Authors: Colin Tudge
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.
H
EAT
, L
IGHT
,
AND
L
OGISTICS
“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.
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 outcompete 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.
Diversity Makes Diversity
A second kind of idea—of a logistic kind—was first proposed formally by Alfred Russel Wallace and was then taken forward by two great evolutionary biologists of the twentieth century, the Ukrainian-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; they 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 rain forests, it will typically be wet around the roots of trees, while their tops, perhaps thirty meters up, will be in burning sun, potentially as strapped for water as any desert. Thus any one tree provides a host of microworlds 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.