by Colin Tudge
The northern hemisphere has comparable stories to tell. Thus Michael Donoghue of Yale University has examined the relationships (as revealed by their DNA) between the plants of eastern Asia (mainly China) and those of eastern and western North America.
We know that a few tens of millions of years ago, all of the present land mass of Europe and Asia was linked at its western extreme to Greenland and Iceland, which in turn were linked to what is now North America. Eastern North America was then much nearer to east Asia than western North America was. Common sense says that the easiest way for plants to get from eastern Asia to North America in the deep past would have been via Europe and Greenland. So we would expect the trees of eastern North America to be more like those of eastern Asia than the trees of western North America are. (It’s useful to have an atlas handy when reading this account.)
In fact, though, says Professor Donoghue, the trees of western North America are more like those of China than those of eastern North America are. It seems, indeed, that many of the trees in the west of North America arose in China — and then evidently crossed what is now the Pacific Ocean. This seems most unlikely until we consult the atlas and perceive that the gap between Siberia, to the extreme north-east of Asia, and Alaska, to the extreme north-west of North America, is small. At present that gap is linked by a chain of islands. But when the ice ages descended in the past, the sea level fell by up to 600 metres (since so much water was trapped, as ice, on the continents both of the extreme north and the extreme south – including Antarctica of course, but also Australia). During those times, there was dry land between Siberia and Alaska – a land known as Beringia, which at times was huge: the size of present-day Poland. Many animals are known to have crossed from Eurasia to North America via that route, including lions, bison and ancient elephants; and many others crossed from the Americas to Eurasia, including dogs and rhinoceroses. Human beings also reached America via the Beringian land bridge. Professor Donoghue’s studies suggest that many plants made use of this bridge too. In short, many of North America’s present-day plants, including many trees, seem to have originated in eastern Asia (notably China). But they did not (as the history of continental drift would lead us to expect), move west to North America across Eurasia. They moved east to North America via Beringia.
More generally, we see that the broad general principles do indeed explain a great deal. Each lineage of plants did arise in one particular place; each group may then have spread to secondary centres, and diversified again; and all the time the stage has shifted, as the continents processed around the world. But if we set too much store by the broad principles, we are deceived. The actuality of each tree’s history, unearthed as best we can by their fossil spoor and their relationships as reflected in their DNA, reveal layer upon layer of complexity. We could tell quite a good tale now of the origins and migrations of trees. But in twenty years it will be different and surely richer; and in a hundred years it will be different (and richer) again. Of course we can never be sure of anything, and least of all of events in the deep past. But it is tremendous fun finding out – or simply enjoying the fruits of others’ scholarship.
What of the other question – why there are so many more species in the tropics than in high latitudes?
WHY SO MANY TREES IN THE TROPICS?
In print at present are approximately 120 recognizably distinct attempts to explain why the tropics are so various, and why they are so much more various than the high latitudes. Many if not all them are bona fide scientific hypotheses – not just top-of-the head speculations that may or may not be true, but ideas that give rise to predictions that can be tested. In practice, some of the predictions remain untested, and the tests that have been done sometimes seem to support the underlying hypotheses and sometimes simply raise more questions. Some of the explanations complement each other, while others are definitely at odds. It really isn’t easy to convert the observations of natural history (in this case, that there are huge numbers of species in the tropics and many fewer in temperate lands) into hard science.
In a nutshell, the accounts are of three kinds. Some ascribe the diversity of the tropics to physical factors, notably the abundance of heat and light. Some home in on logistics – the notion of complexity, the outworking of natural selection, and so on. Some cite history, suggesting that the diversity of tropical forests and the relative impoverishment of temperate ones depend on what has happened in and to tropical countries over the past few decades or millennia or millions or even hundreds of millions of years. In truth, of course, all phenomena of all kinds should be discussed from these three angles: the physical facts of the case; logistics; and history. In the following I will discuss the first two kinds of ideas together, and treat history – always the joker in the pack – separately.
Heat, Light and Logistics
‘Energy is one of the best predictors of diversity’: so says Douglas Schemske of Michigan State University records in ‘Ecological and evolutionary perspectives on the origins of tropical diversity’ (Foundations of Tropical Forest Biology, 163–73). Energy, in this context, means warmth and light, including ultra-violet. This seems to be true everywhere. Even within Britain, places that have more sunshine tend to have more species than places with less. Why should this be?
For starters, and most obviously, more warmth should mean that more is happening. Plants certainly grow quicker in warm places. With such thoughts in mind, some biologists have suggested that if creatures grow quicker, then they can reach maturity more quickly. This means they can fit in more generations in a given time – and so, we might expect, they can evolve more quickly.
