The Tree

by Colin Tudge

  Yet the relationships between creatures of the same species—parents and offspring, friends and rivals, males and females—are only a part of life’s complexities. Each creature must perforce interact with all the other species that share its environment, and especially for those that live in forests, the catalog of cohabiting species is very long indeed. Sociology merges with ecology. It is not true (as Tennyson’s line implies) that each individual or each species is inevitably in conflict with all the others. No species, to extend Donne’s metaphor, can ever be an island. Cooperation is often the best survival tactic, as Darwin himself emphasized—and so it is that many pairs and groups of different species are locked in mutualistic relationships that are vital to all participants. Yet even here there is tension. Figs need fig wasps, and fig wasps need figs. Their interdependence is absolute. But as we will shortly see, no once-and-for-all peace treaty has been signed, or ever can be. The relationship is always liable to break down as each partner begins to take advantage of the other—and “freeloaders” (a technical biological term) cash in on both. Machiavelli spelled out the intricacies of such relationships in The Prince. The themes of literature are indeed the themes of nature. Still, though, it is not true, as has often been argued of late, that human beings need to reject their own biology in order to behave unselfishly, as moral beings. Cooperativeness and amity are at least as much a part of us as viciousness. The point is not to override our own nature, but simply to give the positives a chance.

  So what does all this mean for trees?

  To begin with, for a tree to reproduce sexually, it must transfer pollen from anthers to stigmas. In theory, a hermaphroditic flower might achieve this easily enough by self-fertilization, but in reality this is rare. Most wild trees are “outbred”—meaning that even when they are hermaphroditic, they far prefer to have their flowers fertilized by pollen from other trees of the same species (and sometimes, as we saw in Chapter 1, they wind up being fertilized by trees of different though closely related species, and so form hybrids). Pollen travels from the male flowers (or male parts of hermaphroditic flowers) of one tree to the female flowers (or female flower parts) of another tree. Since trees do not move, they must employ couriers. In mangrove swamps, water sometimes serves as the vehicle. In temperate forests, where any one tree is liable to be surrounded by others of its own species, and the weather in general tends to be breezy, the wind does the job. This is hit-and-miss, of course, and trees that do reproduce by wind tend to produce prodigious quantities of pollen. Flick a young male pinecone and the pollen swirls out like orange smoke. One early summer in Oxfordshire I watched two wood pigeons jostle for position in a birch tree. From a field away I could see the thick yellow puffs of pollen that they dislodged—illustrating, incidentally, that wind pollination may be animal assisted.

  In a tropical forest, however, where any two trees of the same species may be a third of a mile apart, with thousands of trees of other species in between, it just will not do to scatter pollen literally to the four winds and hope for the best. In the tropics, only some trees of the open savannah or the Cerrado practice wind pollination—apart from a few, like Cecropia, that grow only in forest clearings. Most tropical trees rely on animals to carry their pollen. This has led to some of the most spectacular examples of mutualism in all of nature.

  ANIMALS AS GO-BETWEENS

  Transmission of pollen by animals requires many layers of coevolution. The flowers, both male and female, must have the shape and color that the chosen pollinator will respond to, and they must be displayed appropriately. The pollinator, in turn, must be geared to the plant’s signals. When bees are at work in a rose garden it all looks simple enough, but in a tropical forest the bee or wasp or fly or bird or bat must seek a particular glint of color, shape, or whiff of scent among a cacophony of colors, shapes, and scents, as a million different organisms send a million different signals to their potential mates, allies, predators, or prey; and there are many other smells besides to sow confusion, including those of general decay.

