by Colin Tudge
All the time the continents have been shifting, so the modern groups of plants and animals have been emerging. Modern land mammals, which could not easily cross the widening oceans, still show its effects very clearly. So it was that in the nineteenth century Alfred Russel Wallace pointed out that the mammals of Southeast Asia were quite distinct from those of Australia and New Guinea. In Asia there were cats, pigs, and deer; while Australia was dominated by marsupials and contained the unique egg-laying monotremes—the platypus and echidnas (though there is also an echidna in New Guinea). The boundary between the two faunas became known as Wallace’s line. Although Wallace didn’t know it, his line marks the border of Laurasia and Gondwana.
Continental drift must also enrich our attempts to explain why modern trees are where they are. To be sure, each kind must have arisen in one particular place, its center of origin. Each may then have spread to other places, and then spread again, and perhaps the earliest ones in the original center went extinct. But all the time this was happening the ground itself was shifting, as the continents wandered the globe, breaking apart and sometimes joining up with others.
Through such ideas we begin to see why different modern-day islands have such very different characters. Madagascar has been an island for a very long time, but it used to be part of Gondwana. When it broke from the mother continent it contained a random collection of plants and animals—the latter (perhaps) including the ancestors of lemurs, but not of modern carnivores or hoofed animals. Whatever plants it had on board were free to evolve in all kinds of strange directions, without competition from continental types. The traveler’s palm, Ravenala, the baobabs, and the Didiereaceae are among the results. New Caledonia too is a fair-sized, long-isolated fragment of the erstwhile Gondwana; but New Caledonia began its island existence with a different collection of Gondwanan creatures on board, which now are almost as spectacularly strange as those of Madagascar. In contrast, Hawaii, the Galápagos, the Azores, and the Canaries did not begin as bits of continents. They arose from the bottom of the ocean, in volcanic eruptions, and began life as bare rock. They, too, have their own unique collections of endemic creatures. But all their inhabitants are descended from ancestors that were brought in by wind or water, or flew or swam, or hitched a ride on the fliers or swimmers. Creatures with no such abilities stayed away. Conifers (apart from one bird-dispersed juniper on the Azores) are notably absent from midoceanic volcanic islands. New Caledonia is in the middle of nowhere but it is a fragment of continent, and its indigenous conifers, borne from Gondwana, are among the wonders of the world.
About 225 million years ago the world’s continents were joined to form a supercontinent called Pangaea.
Eventually Pangaea started to break into two separate continents.
Gondwana broke up to form Antarctica, Africa, Australia, and South America, plus India, Madagascar, New Zealand, and New Caledonia. Many (though by no means all) of the trees that now live in those southern lands were Gondwanan in origin.
The continents are still drifting. Perhaps in a few million years Australia will collide with Southern Asia, as India once did. Perhaps it will crunch into Japan. Or perhaps it will slide past Japan and into the North Pacific. Each of the possible scenarios will be dramatic, though none is urgent.
Britain was also part of an ancient continent, as Madagascar was, and has been linked to mainland Eurasia from time to time in the past few million years as the sea level fell during the ice ages. Yet it lacks much of the variety found in neighboring Eurasia and has no endemics to speak of, apart from a few fish, including various subspecies of char and trout. But here we encounter yet another complication. Britain endured the rigors of the ice ages. During the ice ages the British Isles were linked to mainland Europe, which should have enriched its flora and fauna. Instead, or at least equally, the ice wiped out what was there. Madagascar has long been in tropical latitudes and escaped any such purge. This story belongs to the next discussion, however: why the tropics are so much more various than temperate latitudes.
With these general principles in place (center of origin, secondary centers of diversity, continental drift), and aided by modern techniques (analysis of DNA and the steadily improving fossil record), biologists are now putting flesh on the theoretical bones. The Royal Society meeting of March 2004 showed how rich—and unexpected—the realities are now proving to be.
REALITY: A FEW CASE HISTORIES
Where Did the Amazon Forest Come From?
Take, for example, the American tropics (the neotropics), the most diverse ecosystem of all. South America is a fragment of Gondwana that broke away from Antarctica sometime in the Cretaceous, rafted north as an island for the best part of 100 million years, and finally docked with North America around three million years ago. (Although “finally” in this context simply means “most recently.” The travels of continents, like the Flying Dutchman’s, will never end.) The relationships of animals can be easier to see than those of plants, and it’s clear that South America has many strange animals—notably the sloths, anteaters, armadillos, and its own suite of marsupials—that are peculiar to itself and reflect the Gondwanan origins of South America and its long isolation as an island. But South America also contains northerners, including jaguars, pumas, and various deer, which came in from North America. So we might expect to see the same pattern among trees: basically a Gondwanan island flora with a few incursions, mainly from North America.
