The Tree (45 page)

Most of the feasters are bad news for the almendro: they eat, but they do not disperse. Squirrels eat but are poor dispersers. Monkeys can be useful: sometimes they eat the fruit where they find it, but sometimes they carry it away from the tree. A young almendro, as is commonly the way with tropical-forest trees, will not grow close to its parent. Wide dispersal is necessary. For this, the most important disperser by far is Barro Colorado’s largest fruit-eating bat (although it still weighs less than two and a half ounces). Fruit bats do not hang around on fruiting trees. If they did, they would be picked off by the meat-eating predators that also lurk in trees (waiting for the fruit predators) or by owls. Instead bats carry the fruit some distance away, to a quiet roost where, says Dr. Leigh, “they can chew off the pulp in peace.”

But mere dispersal is not enough. The seeds of the almendro also have to be planted. Bats do no planting. However, when they drop the pods (they are interested only in the pulp around the outside), these are found by agoutis, which eat some of them with the seeds inside, but also—like temperate squirrels with acorns—bury some against leaner times. In some years almendros bear fruit while other trees bear very little, and then the agoutis eat all the almendro fruit. If other types of fruit are available, then some almendro fruit pull through.

Clearly, this process of seed dispersal is extremely chancy. The fruits and seeds of the almendro must first run the gauntlet of a whole range of animals, most of which simply gobble them up. Eventual success depends on the good offices of two very different kinds of animals: the fruit bat in the air and the agouti on the ground. The bat does not eat the seeds themselves, and so is a reasonably safe ally. But the agouti does eat the seeds and is useful to the tree only because it sometimes fails to eat all of them. Partly this may be because it is simply forgetful (although it is always likely to find the young germinating almendros). Agoutis may fail to recover all their buried booty too because, between the burial and exhumation, they are themselves eaten, notably by ocelots, the midsized spotted cats of tropical America. But the almendro also contrives to satiate the agoutis—to produce more seeds in a given year than the agouti ever gets around to eating. Big crops matter. This is why the almendro and other such trees
need
to produce good crops. A poor crop (caused by quirky weather) means total wipeout for the particular year.

But, says Dr. Leigh, the almendro has not been replacing itself on Barro Colorado. There are very few young trees. Perhaps, he says, this is because the island has too few ocelots, and so has too many agoutis: too much of a good thing. So perhaps we should say that the safe dispersal of almendro seeds requires three kinds of animals: fruit bats, agoutis, and cats to keep the agoutis in check. It seems a very precarious existence. But up to now it has clearly worked, or there would be no almendros—and as of 2004 there were some saplings, since such ocelots as there were had apparently reduced the agoutis.

Clearly, though, overall diversity is necessary for the survival of any one species. The elusive concept of natural balance matters. Trees are not adapted simply to the presence of particular animals. They are adapted to their whole environment: climate, flora, and fauna. But on the whole, they are particularly reliant on particular allies.

My second tale is an anecdote from a different continent—but it again shows how the fate of trees depends so much on the caprices of environment and (increasingly) on the whims of human beings.

At the magnificent Forestry Research Institute in Dehra Dun, near the foothills of the Himalayas, Dr. Sas Biswas likes to show his students an impressive row of
Chukrasia velutina
trees that form one side of an avenue along one of the main streets across the institute’s huge campus.
Chukrasia
is a relative of the mahogany, and these trees grow tall and straight—and, along this roadside, they are perfectly evenly spaced. The question he puts to the students—and to me—is “Who do you think planted them?” All who are asked pluck various plausible bigwigs out of the air while Dr. Biswas looks on with mounting glee. Pandit Nehru? Gandhi himself? Some passing British royal? When the students finally run out of steam he reveals the answer: “Ants!”

How can it be? It is easy to see how ants might help plant a tree. They could carry the seeds to their nests if the seeds are small enough. Those of
Chukrasia
are only a couple of millimeters long, and ants are prodigiously strong. But how could they space the seeds so neatly? Colonies of ants are often compared to armies, yet they receive no military training. They do not naturally distribute themselves so evenly, or in lines as straight as the cavalry’s tents at Balaklava.