  How is this possible? It’s as if any of us could pick out the reedy squeak of the oboe when the New York Philharmonic was in full flow—but then, of course, we can; or at least the conductor does. Some biologists have suggested that the ability of a wasp to detect its particular fig or of a nighttime moth to pick up the ultrasound pulse of a bat implies an advanced ability to filter out all extraneous scents or sounds, as any of us can do (up to a point) when chatting at a cocktail party. But perhaps the truth is the other way around. Perhaps particular animals are geared exclusively to the particular sights, smells, or sounds of the flowers they feed from or the predators they seek to avoid, and register nothing else. In the same kind of way we see light and yet are not at all fazed by radio waves and ultraviolet and cosmic rays, not to mention the swarms of neutrinos, that assail us all the time. Our senses are simply not aware of them. No filtering is required.

  We see, too, that coevolution is an exercise in give and take. The tree must buy the animal’s help. Many primitive plants, such as waterlilies and the trees of the custard-apple family, the Annonaceae, allow or encourage the pollinating insect to eat great chunks of the flower itself. Others offer custom-made, especially attractive food, which commonly but not always takes the form of nectar, both sweet and nourishing. Sometimes they lure the pollinators with aromatic oils. Often the pollinating animals eat a lot of the pollen itself. In short, trees that seek insect help must pay a double price. First they must produce the pollen and ovules, and all the supporting apparatus of petals and sepals—but then they must make a surplus, to bribe the pollinators.

  Some of the relationships between trees and their pollinators are somewhat loose: many trees solicit the help of several or many animal helpers, and many animals seem happy to pollinate many different trees—domestic honeybees, for example, are generalist pollinators. But often the relationship is specific. Often a particular plant is completely committed to one particular pollinator (a bee, a moth, a wasp, a hummingbird), while each pollinator depends absolutely upon the particular tree. Generalism spreads the options but reduces precision. Specialization improves the accuracy but also means that the fate of any one creature is linked absolutely to the fate of another. Lose the pollinator (for example, through some overenthusiastic attack with insecticide) and you lose the plants that depend on it.

  Insects and birds are the chief animal pollinators, and for them (unlike mammals, which tend to be color-blind) color is critical. They each have their preferences. Beetles were probably the world’s first animal pollinators (they pollinated cycads long before flowering plants came on the scene), and beetles prefer white flowers. They ignore red. Bees, too, prefer white—although bees are perhaps most alert to ultraviolet, which we don’t see at all: often the flowers that seem to us to be plain white turn out to be colored when seen in ultraviolet light (and intricately patterned—sometimes with a road map to the nectaries).

  Red and purple flowers attract butterflies—and may repel all the insects that prefer white, including the potential freeloaders. Moths are closely related to butterflies, and yet, like beetles and bees, they prefer white. The color preferences of butterflies and moths are reflected in the related Amazonian trees Hirtella and Coupeia. (They are both in the family Chrysobalanaceae, in the Malpighiales.)

  The many species of Hirtella are geared up perfectly for butterflies. They open by day; they are pink or purple (rarely white); they have hoods, which provide the butterflies with a place to land; they reward their pollinators with copious nectar; and they have only a few stamens (the organs that bear the pollen), which are neatly and widely spaced. Butterflies of many species, guided by sight as well as scent, land at leisure on the flowers of Hirtella and feed decorously, coating themselves in pollen as they do so. Coupeia puts its trust in moths—particularly big hawkmoths, which fly by night and hover like hummingbirds to feed. Coupeia flowers open at night, just a few at a time, and are always white. They have a great
many stamens—from ten to three hundred. They coat the hovering hawkmoth (and occasional hummingbird) with liberal quantities of pollen as it probes among the tangled stamens for the nectar—which Coupeia provides even more generously than Hirtella.

  As a relative of the custard apple, the Amazonian tree Annona sericea is pretty primitive; and it is pollinated mainly by beetles, which as insects go are primitive too. But “primitive” does not mean “simple,” or merely “prototype”; and the degree of coadaptation between A. sericea and its pollinators is extraordinary. To be sure, the flowers of A. sericea are simple: three fleshy petals that never fully open, grouped around a central conical knob that bears both the stigma (female) and the stamens (male).

