The Tree (38 page)

But here we get into some more twists, this time of a genetic nature.

First, if many different species are crammed into any one place, 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), 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 bedeviled many an isolated village (in turn inspiring many a gothic novel). Genetic drift, in short, often leads to extinction.

But in 1966 A. A. Fedorov 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 color 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 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.

Are the Hills of the Tropics Really Higher?

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 favorable 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 tropical 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 at even 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 Fedorov’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 organisms—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
trees.
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. 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 neighbors fall and so let in the light. Thus Emanuel Fritz observed a coastal redwood 160 years old that was a mere one hundred feet high, which meant it had been growing by only 1 percent per year. But then a gap appeared in the canopy, and for the following decade it grew at an astonishing 20 percent per year—to take its place almost instantly as a respectable, three-hundred-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 in a wide variety of soils. Nick Brown from the Oxford Forestry Institute, 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 suboptimal. But they grow just the same. Yet 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. 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 that suppose that tropical forests are varied because the different kinds of tree require extra-special niches.

Is the World Driven by Parasites?

There is, however, 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 attacks 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 orangutans.

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 orangutans 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 farther 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 their own kin, then those of the same kind will wind 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 end up with enormous diversity and enormous distance—a third of a mile is commonplace—between any two of the same species.

So the secret of tropical diversity—or, at least, of the diversity of the trees themselves—may lie with parasites. This may seem humbling: that such magnificence and 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 unsavory idea seems to stand up more firmly than most, and perhaps it 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 a third of a mile 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.

Also: if disease causes tropical forests to be so diverse, why doesn’t it have the same effect on temperate forests? 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 the 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.

H
ISTORY

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. 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, landslides, 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 that will have a tremendous effect on trees everywhere.

In particular, the world has grown steadily cooler over the past forty million years or so, albeit interrupted by a few warm spells, culminating in the ice ages of the past two 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, the surface temperature goes up. This is what seems to be happening now, and it 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 infrared radiation. The earth is warmed in the day by sunlight and loses the heat again at night in the form of infrared radiation. But carbon dioxide inhibits the loss of infrared, 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 (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 during the past forty million years or so because the concentration of atmospheric carbon dioxide has steadily gone down.