit is more circular. When the Earth is more distant, said Milankovitch, the amount of sunlight striking it can go down by 30 per cent, and the climate then is obviously cooler. The orbit passes from near circular to more elliptical in cycles, known as Milankovitch cycles, that last around 100,000 years. So, said Milankovitch, we can expect periods of relative cold to alternate with periods of relative warmth every 100,000 years.
When the general level of carbon dioxide in the atmosphere is high, the periodic cooling caused by the Milankovitch effect may not be too disturbing. But when atmospheric carbon dioxide is low, the extra cooling can change the world radically. By about 2 million years ago (thanks to the continued depletion of atmospheric carbon dioxide by the rocks of the Tibetan Plateau), the earth was on the point of freezing over. In practice, over the past 2 million years the earth has frozen over, at least partially, every 100,000 years or so. These periodic freezings were the ice ages. In the northern continents, glaciers and mountains of ice in places reached depths of several miles, extending through Europe to southern Britain and France and through North America to what is now New York and beyond. Ice sheets centred on Antarctica encased much of the Southern Ocean, and encroached at least into the south of Australia. During the ice ages, the world was cooler, or course. But it was also much drier, as much of the world’s fresh water was locked into ice, and the cold oceans evaporated less freely. The creatures that lived around the poles (particularly in the north) and around the equator were both affected by the combined effects of cooling and drying. But the biodiversity of the north and in the tropics was affected in totally opposite directions.
In the north, the cumulative effect of advancing and retreating ice, in the roughly 100,000 year cycles, was devastating. In North America, Greenland, Iceland and Eurarasia, as the ice encroached from the north it simply obliterated all the creatures in its path that could not move away and caused all the ones that could move to migrate to the south. Reindeer and lynx, archetypal Arctic creatures, came down into southern Europe. Some species of trees were wiped out. Others scattered at least some of their seeds ahead of the ice, and so their descendants lived on, but further and further south. At the end of each ice age, as the glaciers retreated, the remaining creatures could extend north again. The plants generally did so in a fairly orderly sequence. Often the land had been scoured of soil, and the vascular plants had to wait for the hardiest pioneers, like moss, to build it up again. The trees followed in their wake, with the hardiest first: birches, aspens, pines, then oaks, beeches and chestnuts. Records of ancient pollen in the north show this sequence, or ‘succession’, over and over again. Oaks and ash were typically among the later species to be installed and often became dominant, as in Britain. When any particular group of plants become dominant they and their undergrowth are commonly said to form the ‘climax vegetation’. In more recent years, however, the classic concept of ‘climax’ has been challenged somewhat. Everything is transient, seen on the geological timescale. Oak forest may be ancient, but it is merely the latest instalment in a sequence that’s continuously unfolding.
Clearly, the advancing and retreating ice in the north reduced diversity. As the ice came south, it caused extinctions. The trees that followed it north again were a selection of those that had survived the southern putsch: only the tough could undertake the northward journey at all, and the race was to the swift. Whatever diverse creatures occupied the northern lands before each ice age were weeded out. In the past 2. million years the weeding has been repeated at least a dozen times.
In tropical latitudes, by contrast, the cooling was less than devastating. Even in an ice age, the equator was still warm. But tropical creatures were affected by the drying caused by the general cooling and the entrapment of water in polar ice. The vast equatorial forests, which in interglacial times are continuous within each continent, became patchy. Patchiness provides precisely the conditions required to produce more species, as different populations become repro-ductively isolated one from another, and each evolves along its own lines. Then, when each ice age ended, the patches expanded again, and the newly evolved species from each different place were brought into contact again – to form astonishingly diverse assemblages of the kind we see now in the Amazon. That at least is the theory, and it is highly plausible. We see the same phenomenon among fishes – particularly the cichlid (pronounced sick-lid) fishes of the great African lakes. In glacial times they were reduced to a series of (big) ponds, each developing its own cichlid variants, which then reconverged to form the great inland water masses of today. Various human incursions have caused extinctions, but until recent years Lake Victoria had at least 300 species of cichlid and Lake Malawi had more than 500.
In short, both the northern and the equatorial forests felt the effects of the ice ages. But as the ice came and went, the northern forests lost more and more species, while the tropical forest became more and more diverse.
The ice ages, too, had one more dramatic effect. They caused forests to disappear in some places, but allowed them to flourish in others. Thus if we could look at the world’s forests in geological time we would see them racing over the surface of the globe. The rainforest of Queensland seems to have been there for ever. But, like the Great Barrier Reef just offshore to the north-east, it has been there only since the last ice age ended, less than 10,000 years ago. The people of 10,000 years ago were modern, like us. Some people were building cities – Jericho is about that age. Many had long been navigators. Farming was beginning on a settled scale. Doubtless they had priests and paid taxes. Some at least of the stories in the Bible and the memories of the ancient Egyptians and of present-day Australian Aborigines, extend back that far.
It seems to me that all of the ideas outlined above to explain the diversity of tropical forest and the impoverishment of temperate forest could apply at any one time. Each kind of influence could build on the others. Two ideas seem most cogent, however (at least to me). The first is the impoverishment of northern faunas and floras, and the diversification of tropical creatures, by the fluctuations of the ice ages. The second is the grand if simple idea of Dobzhansky: that in the tropics the main pressures are biological, while in temperate and boreal lands the stresses are mainly physical. In the tropics the critical pressure comes from parasites, which make it advantageous for any one species to be rare, and for the individuals to be widely spaced. In the north, the physical pressures are of various kinds, and only the toughest or the most adept survive. The aspens, jack pines and coastal redwoods of North America make the point beautifully.
A TALE OF THREE NORTHERNERS
Canada is the world’s second biggest country, at nearly ten million square kilometres, and more than a third of it is boreal forest. Yet this huge northern forest, nearly fifteen times the size of Britain, is dominated by nine species of tree. There are six conifers – jack and lodgepole pines; black and white spruce; balsam fir; and the larch
known as tamarack (Larix laricina). The three broadleaves are aspen, balsam poplar and paper birch, which sometimes form pure stands and sometimes are mixed in with the conifers. There is more diversity to the south of the boreal forest (including more broadleaves). The beauty is haunting, but life is hard. Only a few species can cope with the northern winters.
Odd though it may seem, a crucial feature of this coldest of all lands is fire. Those species that are especially equipped to cope with it can steal a very large advantage over those that are less adept. One adept is the aspen, sometimes known as the trembling or quaking aspen, Populus tremuloiodes.
You would not immediately suspect, if you confronted an aspen in an urban park, that it is among nature’s most resourceful trees. It has a languid air, with wanly fluttering leaves on long flat stalks, which in autumn turn a melancholic yellow. Its trunks, at least when young, are ghostly smooth and greenish-white (though deeply grooved near the base when older). Yet for all its bloodless foppishness the quaking aspen has the widest distribution of any North American tree and in large stretches of the far north it is the dominant and at times the only species. How come?