  At about seven o’clock in the evening—soon after dark in the tropics—the flowers begin to warm up, to about 6°C (11°F) above ambient. You may well find this surprising: after all, we all learn at school that only mammals and birds are warm-blooded, able to raise their body temperatures just for the sake of it. In truth, though, many creatures can do this—probably including some dinosaurs, and certainly including some modern insects and some sharks. Some flowers can do it too. The rise in temperature helps intensify their odor. The flowers of A. sericea do not breathe out the sweet smell of violets and honeysuckle that entranced lovers in Shakespeare’s comedies but (says Ghillean Prance) a perfume more “like chloroform and ether.” Nevertheless, it serves its purpose. Beetles (of the particular kind known as chrysomelids), and also some flies, come flocking in.

  The beetles squeeze their way past the fleshy petals to the cone of male and female organs within. This is a common device among insect-pollinated flowers: provide an obstacle that only the desirable insects can overcome. The stigmas at this time are ready to receive pollen, but the anthers, which provide pollen, are still closed. Any pollen that the beetle has about its person may thus be transferred to the stigmas. But the beetle, at this stage, cannot obtain pollen from the same flower that it is pollinating; so there can be no self-fertilization. The beetles often stay to copulate within the flowers, and as they mill about they transfer even more pollen.

  When the beetle has transferred its pollen, the stigmas at the top of the central cone fall off. Then the anthers become erect and release their pollen, and so the beetles become coated in it. Then the stamens drop off. The beetles eat the bases of the petals, and then the petals fall off. Then the beetles can escape (they could escape before the petals fall off, but they generally do not) and fly to another flower—now carrying pollen from the flower they have just pollinated. The flies that may visit also serve as pollinators in passing, and may lay their eggs on the sepals, but the beetles are the main players. Note, in this account, that the flower is seriously damaged, not to say destroyed by the beetle: the flower sacrifices bits of itself to bribe the beetle. But so what? The flower is only a lure. Once pollination is effected, its job is done.

  Flies are only bit players in the life of Annona sericea, but flies including midges are the prime pollinators of many a plant, including many a tree—and including the cacao tree, Theobroma cacao. Again, the cacao flower sacrifices part of itself—sterile parts of the flower—to encourage the midges; and, again, the flower is organized in such a way that as the midge feeds it is brushed with pollen. The flowers are produced on the trunks and branches (this is “cauliflory,” so typical of tropical-forest trees), and the midges breed mainly in the fruit pods that fall to the ground and decay. If the cacao grower is too tidy and clears away the pods, the cacao loses its pollinators. Here, as in all of life, too much hygiene doesn’t pay. Many flowers that are pollinated by flies smell horrible, incidentally, imitating the rotting flesh that flies prefer; and some of them heat up to make it worse. (The most famous is the world’s biggest flower, Rafflesia, from Indonesia.)

  But the best-known insect pollinators, and probably the most important, are the bees. Some bees are very small. Some are extremely large, like the carpenter bees and bumblebees. Others are in between, like honeybees and the long-tongued (“euglossine”) bees. Many are solitary, some live in small colonies like the bumblebee, and some in very large colonies like the honeybee. Many are generalist pollinators, but some—especially among solitary species—are adapted to pollinate particular flowers, which, in turn, are highly adapted to them. Bees are strong fliers: studies in the Amazon in the early 1970s showed bees of the genus Euplasia returning to their nests when released from a distance of fourteen miles. In the normal course of foraging they commonly fly many miles in a day, following a regular route from flowering tree to flowering tree according to the strategy known as “traplining.”

  Some trees attract many species of bees: one study in Costa Rica in the 1970s showed that one leguminous tree (Andira inermis) attracted seventy different kinds, from middle-sized long-tongued bees to big carpenter bees. By the same token, many species of bees visit many different species of plant. But a few—particularly solitary bees—do have very close, specific relationships with particular trees, requiring a great deal of coevolution between the two.