The first bit of this argument (that organisms can grow quicker when warmer) stands up to an extent, but is not simple. For example, mammals and birds are ‘warm-blooded’, meaning they achieve some independence from background temperature by creating their own body heat. By the same token, they may grow very quickly even in the cold. Nothing grows quicker than a baby blue whale out in the chilly ocean, while the growth rate of Arctic goslings or of baby seals on ice floes is prodigious – it has to be, because they have only a few brief weeks to grow before they must take to the air or put to sea. Plants, however, clearly can and do grow much quicker when it’s warm (but not too warm), and in general, as we might expect, the tropics do produce much more biomass per unit time and space.
But the idea that creatures that can grow fast are likely to have shorter generation times and so may evolve more quickly, is far more equivocal. Many tropical forest trees take an unconscionably long time before they set their first seeds (bamboos take several decades) and many animals that live in the tropics, from scorpions to elephants, can be slow to mature and even then produce very few offspring, and only at long intervals. Human beings first evolved in the tropics, and we too take our time to reproduce. In short, although common sense suggests that tropical creatures might mature earlier and reproduce more quickly, in practice nature is far more subtle than that. There is no simple correlation.
Still, though, where there is more energy there is in general more life. Greater biomass is liable to be generated in a given space and time in the tropics than in temperate climates. The biomass is divided among many different individuals, which suggests there will be many more individuals. More individuals means more competition, and competition is the stuff of evolutionary change by means of natural selection; so we might expect more and more new types to be introduced after all, as the plethora of individuals battle it out.
But why should that produce more species? After all, we could perfectly well envisage that the individuals of one species would out-compete those of other species, so that the less-adapted species would disappear altogether. Then we would have a lot of individuals right enough – but they might all be of one species: the one that has adapted most adeptly, and succeeded at the others’ expense. So greater biomass in a given time and space, and more individuals, doesn’t necessarily lead to more species. Again, at high
latitudes, we find forests with huge biomass but very few species; and in the cold oceans, too, we find some of the greatest concentrations of biomass in all the world, in the form of the planktonic, shrimp-like crustaceans known as krill. But the krill is all of one species.
A second kind of idea – of a logistic kind – was first proposed formally by Alfred Russell Wallace and was then taken forward by two great evolutionary biologists of the twentieth century, the Ukranian-American Theodosius Dobzhansky and Britain’s R.A. (Sir Ronald) Fisher. In essence, it says that complexity builds on complexity. Every individual in a community (a community is a collection of creatures in one place, that may or may not be of the same species) obviously limits the space and resources available to other individuals. But at the same time, each individual may provide new niches for other individuals. Trees provide the supreme example. They have leaves, buds, flowers, fruits, twigs, a trunk with wood and bark, plus roots which create a special environment around themselves (the ‘rhizosphere’). Trees provide heavy or partial or intermittent shade – a variety of light regimes. In rainforest, it will typically be wet around the roots of trees while their tops, perhaps 30 metres up, will be in burning sun, potentially as strapped for water as any desert. Thus any one tree provides a host of micro-worlds, and a host of feeding opportunities: on the leaves, in the leaves, under the leaves, in the wood, on the fungi and protozoa that grow on bark or leaf, and so on. Each creature that exploits any of those potential niches in turn provides opportunities – and raises problems – for others. Thus every beetle on every leaf has its retinue of parasites and predators. Any one niche may be exploited in a host of different ways (some insects bite, some suck, and so on). Each method of exploitation provides new niches for other creatures. For instance, suckers of plant sap may suck up parasites at the same time (as aphids suck up viruses) and then pass them on to new hosts. Thus we have a positive feedback loop: as diversity increases, so it encourages further diversity.
But here we get into some more twists, this time of a genetic nature.
First, if many different species are crammed in to any one place, then the population of each species is bound to be small. But when populations are small, they start to lose genetic variation as the generations pass. Each parent in each generation passes on only half of his or her genes to each offspring. If the total number of offspring produced is low (as it will be if the population of parents is small), then it becomes very possible that some of the parents’ genes will not be passed on at all. Thus, as the generations pass, small populations tend to become more and more genetically uniform, as the rarer types of genes within the population fail to get passed on. This loss is called ‘genetic drift’.
Genetic drift generally has negative effects. For as a population becomes more genetically uniform, so the different individuals within it become more and more genetically similar. When individuals that are too genetically similar mate together, the offspring are liable to suffer from inbreeding; the same phenomenon that has wiped out many an aristocratic family who loftily refused to conjugate with commoners, and bedevilled many an isolated village (in turn inspiring many a gothic novel). Genetic drift, in short, often leads to extinction.