Yet this commonsense assumption holds up only to a very limited extent. Thus, the characteristic flora of northern South America at present is tropical rain forest. If the trees of the forest are indeed Gondwanan in origin, their ancestors ought to have been there tens of millions of years ago. But Dr. Robyn Burnham of the University of Michigan has studied fossil plants in Bolivia and found very little evidence of any tropical rain forest at all before about sixty million years ago. The oldest rain-forest fossils seem to date from around fifty million years ago. She has, however, found clear evidence of tropical rain forests in several sites in North America that are more than sixty million years old. This suggests (it does not prove, but it suggests) that the great rain forests of the neotropics, including South America, arose in the northern continent, Laurasia; and that the trees found their way to South America long before South America met up with North America around three million years ago, in the Pliocene. This fits in with other evidence of many kinds that suggests that South America had contact of a kind with northern continents long before the Pliocene. For example, the small South American cats—the ocelot and margay—seem to have been in place since well before the Pliocene. So, too, have South America’s monkeys, like the howlers, capucins, and tamarins. The ancestors of these animals also arose on other continents, but they got to South America before North and South America most recently collided. Presumably there were other land bridges in the past, now long gone—or if not literal bridges, then at least chains of islands, as are still to be seen through the Caribbean.
However, molecular evidence from Dr. Toby Pennington, at the Royal Botanic Garden Edinburgh, strongly suggests that the present-day trees of the South and Central American forest did not come from North America either. Their closest relatives, at least of the groups he has looked at, often seem to be in Africa. This seems to take us back to the original idea—that the plants of Africa and South America are both simply Gondwanan. But close comparison of DNA suggests that many of the present-day South American plants originated after South America broke from Gondwana—and that by some means or other they entered from Africa. The picture grows more and more mysterious.
True Southerners? The Trees of Australia
As with South America, so with Australia—but with a different twist. As with South America, we would expect from first principles that its flora, including its trees, would basically be a selection of those that were originally on Gondwana. Again, up to a point, this is true. But, again, there are some serious complications.
Thus it
is that the present-day vegetation of southern Australia is very varied. Some is alpine (specifically, low mountain). Much is shrubland and grassland. Grasses came only very recently to Australia, but much of the rest, as we would expect, is basically Gondwanan.
Robert Hill, at the University of Adelaide, concludes from the fossils that when Australia first parted company with the rest of Gondwana, around seventy million years ago, the south of Australia was largely covered in rain forest. (Strange to think of rain forest as inherited from Antarctica; but then the world is strange.) As Australia moved north it was cooled by the cold current that began to circulate around Antarctica. As the island continent cooled so it dried, and by five million years ago much of it was arid. Australia, too, is heavily eroded and short of nutrients—so the plants had to cope both with lack of nutrients and with lack of water. In addition, Australia has been beset by ice ages.
Specifically, Mike Crisp of the Australian National University, in Canberra, concludes that Australia inherited southern beeches from Gondwana, and that these sat around for a bit and then diversified impressively. But then the aridity got to them, and now there are very few. So the southern beeches were largely replaced by the eucalypts, which cope with aridity very well. Professor Hill feels that the eucalypts were a late development, and indeed may have arisen within Australia itself. In contrast, Professor Crisp believes that, like southern beeches and, indeed, like the banksias (in the same family as Grevillea, the Proteaceae), the eucalypts arose in Gondwana and were present in Australia from the start of its career as an island. Where Australia’s acacias (in those parts known as wattles) came from is not obvious, but they clearly diversified mightily as the continent became more arid.
In general, too, the picture of Australia’s trees and other plants has been hugely complicated by various animals, including humans. Australia used to have some bigger mammals than it has now, including a rhino-sized wombat called Diprotodon and a giant kangaroo that stood around three meters tall. There were more huge reptiles, too—which in Australia’s early days as a solo landmass included dinosaurs. Some of these may well have helped to disperse the seeds of Australia’s early trees; with them gone, the trees would suffer. The large mammals (including Diprotodon) evidently became extinct sometime after the Aborigines arrived from Southeast Asia, at least forty thousand years ago and perhaps as long as eighty thousand years ago. Aborigines, too, have long made extensive use of fire to encourage grasses and so attract animals for hunting. Their fires may well have altered the vegetation of central Australia irrevocably. The Europeans, of course, who probably first set foot on Australia in the seventeenth century and finally came to grips with it through James Cook’s voyages in the eighteenth, have transformed much of the country, with a huge array of imported plants and animals, including quasi-wild creatures such as rabbits and foxes (which affect the local marsupials, which in turn interact with the plants) and domestics such as sheep, water buffalo, and cats. Even in the deep past, however—millions of years before human beings arrived, and indeed before they arose as a species—the pristine Gondwanan flora that Australia inherited was complicated and largely transformed by climate, erosion, and the native animals.
The Chinese-American Connection
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 landmass 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 eastern 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. 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, in the extreme northeast of Asia, and Alaska, in the extreme northwest 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 meters (since so much water was trapped, as ice, on the continents of both 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 centers 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 of 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 sections I will discuss the first two kinds of ideas together and treat history—always the joker in the pack—separately.
HEAT, LIGHT, AND LOGISTI
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“Energy is one of the best predictors of diversity”: so says Douglas Schemske of Michigan State University in “Ecological and Evolutionary Perspectives on the Origins of Tropical Diversity” (Foundations of Tropical Forest Biology, 2002, 163–73). Energy, in this context, means warmth and light, including ultraviolet. This seems to be true everywhere: 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 more quickly in warm places. With such thoughts in mind, some biologists have suggested that if creatures grow more quickly, then they can reach maturity sooner. 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 more quickly when warmer) stands up to an extent, but it 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 more quickly than a baby blue whale out in the chilly ocean, and 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 more quickly when it’s warm (but not too warm), and in general, as we might expect, the tropics do produce much more biomass per unit of time and space.