  If a bee, on any one foraging trip, visited many different kinds of plant indiscriminately, it would not be much good as a pollinator. It wouldn’t help England’s wayside roses, for instance, if visiting bees flew off and distributed their pollen among the local clover. But it turns out that on any one trip, most bees (more than 60 percent in one study in the Amazon) focus on only one plant species and very few (only 15 percent) visit three or more species per trip—and this, of course, makes them far more efficient as pollinators. Perhaps this focus reflects an “optimum foraging strategy”: a method by which feeding efficiency is maximized. Optimum foraging strategy has been studied most closely in birds. For example, if a pigeon is given a lot of barley with a few peas, it ignores the peas altogether. Confronted with a lot a peas and a few grains of barley, it ignores the barley. It pays a forager to concentrate on whatever is likely to prove most rewarding on a particular day. By focusing on whatever food source is commonest, and is known to be reliable, the pigeon does not have to waste time in wondering whether any one item is a pea, or a barley grain, or a pebble. By the same token, if a bee once establishes that roses are in bloom, it tends to stick to roses. Let others focus on clover.

  Furthermore, other studies have shown that once back in the hive, colonial bees exchange pollen with one another—not deliberately, but just as they mill about. So a bee that picks up pollen three miles to the east of the hive may pass some pollen to another that is foraging up to three miles to the west—and trees that are six miles apart may thereby find themselves exchanging pollen. Of course, at different times of year the generalist bees switch from one species to another, as each comes into bloom, and thus ensure a year-round supply (or season-round, in temperate latitudes). Then again, although the bees may be generalists, individual species of trees seem to adjust their flowering strategy to the needs of particular types. Thus, in the Amazon, some trees flower in a “big bang” fashion—a brilliant show of flowers in a short season—which ensures that bees in general will notice them. Others, however, favor the “steady state” approach. They produce only one or a few inflorescences per day, over a long period—and this tends to attract the kinds of bees, like the carpenter bees, that habitually fly long distances and follow the same kinds of routes every day. This, then, seems to be a particularly good strategy for trees that are very widely spaced; but, of course, it relies on the regular habits and industry of a few species of bees.

  In the forests of Amazonia the pristine ecology has been much interrupted and to a large extent preempted these past few decades by bees imported from Africa: the so-called killer bees. These are simply a hybrid between an African bee and the familiar European honeybee; and very good honey makers they are too, favored by many beekeepers. They got into South America in the 1970s from a research laboratory in São Paulo, in the south of Brazil, and spread at more than 125 miles per year. By 1982 they were already crossing Colombia, thousands of mi
les to the north of São Paulo. “Killer” is well over the top, but they are certainly aggressive both to people and to other insects. Thus in 1973 Ghillean Prance, near Manaus, observed the insects that came to pollinate Couroupita subsessilis, a relative of the Brazil nut. The visitors included wasps and bees. The only visitor the following year was the honeybee—not necessarily, but very probably, the African interloper. Presumably what Professor Prance saw in Manaus is common all over South America. Presumably, too, the Africanized bees will sometimes do a good job; but in general, it seems unlikely that any one generalist, however aggressive, can pollinate the trees of the neotropics as efficiently as the droves of insects that have evolved specifically to the task.

  Yet the Brazil nut itself might well be a beneficiary. Brazil nut trees are wonderful. They are an emergent species, half as tall again as most canopy trees. Of course, too, their nuts are extremely valuable, and so the Brazil nut is on a short list of Amazonian trees that it is forbidden to fell. So it is that when forests are cleared, the pastureland that is sown or grows up in its wake is punctuated by isolated Brazil nut trees, rather like the big solitary oaks in England’s stately parks, although the parkland oak trees are to the manner born and spread themselves most opulently, while the Brazil nut trees, deprived in middle age of their companions, seem forlorn, magnificent but somewhat haggard. Furthermore, although the Brazil nut trees have been conserved mainly because of their nuts, when they are in the middle of nowhere they are liable to remain unfertilized. Most of their pollinators won’t fly over big open spaces. But isolated oaks in English parks, served by wind that’s laden with pollen from surrounding woodlands, have no comparable problem.