On the sites where the trees now stand, there once were small beds of the white-flowered
Tabernaemontana coesnana,
a relative of the oleander, planted for decoration in brick containers and regularly spaced. Sas Biswas remembers them from the 1970s, as he rode past them every morning on his bicycle (people tend to stay a long time at the FRI). This plant repels most insects. Only the ants have learned to live with it. So they did carry seeds to the beds of flowers—from an old, big, mother tree that’s still growing on the other side of the road—and the seeds that the ants didn’t eat themselves escaped the attentions of other insects too. So the surviving seeds germinated—one or two within each small bed. And so, within the working life of Dr. Biswas, they have sprung up—“Before my eyes!” he says, with a huge smile.

But for one of the most intricate stories of seed dispersal we must return to the figs.

WHY SOME FIGS HAVE BIG FRUITS DESPITE EVERYTHING: THE MYSTERY SOLVED

The syconia of figs qualify as bona fide fruits when the seeds within are mature and ready for scattering. Many animals come to prey upon them. Many, like monkeys, may do some dispersing at the same time but on the whole are destructive. The figs’ main allies at this stage of its life, dispersing its seeds without taking more than their fair share, are various birds and fruit-eating bats.

Each species of fig produces either red fruit, to attract birds, or green fruit, to attract bats. Fruit-eating birds hunt by day and rely on vision, and bright red does the trick. Among the birds, mannikins are the main specialist fruit eaters, while tanagers, tyrant flycatchers, and woodpeckers are opportunist feeders, taking what’s on offer but happy to eat other things as well. Bats hunt by night, and for them plain green will do. In fact, fruit-eating bats are less active on nights of the full moon, when there is more light, than on dark nights. On moonlit nights they are picked off by owls: a gothic encounter indeed. The birds that dispense figs are all of roughly the same size and so, accordingly, are the red fruits that have evolved to attract them. The fruit-eating bats are of various sizes. So, correspondingly, are the fruits they eat. Bats, unlike birds, carry fruit away from the tree to eat it at leisure elsewhere, in some more private roost. Perhaps this too is a defense against predators. Otherwise owls (or civets or leopards) might simply hang around the fruit trees in wait.

Here, at last—or so it seems—comes the answer to the puzzle: why figs bother to produce big fruits, which seem to cause them so much trouble. Big bats fly farther than small bats. So, on average, big fruits are dispersed more widely than small fruits. Dispersal, in tropical forests, needs to be as wide as possible. Big fruits seem to lose out at every turn, but in the end, they are worth it.

This, at least in outline, is the fig story so far. Several morals are attached to it. The point that Allen Herre emphasizes is how hard it is, in biology, to relate theory to what happens in the field. The basic interplay of figs and wasps is complicated enough; but then the complications multiply and multiply again as you stir in the cryptic wasps that look like pollinators but in fact are parasites, and the nematodes, whose virulence depends on the number of foundresses, and so on. However much we know about nature, we can never know enough. Science is wonderful—the studies that have led to the present understanding of fig wasps are breathtaking: a brilliant amalgam of natural history, persistence, imagination, and intellect—and yet the more that’s known, the more it seems there is to know.

Already it is clear, though, that if we do anything to interrupt the lives of the wasps—are too free with insecticide, for instance—then we will kill off the figs, or at least ensure that the present generation is the last. The fruits of figs are essential provender not only for bats and birds but for a host of other creatures too. In Panama, figs of various kinds are in fruit all through the year, while most other fruits are far more seasonal. There are times when figs are all there is. Take away the figs, and half the fauna could be in serious trouble. The whole ecosystem balances on a pinpoint—and we could tip it into oblivion without thinking; or, indeed, we could let it slip through our fingers even if we were trying very hard to save it. On the other hand, precarious though it seems, figs and wasps have maintained their relationship in one form or another without interruption for more than forty million years. There is robustness in the system. If only we can work with it, it might pull through yet.

Finally, given that so many trees rely so heavily on animal pollinators and dispersers, we might ask what happens to them if their allies disappear. One highly intriguing answer comes from the island of Mauritius, in the Indian Ocean.