But in 1966 A.A. Federov put a more positive spin on genetic drift. He pointed out that the loss of genes with each generation produces qualitative shifts in the succeeding generations. For instance, a parent generation of a plant with predominantly white flowers may contain rare individuals that have genes for red flowers, and so produce some offspring with red flowers or indeed (depending on the degree of genetic dominance) in varying shades of pink. But loss by drift is a random process, and, quite randomly, the rarer red gene will be lost. Then all individuals in later generations will have white flowers. A shift in flower colour may not be important – but we can easily envisage other changes that could be. For example, a shift in the assortment of genes could result in a change in mating pattern – so that later generations flowered at a different time from the parents. If this happened, then the later types would no longer be able to mate with any of the parent types that might still happen to be around. Once two groups are reproductively isolated then they will each evolve along different lines – and effectively form new species. Putting the whole thing together: having a lot of species in one place means that the populations of each species are smaller; the different populations lose genetic variation by drift, and so (if they don’t go extinct!) they quickly become qualitatively different as the generations pass; and these qualitative changes can lead to the emergence of new species. Here we have a genetic reason why diversity could lead to still greater diversity.
In 1967 another American biologist, Daniel Janzen, proposed yet another twist, both ecological and genetic in nature. He suggested that since tropical species live in very favourable conditions (of heat, light and moisture), they probably could not tolerate a wide range of different conditions. (This idea was widely held in former times: it was assumed that tropical trees in general must be sensitive plants.) Janzen then proposed that if two populations of trees from the same species were separated by a mountain that was even of modest height, they would probably be completely isolated since they would not be able to live even at the modest altitude of the land that divided them. By contrast, he suggested, temperate plants are much tougher, and although they would be kept apart by bona fide ranges like the Rockies or the Alps they would not let a mere hill come between them. For this reason, he said, there is liable to be more isolation of different populations in the tropics than in temperate lands. Cogently and poetically, Janzen called his paper, ‘Why mountain passes are higher in the tropics’ (American Naturalist, vol. 101, 1967, 233–49).
Intuition suggests that all these forces of change and diversification could be operating, and it’s easy to see how they might all act together: isolation caused by inability to cross apparently innocuous boundaries could lead to small populations and so cause further changes by genetic drift, and so on. Such ideas are very difficult to test, however. It is very hard to find out anything for certain in tropical forests, and to test subtle hypotheses that have to do with the rates of gene loss in small populations is difficult indeed. (Although, as described later, this is precisely what scientists in the Dendrogene Project in Brazil are now doing. But they are doing it for different reasons; not directly to test ideas such as this.)
Yet there are some relevant observations, and they don’t all provide support for ideas like Federov’s and Janzen’s. For instance, it is easy to see how a tree provides niches for hundreds or thousands of species of other kinds of organism – fungi, epiphytic ferns, insects, mites, and so on. But the question here is not why there are so many species of ferns and insects and so on in a tropical forest. We are asking why there should be so many kinds of tree. Why should the presence of any one tree provide more niches for other trees? This clearly is not the case, for instance, in the coastal redwood forests of northern California.
One possible answer is as Janzen suggested: that tropical forest trees are highly specialized. For example, it is clear that the soil in Amazonia, say, may differ markedly from place to place, for example in its mineral content. If the trees were really highly specialized, then we would expect different types to grow in different places. Then again, some trees are shade-lovers, while others like bright sun and are inhibited by shade, and many others prefer shade when young but come into their own when it’s sunny. Pioneer trees are generally sun-lovers, and so they quickly occupy any space that appears when some forest giant collapses or is felled, leaving a clearing. The coastal redwoods of California grow very slowly when shaded but then zoom up as soon as their neighbours fall and so let in the light. Thus Emanuel Fritz observed a coastal redwood 160 years old that was a mere 100 feet high, which meant it had been growing by only 1 per cent per year. But then a gap appeared in the canopy and for the following decade it grew at an astonishing 20 per cent per year – to take its place almost instantly as a respectab
le, 300-foot redwood.1 By such means, we can see how the comings and goings of the different trees would create opportunities for others.
In truth, though, trees in tropical forests seem to be far less specialist than might be supposed. Individuals of any one species are to be found growing on a wide variety of soils. Nick Brown from Oxford’s Institute of Forestry, too, has found that whereas mahoganies in Amazonia are adapted to grow at forest margins – that is, when they are young they flourish in the light – they may soon be overtaken by other trees spreading from the forest behind them, so that by the time they are mature they are in the middle of dense forest, in shade. In other words, most mahoganies, most of the time, are growing in conditions that for them are sub-optimal. But they grow just the same. But if it’s the case (as it seems to be) that tropical forest trees are really quite versatile, then they would not obviously benefit from special niches created by the presence of other trees, or necessarily be separated by modest hills that provided only a slightly different environment. But if the individual trees are versatile, then there seems no obvious reason why any one species, or just a few, that happened to be more robust than the rest, should not take over the whole region – just as seems to happen in northern forests. So the observation that tropical forest trees are more flexible in their tastes than might be supposed, throws doubt on all hypotheses which suppose that tropical forests are varied because the different kinds of tree require extra-special niches.