THE DODO AND THE TAMBALACOQUE: A SAD TALE WITH A FAIRLY HAPPY ENDING

On the island of Mauritius lives
Calvaria major,
known as the tambalacoque, one of the vast tropical family Sapotaceae. But in 1977 Dr. Stanley Temple of the University of Wisconsin reported in the journal
Science
that the only tambalacoque trees left on Mauritius were all over three hundred years old.
²
There were no young ones. Since the tambalacoque lives exclusively on Mauritius, these ancients were the only ones left in the world. Yet the tambalacoque used to be common—common enough to be used for lumber. The remaining trees were fertile and were clearly pollinated, for each year they produced plenty of seeds. So what was going wrong?

A little background is called for. Mauritius is an island—one of the Mascarene Islands—deep in the Indian Ocean, a few hundred miles to the east of Madagascar. As we have already seen (not least among the Didiereaceae and the baobab trees of Madagascar itself) creatures on islands tend to evolve in strange directions; and among the strangest creatures on Mauritius was the dodo. Biologically speaking, the dodo was a flightless pigeon—but huge: as big as a modern farmyard turkey, with a great hooked beak, a bewildered expression, and tiny wings and downy chick-like feathers: a singularly daft-looking animal.

So long as Mauritius remained undiscovered and uninhabited, the dodo thrived. But in the fifteenth century, at the start of the great age of discovery, the first European sailors arrived. Passing ships then stopped regularly at Mauritius for fresh water and fresh meat—not least from the giant Mauritius tortoises. The sailors also ate dodoes, though they have left mixed reports of them: some said they were very tasty, and others complained that they were tough and greasy. Perhaps the time of year mattered. Perhaps the dodoes were succulent after the rainy season when they were well fed and stringy after the lean dry season. In any case, by 1681—barely two hundred years since the first European ships made landing—the dodoes had been exterminated. Indeed, they have become the world’s favorite symbol of extinction: “dead as a dodo.” To be sure, it probably wasn’t the sailors that finished them off. European rats, which ate their eggs, and other creatures that the ships brought with them, including monkeys, probably did the trick.

When the dodo went extinct, the tambalacoques stopped breeding—or so Dr. Temple suggested. For tambalacoques produce very big seeds, about two inches across, surrounded by an enormously hard and woody husk up to nearly a sixteenth of an inch thick: too thick, apparently, to allow the young seedling to emerge unless the walls are first weakened. The dodo, said Dr. Temple, did the necessary weakening.

The dodoes ate the fruit of the tambalacoque. They digested the pulpy exterior, and the big wooden pip passed to the gizzard—the extension of the gut that birds pack with stones and use to crush seeds. Tambalacoques evolved seed coats that were thicker and thicker, in response to the dodo gizzard’s enormous crushing strength. Eventually they became so thick that the seeds could not germinate at all
unless
they had first been eaten by a dodo. Of course, the seeds would fail if they were crushed in its gizzard. But if the pips were merely abraded, or “scarified,” they would germinate much better than if not; indeed, they needed the scarification. Here, then, was another example of coevolution. Dr. Temple tested his theory by feeding tambalacoque seeds to turkeys, which are not related to dodoes but are ecologically equivalent. Wild turkeys eat hickory nuts. The turkeys crushed some of the tambalacoque seeds—seven out of seventeen—but after six days or so they gave up on the other ten, and either coughed them up or passed them through their guts. Those ten seeds, duly abraded,
did
germinate.

As we have already seen, many other seeds benefit from some pretreatment from animals in one way or another. In India, teak seeds are sometimes prepared for sowing by laying them out on the forest floor where the termites can get to them. They germinate better after the insects have nibbled some of the seed walls away (though it’s important not to leave the seeds out too long). On the whole, the tale of the dodo and the tambalacoque has all the elements of a classic.

The story has only one shortcoming. It does not seem to be true. Areas of forest in Mauritius have now been fenced, and cleared of the pigs, deer, and monkeys that meddlesome Europeans have introduced to the island over the past three centuries—and lo, in the cleared areas, young tambalacoques have been springing forth. Evidently, it wasn’t the presence of dodoes they required but an absence of imported herbivores. This is excellent news for the tambalacoque. But it is a pity, indeed, to kill off such an excellent story.